THE SCIENCE OF

PHOTOGRAPHY

THE SCIENCE OF

PHOTOGRAPHY H. BA INES

o.sc. (LONo.), 1'.R.1.c., P.1.B.P., HON. P.R.P.S.

Revised and edited br E. S. Bombaek

FOUNTAIN PRESS Argus Books Ltd., Kings Langley,

Mertfordshirc, England

HALSTED PRESS A Division of JOHN WILEY & SONS, Inc., 6o5 TI1ird Avenue, New York, N.Y. 10016

FountainPrtss, Argus Books Lid.,

Station Road, Kings lAngley, Htrifrrrdshirt, England

Publishtd in fht USA by Halsltd Prm, u dii.Jision of J ohn Wilq and So11s, I11e., Ntw rork

First Publishtd 1958 &eond Impression 196o

&eti11d Edition 1967

Third Edition 1970

&etind Impression 1971 Third /mpre.ssi011 1974 Fo11rth Impression 1976

© Argus Books Lid., 1976

ISBN o 85242 210 5

L.C. No. 13-19208

Printtd and Madt in Grtal Britain by O;rlq Press Limittd, Long Ea/011 and Edillburgh.

EDITOR'S NOTE

AT the time of his death in 1963, Dr. Baines had already siarted on a new edition of 'The Science of Photography', From the notes which he left, it WI!$ obvious tha1 he planned

considerable revisions in those sections which dealt with the practice of photography. I t is therefore chiefly along these lines that I have amplified 1he book.

The introduction of new material, without expanding the physical site of the book, has necessitated some condensation of existing material as well as some rearrangemem of chaptcni. New material includes chapters on Light Sources in Photography, Photographic Exposure, Printing the Negative, Cinematography and Stereo­photography. The chaptcni on the Camera Leos and the Camera have been expanded to include modern equipment.

The task or revision has been undertaken with diffidence since Dr. Baines was an acknowledged master of the art of explaining clearly and simply subjccu of a scientific and technical nature. I am i11dcbtcd to l\fr. E.W. H. Selwyn of the Kodak Rcscan.:h Lab­oratories for much helpful advice and to many other mcmbcn of Kodak Ltd for helping me to obtain suitable illustrations.

E. S. B

CONTENTS CHAPTER I

HOW l'HOTOCRAPH Y 8ECAN

I'r':.~.::l:11i,;~i:;·~~n,~! '!~~·~::r,~~~~u;;-ro!:!.t~!:'~'li~~: process 3nd the gela tin dry pla1e-T hc photogroph><: indu11ry-Roll filrn­}'osi1ivc p rin1 rn111crials.

CHAPTER 2

THE BEHAV IOU R OF LIGHT

CHAl~l'ER 3

LIGHT SOURCES USl::D IN PHOTOGRAPHY

CHAPTER -l

THE CA~I ERA Ll?.."S

Lent abc:rn•ion-Evohuion of the ca men lcnt--1..cnJ al)(r•ure-f/nun•bc._

::~!.;.',~.:~i:.t~~:rit:~~i!~i~~.~;;;~; •'erticalt-Ci~k or confu1ion ar>d depth or focu-n..pth of fldd - Lcns Jl"rl"Ormancc .

CHAPTER ~

THE CA~IERA

CHAPTER 6

THE CHE~llSTRY f"lf l' HOT OCRA PH\'

..

EM ULSION MA KI NG AND COATINC

CHAPTER 8

THE PHOTOGRAl'H IC LATENT IMACf. .

Dcvdopmcnt Ccntrc.....Sl>cpP.anl'• da-.,kal work-Structure or tl>c atom­Elcc trkal conduc1h•i1y of 11kc r halidc--l'hotoeonduc1ance-Curncy·Mo11 cheo11~La1cntinll!.gcdiJ1ribu.1ion- L..ucn1•ub-ima,11c.

CHAPTER 9

DE\'El.Ol'ERS

Cl-L\PTER 10

Tll E ~ll!CHAN ISM 01' DEVELOPMENT

.. 1'23

Solubility and su.,.:na1unuion--011 wakl·A~gg theory of de•·clopment­A parallcl problem.

CllAl"'rER II

FIX,\TION AND STABl l. ISATION

~~iid tt;t~~~~:i:::"i;:!,!~~~~:-~!~1~d -~~i~.!i~= -S1ab1li.a1ion.

CHAPTER 1'2

SENSITOMETR Y . ,, 15?

~...'i:1~~n~~;'!~~~·ac 1d~~~~~YC:.r:;;-~~.;':.,";:~~c~Pa;;tf°'."~ diffuse dcno.i tics-Rcflcction dcnoicy-P~pe• dcmicy range. l

CHArTER l3

THE CHAR1\C1'£R ISTIC CU RVE ANO ITS INTERPRETATION: TO:'<F. REPRODUCTION . . . . . . . . . . . . 170

~=;~~~11!°J:,~ t!,';'!!:~' ,,::c;;;::~ 1~~~~~1"·~~;;;·~h:e r;t::; pri nt.

C.:llAf'TER 1'4

CO i.OUR SEN'S IT IVl'rY A:-<0 fl t. T ERS .• l8?

Cll1\PTt:R lS

RECll'ROCIT\' •'All.Uk£ ,\ NO OTHER l'HOTOCRAPlll C f.ff ECTS.

(:llAPTEk 16

PHOTOCRAPHIC EXPOSUR~:

CHAf'TEK l7

A•,'ER·TREATMENT o•· T HE NEGATIVE

CllA 1~rnR is

T l!£ C RANU l,r\ R STRUCTUR£ O•' Tllf. IMAGE

~~~~~5§l1~~~~~¥~3l~~f·~~~ CHAl'T ER 19

l'RINTl:'\C T HE NEGATIVE .

COLOUR PHOTOCR,\Pt!Y- T HE ADDITIVE PROCESS

l'crccntaj!:ctcftccciont--Scp.aratccolourocrccn- Randomocrccnrnc1hod­Cwmetnc S<:tten rncthodJ-1.enticular S<:rccn rne1hod1-0i ... dvantagct of thcaddiiivcproectSoell.

COi.OUR PHOTQCRAPl lV- THE SUBTRACTIVE PROCESS

Colour development- Exposing ihc three ttcord-Mulli·lnycr fih11J-­Colour b)' couplcr-dc•·clopcr...--Colour film with couplers in d•c: emul•ion.

CllAl'TER 22

COLOUR PHOTOCRAPUV COLOUR PRINTS AND COi.OUR MASKING .. 270

CHAPTER 23

.• 277

Pe..W1encc of vision-Film 1iic and piclutt format-Blad-and-white: filn1J-­Colour filrn-.5ound rccordin!(.

STER EO·l'HOTOCRAPHV

Binocular •·ision-T ht making of 11Crw-photograpt.-Vic: ... in1 l)"S1cm1.

CHM'TER 2.'.I

TH E APPLIC,\TIO:>:S Ol' l' l lOTOCRAPHV .. .. 28.'.I

!':~~~~~:·~~\~~11~.r ... h;,-;~~;t~hJ;~~~:;=i-~~::: JT*Phy-Pho!Ofll':llmmccry-Lqal ., .. 1 rorc.uic phol;ograph)~l'hoiography in nuclear tt11:arch-Photo-re<ording- Phot01Cmiti.-c ,..,,i11-Pho1ogrophy in educa tion.

CliAl~rER 26

oocu:-.n:!\-r COl'Vl:>:c •• 30.'.I

Microphotography-!\licrncopyin1-:'>liero print- Nornu•l 1iu copU.­Offie<: mcchodJ.

8181. IOCRAPl lV

IN"Of.X.

Chapta I

HOW PHOTOGRAPHY BEGAN

F THE F I VE SENSES man J>05$e$$CS, perhaps that which gh·a him 1he most u.tisfaction and pleasure is sigh1. Certainly some or the early records or liis existence on 1his plane! consist

or his attcmpu to perpetuate in 1he form or paintings on flat (two dimensional) surfaces the three-dimensional Kencs around him. Ah hough 1his method or rcprcscma1ion seems so commonplace, we should nol under-estimate 1he gcniui of 1hc first invcn1or. We can imagine his primi1ive companions examining 1hc resulu-to some they may have seemed no more than marks on stone, while others would smrc umil by some quirk of the brain the marks suddenly be· came transformed into deer, biwn, and mammoth. Many such paintings must have been oblitera1cd, but some or 1hosc painted on ca\·c walls have survived some 15,000 years or more. Methods or rep­resenting three-dimensional subjccu in a 1wo-dimcnsional plane, by pain1ing on a flai surface, improved, until in the scuentccnth and eighteenth ccnturia artists were so skilful that further improvement in rcalislic rendering may well have seemed impcmiblc. Perhaps for this reason 1he las1 two or three ccnturia have seen a movemenl in the dircc1ion or non-realistic reprcscnuuion in an aucmpt to caplUrc and por1ray the essential reeling or a p.1.rticular subject. The cfforu themsclva have ccrlainly been successful in c•·oking a variety of cmo1ioni-in1cnded and otherwise.

J."or at least rour centuries, however, we have known an alternative method of representing a view on a fiat surface, which giva results truer to the original view in form, colour and range ortona than any pai111ing C."l.n pcmibly give. This is achic\·ed by throwing an optical image or a view on to a fiat surface by means ora lens. The apparatus ~ for this purpose is called a callllTa abKUro, and consisu or a box or room with a lens al one end and a flat surface at the 01hcr. If the camera obswra is so small 1hat the image muSI be viewed from OU1$ide, a translucent K rccn such as ground glass is U5Cd, hut if the ap1>aratus is large enough to enter, 1he image is thrown on to a white reflecting surface for viewing (J."ig. 1 ). Such an image, C\•cn !hough

II

Tl/£ SCIENCE OF PllOTOGRAPHl"

invcrled, takes on a kind or magical rca[ily and i1 is no wonder tha t ar1ists despaired when they saw suc:h perfection of drawing, de ta il and colour. I t is not 5urprising that many or them used it as an aid to drawing or that some thought in 1cnm or fixing 1his transient image 10 make a permanent, 1rarupormblc picture.

It is doubtless to such cmo1ions tha t photography owes its exist· cncc, since the first two succcuful proccs.scs or photography were invented, a lmost simultaneously, by men familiar with 1he 'amera obs"'ra: Daguerre, a French painter or diorama.1; and Fox Talbot, a \Viltshire squire, who had been using the camera ludda, which, like the camtrao/lseura, isanaidtosketching.

Pho1ography coiuists essentially in producing an image ora sub. jeer by physical mcalU, and a permanent facsimile or lha1 image by chemical mcaiu. The image may be 1he shadow of an obj eel cast by X-rays or gamma rays or the trace of a moving beam or cathode rays, but in more normal photographic: procedure, it is the optical image produced by means of a glass lciu in 1he camera, an apparatus similar to a tamera obK111a.

The Camera

Perhaps the first reference to the camera obseura wi1h lciu is that or Cardano in the sixteenth century, but similar devices may have cxiued much earlier. G lass has been knnwn frorn ''CQ' ancjc;nt times -as early as 1400 11.c glass marrnfricmrc ju Egyp~

j~~l~~:S~1=~:;ii~1 :J?c"~t~~:;·~i~:;;;s';n"J~~: ~I purposes of glass globel filled with waler was common p.r:acticc.for ~me •hree rhomand l eao The science of oplies, 100, is an ancient one-the formation of images by pinholes (p. 27) was mentioned by Aristotle abou1 350 u.c., and some fifty ycan later Euclid published a treatise on optics which con1ains the fint known corutruction or an image by gcomeiric means. Some fi•·e hundred ycan later, in the second century A.O., Ptolemy published tables showing the amount orbendingor lightal an air-water interface, but he failed to deduce the law governing such refraction. Although glass lenses may ha\'e been used for image formation and as an aid to vision for many centuries previously, it was not until about the end of lhe sixteenth century that they were combined lo produce scientific intrumcnts such as the telescope and microscope. The laws governing combinations or lenses were discovered by Kepler about the end or this century.

11011' PllOTQ G RAPllr BEGAN "

Fi,. L F.arly drawing of C<1"'"• Obse>1Ta.

Thus, knowledge of prac1ical optics four hundred years ago, and perhaps much earlier, was adequate for the construc1ion ofa crude camera similar in essentials and principle to the modern photo­graphic camera. Nearly three hundred years elapsed, however, before serious attcmpu were made to render 1hc optical image pennanen1.

The Evolution of the Photo11raphic Process

To make a reproduction of an optical image by utililing the ligh1 energy, one must employ a substance which undergoes some chcmi· a l ·

.undergo on exposure 10 lighl will be described in later chapten, but in order to understand the evolution of photography, it should be know11 that they arc insoluble in water, and a rc formed when silver n itra te (a soluble si lver salt) reacts with soluble chlorides, bromides

TllH SCllf.NClf. OF l'llOTOCRAP/11"

or iodides such as sodium chloride (common sah ), potassium bro· mide or potassium iodide.

Silver nitrate was known to 1\lbertus Magnus before 1280. Schuhze in l 727 rCCQrded shadow image5 of opaque stencils on bottles co11taining chalk and si l\•er 11itra1e, and Thomas \\ledgwood and Sir Humphry Oa\'Y in 1802 described the production of shadow images of such objects as feathers and leaves on leather and paper impregnated with silver salts. Howe\'er, no method was known of fixing the image to preve11t the picmre from darkening further when it was examined in the light. Attemp1s to m:1ke prinu in a camua ol:isrnra were unsucccssful.

Early Experiments In France

Meanwhile in France experimcnu on \'Cry dilfcrelll lines were pro­ceeding, with greater success. Nicephore Niepce in 1826 had sue· cccdcd in obtaining a record of a \•iew by exposing in a camera asphahum coated on to metal or stone (Jo' ig. 2). Light renders asphaltum less soluble in aromatic oils, and after dissolving the unexposed portions of the layer, the plate could be inked up and used for printing. The process wu not very practicable, and Niepce spent much time trying to impro\·e it , both alone and after I 829 in partnership with Daguerre. When Niepce died in 1833, Daguerre reverted to the use of silver sahs as the light-sensitive mcdiu1n. In 1839 came the announcement of the Daguerreotype process whereby a silvered copper plate was exposed to the action of iodine vapour to form a surface la)'Cr of light-sensit ive sih•cr iodide. After an exposure in the camera insufficient to produce a visible effect, the image could be revealed by t ~ating it with mercury vapour. The mercury was deposited preferentially on the exposed portions to give a l>O$ith•c image. The Oag\1errco1ypc process l>ceame very popular, but in competition with the wc1 collodion process invented in 1851 it witlu;rcd and died in less than two decades.

The Beginnings of the Modern Photographic Process

h is to Henry Fox Talbot that the tide inventor of moch:rn photo­Kraphy is given. Abo working with a tamtra obsC11ra and a 'negative'

11011' f'llOTOCR.~PllY BEC.-IN "

Fif. 2. Nitpet:'• li rs1 pho1ograph (1826) . From 1hc •hBdow po<ilimo<. ii i1 deduccdtha11hepl•lt"'Hcxposcdfor1hcwholeofa1unnyday.

FO,. 3. Louiijacquet Man<tc Daguerrc.

THE SC I ENCE 01' l'HOTOGRAPHT

l'if.ol. William H~nry Fox Talbot

~·;,. 5. Talbo1's first phoiograph( l835).

HOW l'llOTOGRAl'H1" BEGA ;\'

Vi1.6.0ncofTalbo1't firs! camc:nu •ho""" roughly life-size. They were nick.named

'~lousc:1raps'.

material consisting or paper coated with silver salts, he produced in 1835 a number of views of Lacock Abbey.

Unaware of Davy's work, his early experiments consisted in $0aking paper in common salt solution and then in silver nitrate $0lution, thus forming silver chloride in the body of the paper. The dried paper required an hour's exposure before it was sufficiently darkened, but by repeating the impregnation of the paper in this order several times, and exposing wet, he was able 10 reduce expo­sure to ten minutes. He fixed the image by treatment with either potassium iodide or sodium chloride. In 1839 he used sodium thio­sulphatc (hypo) for fixation, and by 1840 he had substituted silver iodide for silver chloride as the light-sensitive medium. In his attempts to reduce the required exposure, he made a discovery of great imporiancc. He found that he could make his silver iodide paper more sensitive by treating it with a mixture of silver nitra te and gallic acid. Jl.foroovcr, after giving a short exposure (half min­ute), sufficient to produc<: only a faint image, this image could be built up or dectl1JjJtd by further treatment of the exposed paper with ~is sensitising w lu1ion-silvcr nitrate and gallic acid. From this

TJIE SC/IJ.",VCE 01'" PllOTOGRAPllY

observation has evolved the modem technique of exposiog to give an invisible laknt image, and its subsequent dcvclopmem.

The paper ncgatke wu impregnated with \\"ax to make it more translucent, and prints were obtained by printing through the waxed negative on to similar sensitive paper, the whole process being known as the calotypc process.

In essential principles it was identical with modern black-and­while photography in that a layer of light-scruitive si lver 53. lts is applied to a translucent support, exposed, developed, fixed and washed to give a negative, which is then printed on an essentially similar material to gi••e a posilive print. A series of impro•·ements, discoveries and invemioiu has brought about the evolution of the ca\otypc process into modem practice. The most obvious way of improving the calotypc process was to replace paper as a support for the negative material by something more transparent, for example by glass. But since the silver 53.its could not be formed in­side the glass, it was e»cntial to devise another medium in which the silver 5alts could be suspended, and to apply thiJ to the surface of glass. In 1847, Niepce St. Victor used white of egg (albumen) for this purpose. Albumen containing potassium iodide was coated on gla.$$, sensitised immediately before exposure by bathing in silver nitrate, and aposcd in the wet state.

The Collodlon Process and the Gelatin Dl"J Plate

Numerous apcrimenters were ancmpting to apply media other than albumen to photographic practice, and two materiab anracted par­ticular attention. One of these was ul/odion, a solution in alcohol­ethcr of a material called pyroxylm or ullulose nitrafl (sometimes erroneously termed nilroullulose), which was fint produced by Schocnbein in 1846. The other was gelatin, obtained from the hides, hooves and bones of animals. Success wu first achieved with collo­dion, when Scott-Archer, in 1851, described the wtt collodion proct.SJ (Fig. 7). A solution of potas.sium iodide in collodion was coated on glass, allowed to set, and soaked in si lver nitrate solution, thus form ­ing silver iodide in the collodion layer, the accn silver nitrate acting as a sensitiJcr. If the collodion layer were allowed to dry, it beca.mc impervious to processing solutions, and therefore the platC$ had to be exposed and pl'OCC$$Cd immediately after manufacture, while in the wet state. They were developed in ferrous sulphate solution, inten­sified and fu:ed in potassium cyanide.

11011' l'HOTOGRAPJIJ' BEGAN

~·.,. 7, Frederick Scon Arclw: r. ( l n\'cnror of 1hewe1collodionpro-

u~)

r.,. 8. App.a .... 1ut for the: "·c1 collodion P<ncu.. (From a con­lcmporary u1a loguc.)

"

TllE SCIENCb' 01: f'llOTOCRAPllT

,.-;1.9. T1>ew.,1 c0Uodionpror-ir11helidd.

The wet oollodion process was fortunate in receiving unusual and immediate publicity, as examples were shown at th~ 1851 Exhibition, and it speedily displaced a.ti other proccs.scs and rc1g_ii.ed supreme. for nearly thirty years. Photography became .a .nuhionablc pastime among the leisured and moneyed class; soc1et1es were forn~ed (the Photographic Society of London in 1853-latcr bcconung the Royal Photographic Society) and journab devoted to p~1?tography appeared (T"4 l'hotographit: Journol in 1853, and T"' llT1/1Sh Journal ef PholfJgrapli.J in 1854). . .

The necessity for the photographer to manufacture hu plates ~ust before use was .l great inconvenience in studio work, bul the d1ffi. culties were greatly enhanced when the wet collodio11 process was applied to work in the field. Neverthel~ enthusiasts were prepared to transport in a donkey-cart or on their backs not merely a pro­cessing dark room, but a small photographic facto_ry, and cataloS'_les of the period reveal an astonishing and amusmg r.ontrast with modern miniature camera technique (Fi~. 8 and 9).

Another disadvantage was the extreme care n~ry when _hand· ling the wet oollodion coating, which. was so fragile that 1l was damaged by the slightest touch. £xper1ments t? produ~ the more robust dry plates using either collodion or gelatm as medm gave, at the time, much less sciu.itivc materials. The first su~ful atte~pt w;u the dry collodion plate of Dr. J. M. Taupenot 111 1855, which required an exposure of about 30scoonds atf/ 16. The first succ~f~ l use of gelatin in photographic emulsions (lhc name tmulnen 1s

110 II' PllOTOGRAPllT BEGAN

gh•en to the. light-scruitivc preparation ~led o.n glass~ film ?r paper) is ascnbcd to Dr. Maddox, who described hu cxpenmcng JI\

187 1. This was the forerunner of modern emulsions, and caused the downfall of the wet collodion process. The latter, however, was so well established that several years elapsed before lhe dry plate gained ascendency, and indeed wet collodion pla tes arc st ill used in the photo-mechanical industry.

The Photographic Industry

TI1e invention of the gelatin emulsion caused a revolution in photo­graphic practice. Wet collodion practitioners had perforce to manu­facture their own plates immediately before use. Gclmin dry plates could be made at leisure and stored for future use. The photographer skilled in emulsion making could now produce more plmcs than he required and find a ready market for the excess among those un­skilled or uninterested in 1his ac tivity. Indeed some made the manu­facture, rather than the use, of liglu-sensitive material their sole activity. Thus began the photographic industry as we know it today. A secondary change na1urally followed. Prior to the 1870's it had been the usual practice to publish improvements in emulsion-making technique so that they could be generally adopted. Now that live­lihoods depended upon the quality of ma1crial, the tendency to publish methods of i1nprovemcnt diminished, and as early as 1873, Burgess marketed a dry plate made according to a secret formula, thus setting the pattern for subsequent manufacturers.

~faterials were sold in the fonn of emulsion (to be coated on plates by the user), of dried emulsion pellicles (emulsion sheets stripped dry from glass after coating, and sold in packets), or of coated glass plates. Among the early, successful manufacturers of dry plates was the firm of Wrattcn and Wainwright who marketed their London Ordinary Gelatin Dry Plate in 1877. It is recorded by Dr. C. E. K. ?.<fees {who became a panner of the firm) that the emulsion was at one time made by Mrs. \Vratten in her kitchen and was nowed on to the glass plates from the spout of a teapot!

The Roll Film

The second revolution in photographic practice Wa.$ brought about by George Eastman, who had embarked on dry-plate manufacturing

TUE SCIENCE OF PUOTOGRAPJ/r

F;,. 10. The first Kodak camera, produced in 1888.

in Rochester, New York, U.S.A., in 1880. Although photograph· crs had been relieved of the nCCC$$ity of m:mufacwring their own sciuitive material, they 5l ill had IO do lheir own processing, which confined the arl to those wi th some practical dexterity and sufficient capital to equip a darkroom. Eastm;m visualised a system whereby those with no inclination for processing material could load and unload a camera in daylight, and have the exposed material pro. ccsscd and printed commercially. For daylight loading, he used a method originally proposed by Spencer and '.\lclhuish in 1854, when:by emulsion-coated paper was fitted into a camera in lhe form of a light-tight roll. The paper roll was waxed to give a 1raruparcn1 negative, but soon gave p!ace, in 1885, to an emulsion w~ich could be stripped from iUI paper support, and finally to emulsion COl\ted on to cellulose nitrate film. T he rnbslitution of 1-011 films for glau plates, the introduc1ion of inexpensive but efficient hand cameras (the first 'Kodak' roll-film camera appeared in I~) and film processing by the manufac1un:r made photography available 10 the masses anxious to take advantage of Eastman's proposition, 'You prCS$ thc button-we do the rest'. (Fig. 10). The roll-film has gained in popularity and has been progressively improved. 'Non-curling'

I/OW PJIOrOGRAPJIY BEGAN

I-if. ll. George t:asun.an {185'i- 1932). 1hcfoundcrof

chc Kodak Company.

fil m appeared in 1903, panchro1natic roll-film in 1928, antihalo­backed roll-film in 1931, 35mm. perforated film on safety base for special miniature cameras in 1929; 'safety' (non-inflammable) base of ccllul05C acc1ate has of rccenl ycan been applied to all film producu.

Positive Print Materials

Until about 1860 the calotype process used a similar material for both negative and positive stages. Parallel with the specialisation of negative malerials which resulted in modern roll-films, sheet· films and pla1cs, 1hen: has been an evolution of the positive paper to give the prcscm chloride, bromide and chlorobromide printing papers. The decades 1850--70 saw the introduction of albumen­silver chloride, and of collodio-silver chloride papen, and in the following decade the clcganl pla1inotypc process and the silver bromide development paper made their appearances. Gelatin print-out paper was introduced by Abney in 1882, the image being produced solely by con1ac1 priming in bright daylight. The excess silver salt was n:mo"cd by fixa1ion in hypo, and the inclusion of a gold salt in the hypo bath COn\'er1ed the unpleasant colour of the fi nely divided silve1image10 the more satisfactory colour ofa gold image.

Tl/£ SCIENCE OF f'llOTOGR11J'llr

Fur1her advances were Eder's gaslight paper in 1873, and llacke­land's 'Vclox' paper in 1894. These were slow chloride development papen and were much more efficien1 in producing results than the printing-out papen, yet could be handled iafely in weak artificial light thus obviating 1he need of'darkroom' technique. Also the tone of the image- a deep blue-black-was held in great esteem. The invention of gaslight paper had far-reaching results, as ii was ideally tuitcd to the mass-production of prints. ' l11us it can be said to be one of the main causes of the rise and growth of commercial photo­finishing which greatly popularised photography.

Other prOCC5SCS were introduced. The ca1bou proct.D of 1864 and ~lanley's modification, the Ca1bro proct.Dof 1905, depend on the pro­ductioo of reliefimages in pigmented gclatio (by rendering insoluble bichromated gelatin) ; \\'clborne-Piper's bromoil prouu of 1907, de­pending on the differential at.ecptance of ink by image-wise harden­ing of gelatin, gives superb results and is still practised by pictorial worken.

The ch\orobromide papen, in1ermcdiatc in speed between chlor­ide and bromide papen, were introduced in 1914. They are capable of gi\•ing a wide range of image colours from warm black to red, and arc deservedly s1ill growing in popularit)'·

It has been common practice for many )'tan to eliminate the natural grain of paper by pre-coating the paper with a layer of finely divided barium sulphate suspended in gelatin (the so-called baryta coating). This gives a highly reflecting white surface which can be made man or glossy at will. The slightly )'Cllow tinge d11c to the gelatin of baryta and emulsion coats was counterbalanced by incorporating a blue dye, but in recc:nt )'Ca"' 1he blue d)'e has general!)' been replaced by a fluorescent dyestuff which converts invisible ultra-violet radia1ion into blue light, and thus gives an overall higher visible reflectance.

The evolution of the calotype process into modern pictorial black­and-white photography is thus a straightforward story. When we consider colour photography, <:inc photography, radiography, nuclear track recording, .documcn1 copying and a score of other applications, we find lhat both materials and processing ha,·e been greatly modified 10 suit the manifold applications of photography toda)'. Bul the fundamental basis, though disguised, is still that of Fox Talbot's calotype process--1hc exposure of a light-sensitive layer containing a siker salt, its development to give a desired image, and the fixation of the image in a manner which will give the required permanence.

THE BEHAVIOUR OF LIGHT

I N THE PREVIOUS chapter we saw that photography consists essentially of the production of an image by physical means, and the formation of a permanent facsimile or that image by chemi­

cal means. In pictorial pho1ograph)•, the physical image is an optical one, produced in a camera by a lens. Before we can study the fu nctioning of a camera in detail, we must learn something or die fundamental nature of light, and the formation of optical images.

Light has always had a f.'UCinalion for man. i\r1i.sts ha\·c striven to reproduce iu effects, poets have praised it and philosophcn have speculated on its nature. But experimental science alone has provi· dcd reliable evidence to give il substance. In 1he seventeenth cen­tury, Newton concluded from his smdies that light consists of tiny particles emitted at high speed from a light source, and that 1he impact of these corpuscles on the eye produces the sensation of sight. Huygens on the other hand considered light as a form or wave mo1ion in a hypothetical medium which pervades all space-empty or full-and which is called the tllitr. Further evidence accumulated during the succeeding lwo centuries supported the wave theory, so that ninctcc11th-ccntury physicists were convinced of lhc correctness or Huyge1u' wave theory; and the incorrectness of Newton's cor­puscular theory. The present century has produced a great deal of further evidence which can be adequately explained only if light is emitted in discrete particles, a postulate which is known as the Qpantm11 Tkory. It is now considered that both theories are correct­therc is a border region where subatomic particles show wave-like characterist ics; where waves and particles are indistinguishable.

Mos1 of the normal optical phenomena, howc\•er, are adequmcly explained on 1hc wave theorr. Indeed, energy may be tra1umiucd through space by a fonn nf wave motion of elcc1romagnctic origin, the energy manifesting itself in different ways according 10 1hc wave-length (that is, the distance between corresponding parts of adjacem waves, as for example from crest lo crest), though the velocit)' of the wavC!I through empty space is the same for 1111 wave­lcngtlu-186,000 miles per second.

"

TllE SCIENCE 01: PHOTOGRAPJJ1"

Prior to 1801, the only form of such energy tra11sfcrence recognised by man was 1hat of visible light, which has a range of wave-lengths from 400 to 700 mµ, an mµ or millimicron being one millionth part of a millimetre. Not only can the eye detect this range of clcc1ro­rnagnetic waves, but it can distinguish between radiations of

Fit. l. Complctc•pec•rum ofckc1rom:1gnctic r;utia1io11 .

different wave-lengths within the range, since these arc perceived a$ differences in colour, from red at 700 mµ through orange, yel low, green, blue and violet at 400 mµ. This range of colours, which is called the visible sputrum, occurs in nature a$ the rainbow.

In 1801 two discoveries of profound importance wel"c made. Herschel discovered tha1 beyond the red end of the visible spectrum were rays which ha\·e a heating effect and which arc termed iiifra-rtd rays. A few. months later Ritter found beyond the violet end rays which could blacken silver chloride, and which are called 11f/ra­vi11/el rays. Thus one can almost claim that even before it was invented, photography discovered the ultra-violet! Since that time means have been found of generating, detec1ing and utilising an enormously wide band of electromagnetic waves ranging in wavc­leng1h from one-millionth of a millionth of a centimc1re to some five hundred miles. They included cosmic rays, gamma rays, X-rays, ultra-violet, visible, infra-red, heat rays, and wirelCS$ waves(l.-ig. I) .

Rectilinear Propagation of Light Light navels from a luminous source in all directions and in siraight lines. A point source, therefore, sends out rapidly expanding spheri­cal waves of light. Normally we should be concerned only with a portion of the wavr.-front such as that passing through an opening, and 1his is referred 10 as a 'beam' (for a wide band) or a 'pencil' (for a narrow band) of light. It is often convenient to consider the behaviour of a beam of light so narrow that it may be considered

Tl/£ IJEllA 11 /0UR OF LIGHT

as a straight line of zero width, and this is called a ' ray' of light­Fig. 2. As a beam of light may be considered as composed of an infinite number of rays of light side by side, the study of one or

LIGHT SOURCE

Fif. 2. l'rop~gation ofligh1.

PENCIL RAY

two critical rays may define the behaviour of a beam. Beams and pencils can, of course, be divergc11t, parallel or co1wergent, but a ray of light having theoretically no dimension other than length can neither diverge nor converge.

The conception ofa ray of light is not easy on the wave theory, as i1 is difficult 10 imagine a wave front of zero dimensions. It is much easier on 1he corpuscular thcorr as it would 1hcn represent a stream of minme bullets. However, our Victorian wave-propagandists

l'if.3. The Pinhole camera.

saw nothing anomalous in using the simple conception of lighl rays to explain light behaviour, so 1hat we, whose faith in the corpuscular theory has been restored will certainly use it in the present chapter and in the chapter on lenses.

Rectilinear propagation of light explains why a pinhole camera will give an inverted image of an illuminated or luminous objcct­Fig. 3.

TllE SC IENCE 01' PllOTOGRAl'llJ0

Reflection Light falling on a flat poli$hcd surface is reflected so that the 011g/1 of intidmu (the angle between the incident ray and the m>rma/ - lhc perpendicular-to the surface at the poim of incidence) is the same u the angle of reflection. 'n1c reflected ray is in the s.amc plane u the incident ray and the normal (Fig. 4). 111is rcflcctio11 is termed 1pmilor reflection.

Fif. 4 . IP g Cu I. lo. II 1111n11cno11. ,\nglc or incidcncefo) - angle

ofrdlcction(/J).

A matt or rough surface, such u that of paper, i$ characterised by micro-irregularities. Different parts of a parallel beam of light therefore m.cct the surface a t different a11gles and may be reflected at almost any angle to the main plane of the paper. Such reflection is termed dijfru1 or uattered. r-.•lany $Urfaces, such u varnished wood, show both specular and diffuse reflection; the former from thcsemi­traruparcnt polished surface, and the lauer from the underlying structure.

Absorption of light In most casa only a portion of the light falling on a surface is reflected; part is absorbed . Light falling on a highly polished surface is strongly reflected a t a certain angle (equa l to its angle of incidence) unchanged in its spectral composition. 111us white liglu is reflected as white light irrespecti\"e of the charactcriuic colour of the surface except in the case of metallic surfaces. For example, specular reflections of white light from copper take on the characteristic colour of the metal. \ Vhite light reflected from a matt white surface is reflected in all directions from the plane of the surface, while surfaces which appear grey or black, appear so because all parts of the spectrum arc absorbed by about the same amount.

T H E BEllAl'IO U R OF LIGIJT

l\·lost surfaces, however, selectively absorb one or more regions of the spectrum more than others giving rise 10 the appearance of colour. Thus a surface appears red when illuminated by white light because it $trongly absorbs blue and green light and reflects

""· With most surfaces part of the tota l light is reflected unchanged in spectral composition, part reflected after penetrating the pigmented surface and part absorbed . Thus the charactcris1ic colour of a sur­face will appear more saturated (unmixed with white light) if it is highly polished as in the case of a varnished or wet surface and viewed at an angle to avoid specula r rcflec1ions. Similarly a coloured surface will appear more saturated if illuminated with s1rongly dircctiooal light.

T ransparcnl substances have the property of a llowing light to pass through them. Clear substances, such as glass, transmit freel y all the visible wave-lengths and a certain par! of both the ult ra-violet and infra-red regions. Coloured glass and photographic filters have the property of selectively absorbing some parts of the spectrum and tliu9 form a valuable means of modifying 'white' light, p. 192.

Refraction and Critical Angle It is sufficicm to note that when light passes from one transparent medium to another (air to glass, water to air, etc.) it is bent /owardJ the normal 011 passing from a rarer to a denser medium, and away from the normal when passing from a denser to a rarer medium.

Fft. 5. ••••1o.tm0>1. ((l ~prctenl.! cri1ical angl(, (#) 101al rcAection and (j) a ~nec1ingglau pritm.

Different media may cause different amounts of deviation depend­ing on the optical property called the rtfratlit-t index of the medium.

\\'hc11 light passes from a denser to a rarer medium it is refracted awa_, from the normal, thus as the incident angle increases, ii will reach a value when the emergent ray skims a long the boundary

TUE SClf:NCE OP PJIOTOGflllPl/J.

plane between the two media. This incident angle is termed the '1ilital 1V1gle, and light striking the OOundary at a greater angle than this suffe'"' toial reflcc;tion. As 1hc critical angle for a glass 10 air surface is 40°, light striking the boundary a1 45 ° is toia lly reflected. This property or glass is utilised in the rcflcc;ting prism found in binocula'"' and documem copying cameras (Fig. 5).

If light p:wcs normally through a block or glass with parallel p lane sides, it will be undeviated, but if it p:wcs at an angle, the emergent ray will be parallel to the incident ray, but displaced (Fig. 6). (This effcc;t is utilised in llOme high-speed cameras, in which a rotating block or glass causes the image to mo,·e in line with continuously moving fil m).

Fi(.6. Rcfraction bygla.stblockwith pualldoides.

A block or glass with non-parallel plane sides, for example a prism, will calUC a change in d irect ion or the light 1owards the base or the prism (Fig. 7). The amount or refraction, or bending or the light will depend upon the angle or the prUm, the angle or incidenc<: or the light, the refractive indc:c or the gl;us and, finalty, the wave­length of the light.

FiJ. 7. Refraction by glanpril'll.

TllE 8£/l;ll'IOUR 01' UCllT

Dispersion Since different wa,·c-lcngths are refracted to different cxtenu, white light, which is a mixture of all wa,·e-lcngths, will be anal~'Sed imo iu componem wave-lengths on passing through a glass prism. This is known as dispc'"'ion. The resultant beam, projected on to a white surface will appear as a spectrum, the longer wa,·c-lcngths (red) showing less refraction than the shorter (violer) ones (l.'ig. 8).

The lens A lens is a devict: whercb)' a large cone or light from a point may traverse a large disc and ()C 1·efractcd so that it is once more con­centr11ted to 11 point 10 gke a very bright, sharp i1nage. The ra)'S passing through the centre or the lens must not l>C refracted, and the rnngemial planes at the centre of the lens must therefore l>C parallel. The rays passi ng through the other parts of the lens mu.st l>C re­fracted as through prisnu of increasing angle as they approach the

F1i. g. Rdruti<:in of light by (a) corm:x lens (b) concave kru..

edge of the lens. T he typical lens-shape is given by 1he continuous merging or an infinite number of such prisms (Fig. 9a).

If a lens were concave instead of being convex, light from a po int source would diverge as though it came from a point between

THE SC IENCE OF PllOTOCR..iPIJr

the lens and the light source. Such lcn$Cll arc termed conca•·c, dispersive, divergent or negative lcn$C11, rui distinct from co1wcx, collcclivc, co1wergcnt or positive lc~-Jo'ig. 9(a) and {b). Our only concern with concave leiua is in their uw :u components in complex photographic lenses, as discussed later.

Focus

The point at which light from a point on the object is converged by a lens is called thefo.cws. The posi tion of the focus depends on the position of the object. I f the object is so distant that the r:l}'S of light from it arc practically 1mrallcl (as for example, light from a star), the foca l poim is called the principal focus, and its distance from the lens is called thcfoc:af lmgth {Fig. 10).

..__f ;

~~~ It is comparatively easy to measure the focal length of a fairly thin

lens. Willi a compound lens of considerable ovcrall thicknCS5, how­ever, we must first determine the position ofa particular plane of the lens {called the 11oa't), and then measure the distance from this plrmc to the focus of parallel light.

Brightness of Image

The brightness of an i!nagc formed by a lens depends on two factors-the amount of light tra111mittcd by the lens, and the area over which that light h:u lo be spread. The amount of light trans­mitted by a lens, assuming that it is perfectly transparcm, is propor­tional to its area, which is proportional to the squart ofilS diameter.

Jo'rom Fig. 10 it is clear that the height of an image of a distant object is proportional to the focal length of the lens; 1he art a of the image {that is, the area over which the light p:used by the leru hrui

to spread) is thus proportional to the squaTt of the focal length. Thus, the wider the lens aperture, the brighter the image; but the 1011gcr

TU£ 11£1/Al'IOUR OF LICllT

~·~ l'i,. 11. lrnagtt formed by lcnse1 of different focal Jeng.ta.

the focal length, the dimmer the image. Htm~e the brightness of an image is proportional to

(diame1t1eflt11J) 1

(focal lmglh) 1

Object Distance: Image Distance

(/,rudi•="')' foc:alltngth

All r."l)'S reaching a perfect lens from a point on the object art refracted so that they intersect at a point. If therefore we can trace the path of two rays and find their point of intersection, we know that this will define the position of the image, since all other rays will al50 intersect at this point. Now we have already seen that 1he ray which passes through the centre of 1hc lens will be undeviated (ac1ually it will be displaced by a very small distance, but will be parallel to the incident ray), and we have aiJo seen that the ray parallel to the axis of the lens will be refracted so that it passes through th<" principal focus of the lens (that is, the focus for parallel light). The intCl"$CCtion of thC$C two rays defines the position of the image (Fig. 12).

l~ 1-'i,. l'l. ~me1ri<:coru1n1C1ionofan image.

It is clear that :u the object approaches the lens from an infinite distance, the image recedes from the principal focus of the lens. By simple geometry, it can be shown that if we call the dista nce of the

THE SCIENCE OF l'llOTOCRA.PllJ'

objccl from 1hc lens 11, 1hc distance of the image from 1hc lens 11, and the foca l length of the lens./, then these three values arc rcl:ued according 10 1hc impormm fundamental equal ion:

I I I 7--;;+-;

Let 11s consider a special case-photographing an object al unit mng11ifiration, that is, so that the image is exactly the same size as the object. It will be seen from the diagrams 1hat the ratio of image size to object size is the same as the ratio of image distance to objccl diSlancc. If image and object sizes a rc to be equal, then image and object must be equidistant from the lens, or 11 must equal 11. Suh· stituting 11for 11 in the equation, we find that:

7-~+~-~ or 11 - 2f

Thus both image and objcc1, when photographi11g at unit magnifi. cation, will be twice 1he focal length away from 1hc lens.

J<'rom this special case we can make another imponant deduction. When the light entering the camera is focused at a plane twice the foca l length from the lens, it is spread over an area 21, or four times tha t of an image at the principal focus, and has therefore only a qunrltr of its brighmcss. This must be taken imo account when copying at unit magnification.

Light and Spect ral Energy Curves

The spcc1ral analysis of a pencil of light can be made by mca11s of a prism in the manner first demonstrated by Newton. There arc other ways of breaking Jiglll into its component wavclcngdis as we shall sec later in this chapter.

The spectral composition of light has important photographic (and scielltific) considerations. For any given light source it can be expressed by measuring the energy at various regions oft he spcclrum by means of a spcctropl~o1omctcr and plotting the values as shown in Fig. 13. This curve shows the energy distribuiion for noonday sunlight and a tungsten filament lamp. II shows at a glance tha1 1unligh1 has a maximum in the grecn region (550 ml'), while tungsten light shows very linlc energy in lhc blue (400-500 ml') but increasing energy lowards lhc red region (600-700 mµ). Both light sources are accepted as white light, ycl when directly compared with each other at the same intensity level, the sunlight will appear bluish and the tungsten yellowish.

TllE 8£/IA l'IOUR OF LICllT

TJ

I ·- -/ -r--. s I r--.

/ I ...._

J / / :/ ~

400mfl "'""' 700mµ

F~. l 3 .Speccralcr>ef1YCur-·~• ofounlight iS) and a u.rngstcn lamp (T ).

The Diffusion of light

\\'hen lighl passes through a foggy or smoky atmosphere, the SUS·

pended particles of water and dust absorb a part and diffuse or scancr a further part. If the particles arc large in comparison with the size of a wa••clcngth, all rcgioiu of 1hc spectrum arc equally scattered and 1he light will appear as white, diffused lighl. If 1he particles arc small cotnpared with the si:tc of wavelengths, diffusion on a selective basis takes place, the shorter wavelengths being affected most. When this happens the diffuse light appears blue and the transmitted light ye llow. This can be readily seen by direc1ing a beam of light through tobacco smoke which consisu of very fine particles. Similarly, diffusion caused by atmospheric haze c11plains \\hy the sky appears blue and why light from the sun appears )"Cllowish or red when it passes through greater dcnsitiC$ of haze "'!ten it is near the horizon. I t also explains why photographs made \\llh infra.red radiation give much better penetration thro11gh hate-sec the illustrations on page 194.

Diffract ion and Inter ference Phenomena

The diffraction of light is of relevance to photography since it can be used as a means of splitting light into its component wave-lengths (e.g. in a wedge spcctograph, an instrumcm for measuring the

Tllli. SClii.NC E 01' PJIOTOGRAP/11"

colour scnsi1ivity of photographic materials, p. 190) and in certain cases can be 1hc cause of loss of sharpness of an optical image.

It occun when light cncoumen a \'Cry small obstacle or passes through a very small aperture. Instead of tra\'Clling in a straight line, ii tends 10 bend, 1he shorter wavc· lcngdu being most affected. Thus if the aperture of a lens is very small compared with iu dis· ta nee from the film, 1he image of a point source takes 1he form of a sma ll disc. The diffrac1ion of light is common!)' seen in the irides· ecol colours reflec ted from the surface of long·playing records.

Interference phenomena can be explained by considering what happens when 1wo rays of light arc out of phase. This commonly occun when light is rc flcc1cd from a ''Cry thin transparcn1 layer such as that of a soap bubble or oil on water. The light reflected from the inner surface travels farther than that from the outer surface and if the extra distance is an odd number of i wave.lengths, part of 1he spectrum is reduced or cancelled ou1 and we sec the sum of the remainder. This effect is pul into use in the coating of optical surfaces as a means of reducing surface reflections. As the effect can only hold good for a particular wavc·lcngth, the thickness of the coating is made tha t best for yellow light. This explains why coated lenses have a bluish colour when viewed wi th reflected light. The Lippman colour process was based on light intcrfcrc11cc.

Polarized light

The cltttromagnctic 11a1urc of light offers an explanation for 1hc polariza1ion of light. We may consider unpolarized or 'natural' light as vibrating in a ll directions in a plane at 90° to the direction of propagation (1-"ig. 14). If such a beam of light is made to pass

, .•. 1 •. 'N•rn~1·1;,,, ~" aftcrpasaingthroughapola~ ing filler (P.1) berome1 pol.anzcd. I Withatee0ndlihcr(l'.2)atrigh1-angle. 101hcpl.o.ncofpolari•ation·i1 i'almo:iteom· .I '2

plc1cly aboorb(d. ~·

through a device which absorbs all vibralion5 C.'CCCpt those in a single plane, lhe transmitted ligl11 will have an cltttrical fi eld of fixed direction and is said to be polarized. If a similar device is

r ue 8£1/A VIOUR OF LJG ll r 37

now placed in the path of a beam of polarized light, it will transmit the maximum amount of polarized light when iu plane ofpo:ariu· rion coincides with that of the light, transmitting less at other angles and zero when the lwo planes arc at right·angles lo each other (Fig. 14).

Light may also be polarized by reflection and refraction. Polariza. tion is almost oomplctc when reflection occurs from a polished non-metallic surface at an angle of about 34° (Fig. 15). A polarizing filter can lhcrcforc be used to con1rol rcflcc1ions from glass, water, g!OMy paint, CIC.

FC,. 1$. l.ight reflected ffo m a iurCacc which i1 r>0n-mc1allic is uro11glr pobri~

atananglc ofabout)4°

Polarization also occun through the scattering of light by the atmosphere, maximum polarization being in a region of the sky al right-angles to a line between the camera and the sun.

The usc of polarizing filtcn is especially valuable in colour photography where it offers control of rcflccliom without inter­feri ng wi th colour rendering, sec Plate JI , p. 258.

Ccrlain crystals, such as Iceland Spar, act as very efficient polarizers, but in photography it is usual to usc a filler which comisu of a plastic material containing minute crys1als of quinonc iodosulphate which arc uniformly oricnta1ed. For practical applica· tions sec p. 198.

Tit£ SCIJ::NC£ 01' PHOTOGR.tPllY

The inverse square law docs not apply lO light sources such as a spotliglu which embodies an optical system whereby the light is focused into a narrow beam of nca.r·parallcl light. Sunlight reaching the earth after a journey of 93,000,000 miles may also be regarded as parallel liglu and therefore unaffected by terrestrial distances. The foct that it varies in intensity al different altitudes above the horizon resulu from atmospheric absorption.

Colour Temperature

The ad\·em of modcn1 colour photography has made it necessary to take into account the spectral composition of a ligh1 source:. This can be done photometrically by measuring the intensity a t various wave-lengths and preseming the values in the form of a curve such as that shown on p. 35, Fig. 13. An alternative method is that of relating its colour to temperature. If we gradually raise the tempera· tu re of a black body, the first visible rays cmined are red- when the material is rtd hot. As the tcm1>eraturc rises, the output of the fi rst ta)" increases, and in addi1ion other ra)" of shorter wave.length successively appear and the region of ma...-.:imum emission moves towards shorter wave.length. \\'hen the temperature is such that appreciable amounu of all the visible radiations arc emitted, the material is while hot . White, however, is a relati,·c term. T he glowing filament of an electric lamp is white hot, but it is yellowish in colour compared with the hotter carbon arc, which is in general yellower than sunlight. The colour ofan incandC$CClll body is thus dependent on iu temperature, becoming less yellow (that is, more blue) as the temperature rises. The colour of white light emitted by an incan· descent body such as a candle flame, an electric light bulb, an arc light, or the sun may be defined in tcnns of the temperature of the incandescent body.

111c temperature of an incandescent body could be defined in dtgrce5 centigrade or Fahrenheit, but these scales arc arbitrary and a more absolute 5ca.le is used. It has been discovered 1hat if we con· tinuously abstract heat from a body, iu temperature will progt"Cf' sh,cly decrease until it reaches a limit of -273 • C. At this 1cmpera· ture it has p.1rted with its whole heat content, and there arc no temperatures below -273° C. This, therefore, rcprcsems zero on an absolute scale. The choice of the value of a degree, howe\'er, is still quite arbitrary, and the centigrade dcgrtt has been selected, one hundred dcgrce5 representing the temperature difference between melting ice and boili11g water. Temperature on the absolute scale

LIG llT SOUHCES USED IN PllOTOGH ,lf'llJ"

is represented in dcgrce5 Absolute or degrees Kclvin•(•A or °K), and the corresponding values arc shown in the table:

Centigrade Absolute or Kelvin I

Absolute ~no JHtfting iu Boiling wain

-213° c o• c 100• c 0°K 273 ° K 373 ° K

1\n)' centigrade temperature is thus converted into absolute degrees by the addition of273.

The qualit)' or colour of light emiucd by an incandescent black body may thus be defined in dcgrce5 f\bsolute by the c(}f(}ur ltmjlcr(}­/utt of the light, which is the temperature of the incandescent body. Some sources of illumination, such as mercury or $Odium vapour lamps, emit light of a quality entirely different from tha1 of an in­candescent black body, so that it is impossible to describe it in terms of colour temperature. Fluoresccm (cold cathode) lighting a(>' proachcs more nearly that of an incandescent body, but cannot be accurately defined by colour temperature. Some fluorescent sources designed to imitate daylight approximate to a body at a tcmperMurc of6,soo • K.

Colour Temperature Filters

Colour films have specific colour sensitivities and arc currently a\"a ilablc in three 'types'. Those balanced for daylight (sunlight plus blue sky) corresponding 10 a colour temperature of 5,SOO°K., those balanced for high efficiency tungsten lamps of 3,400"K. and those balanced for nmgstcn lamps of 3,200"K. For critical colour rendering with tungsten lamps even a difference of 100" becomes importa nt and in cases where it is not possible to use lamps of the correct colour temperature, ii becomes lletes.$.1f)' to use filten which arc designed to modify the colour 1cmper:uure. These filters are known variously as colour temperature, or Jigl1t·balancing fi lters. lnstrumcnu designed to measure colour tcmpera1ure arc tomn1onl)' based on the use of a pho1ocdl which indicates the r~la 1ivc amounts of blue and red light emitted b)' the light source. ·1 hey arc usually calibrated in mirtds, an abbreviated term for the reciprocal of the colour temperature multiplied by 1,000,000. For ~'mmplc, a colour temperature of 3,400"K. equals a mired value of

• After Wm. ThomlJllOn, Lord Kelvin .

THI:.' SCJJ:.'XCE OJ' PJIOTOGRAl'HY

294. It is also Jl0$Siblc to allocate mired-shift values ~o colour tcmpera1ure fillers, the shift value being. roughly the same 1rrcspc_et· ivc of the initial tcmperamrc of the 11ght source. A filter wluch lowers 1he colour 1cmpernturc is s.-iid to have a posi tive shif1 value and one which mises it a minus shift value.

C:Olour temperatures and mired values for a typical range of light sources arc gkcn below in Table I. The mired shift va~ues fi:>r \\' ranen fil ters designed for colour photography arc g1\·en Ill

Table II . To gh·e an example of the use of the mired system let us suppose 1hal a colour film balanced for light of 3,400"K. (29'1 mi reds) is 10 be exposed with lamps of 2,600"K . (357 mircds) . T_o raise the colour temperature, a filter giving a minus shift of 61 is re<1uired . This can be achieved with a combination of \Vratten filters Nos. 62A and 82C (-18 and -45 equals -63).

T ABLE I

C:Olour Temperatures and '.\ !ired Values of Light Sources

Candle 100 wan household lamp Pho1opcarl and C.T.C. s111dio lamps Pho1onood lamps Clear nashbulbs =-:oon sunlight plus blue sky Blue flashbu lbs

OJl011r Mirtd Valiu Tmip.0 K. ---

l,900 530 2,800 357 3,200 312 3,400 294 3,800 263 5,500 182 5,500 182 6,000 167 Electronic flash

Uluesky l l ,000 to 20,000

Tt\BLE II

'.\ lired-i;hifi values for Wranen Filters filter No: 81EF61C 818 8\A 6 1 82 82A 828 82C '.\ l ired·shift: + 53 +35 +27 +18 +10 -10 -18 -32 -45

Daylight Daylight may be considered as 1wo diffcrem_ sources _of light: the light coming directly from the sun and the hg~t ~mmg from t~e skv When the sun is unobscured by cloud or mist II forms the mam so~~cc of light except a1 sunrise and sunset. However, before reach· ing the earth's surface, sunlight must pass through 1he atmosphere

LIGHT SOU RCES USE D JN PllOTOGRAPHl" 4l

and for this reason is subjecl to variations both in intensity and colour. T hus on a clear summer's day, the imensily of sunlight a t middll)' is about 7 times greater than the light fro m lhe sky, fa lling 10 as liule as half that of 1he sky immediately before sunset. At altitudes of 55 ° degrees or more above the horizon, its colour temperature is 5,400°K, but this can drop to as low as 2,000•K at sunset. The r.ombination or sunlight and light from a blue sky gives a colour tempcramrc of about 5,500°K and it is to this value 1ha1 most colour films intended for daylight use are balanced.

Obje<::ls in open shadow illumina1e<I by a blue sky receive light of a considerably differem nature. Due IO the M:at1ering by 1he atmos· phcrc of1he shorter wavelengths (p. 35), it is fa r 1nore rich in blue and the colour temperature may range from l l,000 10 20,000 °K. As compared with sunlight, it is diffused by nature and arrives from a very large area

The term daylight may also refer to various kinds of overcast sk)' where the two light .50urcC$-!lunligh1 and blue sky-become merged U) form a large area of diffused light . The existence ofa cloud layer which covers the sun has three main effects: it reduces the intensity of illumination, it converts all illumination to d iffused light and it changes the colour temperature.

Various attempts ha\·e been made to measure the intensit)' of sunlight on a daily and seasonal bas.is, culminating in the publica· lion of tables by the Bri1ish Standards lnstil\Lte which CO\'ered varia1ions for du: time of day, season of 1he year and geographical la titude.

As well as its visual comcrit, daylight contains variable amounts or ultra-violet radiation which can freq uently have disturbing cffcc;15 in colour photography where ii reacts on the blue-sensi tive layer to record a degree of blueness not visible to the eye, p. 193. However, as i1 is readily scattered by the atmosphere, ii normally only be· comes troublesome al ahiludes greater than 7,000 feet, or when the atmosphere is exci:ptionally 'clean' after a heavy fall of rain. Its presence can be dealt with by using an uhrn·"iolet absorbing filter over the lens, p. 193.

Flashbulbs

f he combustion of magnesium as a photographic ligln .50urcc was practised as early as 1870 and the use of magnesium Hash powder became almost the standard light source in many portrait studios. Its use has now almost died out in favourofftashbulbs, though there arc 11ill a few stalwarts who enjoy the spectacular 'e:<plosion' and

TllE SCJJ!.;\'CJ!. OF l'llOTOGRA/'111"

smoke cloud that follow$ the combustion of a few spoonfuls of magnesium powder.

The flashbulb is a similar but vastly more controllable light souree. Modern flashbulbs consist of a quantity of shredded aluminium foil inside a gla$$ envelope containing oxygen. The metal is ignited by an explosive primer which ill itself fi red by a small electric fila. ment. Recently, :.iirconium has been used as the combustible ma terial for small bulbs as it gives a greater light output.

T he light output of a flashbulb depends on its si:.ie and m11ure, but is extremely high compared with continuous light sources. A small bulb may lie compared with other light sources in terms ofluminous flux, that is, in terms of the rate of light emission, as follows:

Standard candle JOO.wan lamp 275-watt Photoflood Small flashbulb

12·51umcns l,200 8,000

1,000,000

Its short duration is, in most cases, an advantage ill photography. The total light output in lumen-seconds covers a range of from

7,500 (for small bulbs) to 95,000 for a large bulb such as the P. F. 100. Clear flashbulbs emit a light of roughly 3,800°K., a colour

temperature higher than that of tungsten lamps and considerably lower than that of daylight. T his presented something of a problem for the exposure of colour films of the reversal type since these require a dose match between the spectral quality of the light source and the colour balance of the film. One approach to the problem was to coat the glass envelope of the flashbulb with a blue-tinted lacquer so that the light emiucd was roughly the same as daylight. Mowever, this was open to two objections. Firstly, the blue-coating more than halved the light output of the bulb- a serious <lisa<l\•a ntage with the existing slow films, and, secondly, the filtering effect of the blue coating varied considerably, often giving excessively blue results. As ao alternative, some brands of colour film were provided wi th an additional version which was specially balanced for clear flashbulbs. T his was labelled Type F to distinguish it from films balanced for tungsten lighting. However, as the method of coating bulbs improved and the speed of colour films increased, the need for a Type F film vanished a11d today a ll except the very large bulbs arc of the 'blue' type, thus making them equally sui table for exposing black-and-white and colour fi lms intended for daylight exposures. Indeed, with the introduction of :.iirconium as the combustible material, very small flashbulbs can be made having

l. I GllT SOURCES USED JN PllOTO G RAl'Hr

a light output equal to the larger clear bulbs which were based on aluminium foil.

Flash S1nchronisation

Equally as important as its \'Cry large light output, is the feasibility of synchroni!ling the flash wi th the camera shutter. Flashbulbs arc made to have specific burning characteristics so that the firing of the bulb can be coupled with the shutter release mechanism. T he com­bustion curve of a typical flashbulb is given in Fig. 2 using a time

7::.:r:~:~r ______ ---1

1 I ,e I

' I I I / 1 \ I

T\ ::F i ---· J I I I I

Fig.2. CombuJ1i011cur>"cofclass :.1 flashbulb showing 1ynchronisa1ionwi1hdiaphr.igm shutu. r .. 11o'M'.

base in milliseconds. The broken lines show the opening of a diaphragm shuncr. It can be seen that from the closing of the flash circuit to half-peak output there is a time lag of about 14 milli­seconds. Thus if the shutter opening is delayed by this amount, effective use of the flash can be made over a range of shutter speeds from l/50second to the shortest gh'en by the shuner. In this case the effective exposure of the flash is proportional to the time of exposure. Maximum use of the flash is obtained with a shutter speed not less t!1an 1/ 30 second and in this case, assuming 1he flash is the only light source, it does not matter if the shutter is open for a ''Cl/' much longer time-the result will be the same. Under these con­di tio11s any camera, synchronised or 1101, can lie used to make a Hash exposure. . The synchronisation of flashbulbs with focal plane shutters is less

simple owing to the fact that the shorter speeds-1/50 or less-arc

THR SCIENCR OP l'H OTOGRAl'll/"

t!t~~ 11:.~~~·c;;. l!>O "·aus (24 rnl u) •hown ac­ni:dsize. Opcr­Ming characLcr­;,. ;cs are shown in1hcdiagrarn. CoUTW)': At/11.f

J.i1Min1 l.1d.

LLJ-1---J,, --'---:;---'--;"'16 ?J'il 'I .11'9\lD\llll61

---- '"" lumrn•

--= cok.ur Lcmp --- ---- = ""''" ~rmitting full ligh1 output throughout the _life of 1he lamp. Ath ~h~ same time it pre\'ents the rapid destruction of the lamp w ic

~~;~\\:~~:c~,~~i;gy~;1c:n~~i:!~is :~:~:~~:";~'~::; ~~;~n~i~~~~ cycle is that the c1wclopc can be made smaller as a large en\'clopc is no longer needed as a means of reducing the effect of tungsten

e";i:e~~~~~ntage of operating a tun~ten _lamp at higl'. tempera­tures is a two-fold one; it gives a whiter hght, more sui~cd to the re uiremcnts of colour photography, and its efficiency _m lumen~ pc~ \''att is greatly increased. This is shown by the followmg valu:S.

4-0-watt tungsten lamp 10·7 lumcns per watt ;~~g .. ~ 500-watt 19"6 " 3225°K

\~::::::: Pho~~fiood'iamp ;;:~ :: ,. ,, 3475"K T ypical operating characteristics of a tungsten-iodine lamp art

shown in Fig. 5.

t. t G HT SOURCES USED IN PllOTOGRAP/11. 4?

Tun!,<sten lamps designed for photographic illumination often incorporate an internal reflector thus dispensing with the need of a separate reflector. Lamps for general studio work arc usually opera­ted at a colour temperature of3,200 °K as this gives a greatly exten­ded life. However, the output of the lamp and its colour tempcra11.1re depend on the \'Ohage applied to the filament and for exacting work some form or \'Oltage regulator is necessary. For example a drop of 10 volts with a 240-,•olt lamp will cause a reduc1ion of 50 °K in colour temperature and a 10 per cem drop in output.

For photographic lighting tungsten lamps may be used as single uniu, as banks to give a wide area of uniform lighting or as a spot­li~lll. The spotlight, which emplors a frcsnel lens, is designed to give a highly directional beam of light and is often used as a key lighting in displar photography,

Fluoresi::ent Lighting

Although chiefly of use for general illumination, fiuorcscent lighting may be employed as a cheap and relatively efficient means of providing uniform illumination over a large area. One advantage is that it emits very little heat and is therefore ideal for illuminating large colour transparencies of the kind used for display, The fiuorcs­cent lamp is a low-pressure mercury vapour discharge tube genera­ting ultra-violet radiation which is converted imo \'isible light by means of phosphors, Such lamps arc made in a range or colour <1 ualitics (chiefly to meet the needs of interior illumination) which include a \'ariety which approximates to daylight. This, in fact, ha~ a colour tcmi><:raturc which corresponds to 6,500°K.

Mercury Vapour Discharge Lamps

Because of their high emission of ultra-violet, high pressure mercury vapour lamps arc commonly used for exposing process material5 of 1he kind used in preparing line and half-tone printing blocks. For black-and-white printing, process films and plates arc chiefly scnsiti\'e to this region of the spectrum,

A form of mercury \'a pour lamp screened with Wood's glass (glass dyed with nickel oxide), emits only the near ultra-violet in the region of 360mµ and is used tn illuminate fluorescing materials and also in technical and forensic photography making use of ultra­violet records.

Clr.apur 4

THE CAMERA LENS

Tl-IE COST of a camera for amateur use may vary from a few s hillin~ to some hundreds of pounds, a range which may seem very wide co1uidcri11g that even 1hc most expensive

camcrn isessc11t ially a light·tight box incorponuing a lcn$ which ca n focus an image of a view on to a sheet oflight·sensitivc photographic 1naterial. The lens is one of the lll0$t expensive components of a camera, and again the range of prices is \"(:ry wide. If one expccu an expensive lens to gi,·e a greatly improved photograph in normal outdoor use, compared with an inexpensi,·e one, disappointment is in store. A simple box camera CO$ting little more than a pound, will gh'c with its single meniscus lens (one whose cross-section is crescent shaped) surprisingly good results for nearly a ll the photo­graphs that the amateur normally desires to take, and it is an ideal camera for the beginner. An cxpe11sive camera u;ill give better results, but .the difference is not very obvious, and because of mis­adjustmcnt of its many controls it Ui likely to give worse results in incxper1 hands. The main difference is that the expensive camera can be used under condi1ions when an ine:<pcnsivc one cannot; for example, for interior snapsholS, for theatre lighting, for scenes in the shade on dull days, and so on. But even this dis..'ldvantage is dimini· shed by fitting flash comacts to inexpensive cameras. \\le can sum­marise by saying 1hat inexpensive cameras should give good results in perhaps 90 per cent of the average amateur sholS. An expensive camera should gi,·e exeellcnt results in a greater percentage, according to price.

lens Aberrations

A single lcnJ suffers from a number of dcfceu called aberrations. Only the 1nain ones will be mentioned here. In gcncrn.1 the effect or Jens aberrations increases with increasing aperture, and 1hc simple mcniS<:US lens fitted to an inexpensive camera. gives satisfac1ory re5ults only because it is used at small aperture. Most aberrntions ,.

Tiii:: CAMr..'R.~ LB.NS

" arc corrected, or partly corrected, by a suitable ccmbination of positkc and negative (concave) lenses of different types of glass and of carefully computed radii or curvature.

ChromtJli& Abt7ralian.-Sincc different wave-lengths arc refracted to di fferent extenl$, the shorter wave-lengths (violet) will be brought to focu$ nearer 1he lens limn longer (red) wave-lengths. The defect can ~ corrected by combining a positive lens wilh a negative one of different glass. A colour-corrected lens is called an tJ<lr.ramat. In what is ko?wn as t.ransurse&olaur abtrration a light point, off axis, is stretched out mto a lmlc spectrum. It is importam 1hat the lens should be corrcc1~d for this ?cfcc1 w~1en used for colour pho1ography. Sp~mca/ Abtrra/1011 .- A simple lens with spherical surfaces brings

margmal ra)'S to a focus ncare.r to the lens than rays passing th roug l~ the .ccntr~ of the lens. Like chromatic aberration, spherical abcrrauon gives r1~ .to lack ?~definition in the image. The defect is correct~ ~y combmmg posmvc and ncgati,·c lcnse5 in a fashion ra1hcr mmlar 10 that used in constructing achromms. Cim:~lintar Distartion.- Ra ys reaching the edge of an image, that is,

t i:a~·crsmg the lens at a considerable angle to 1he axis, will use d1flc~e~1t. parts of the ~ ens, according to the position of the aperture. lf tlus 1s 11.1 ~ront of a s~mplc lens, barrtl dis tori ion results, for example a square IS 11mged with bowcd·O\ll sides; if the aperture is behind the lens, a1.1 op~ite effect, pi11-cuslr.ion distortion, is obtained. One w~y .of c~nng thu defcc:1 is to place 1he aper1ure midway bc1ween t\\O identical lenses. Tlus method was used in rapid rt,lilii1tar !coses 3:nd is s1ilJ in use in some modern lenses. Fundamcmally the oorrcc~ ~ ion of di$10rtion involves the use of separated clemems, though it is not neccss.1ry that the $)'Siem should be symmetrica l.

0 1hcr aberrations that can be wholly or partly corrcc1ed arc kn~wn as ~oma (which ~ives. a ~~I of skeu:td spherical aberration), turta/urt ef fitld, and ast1gma1um. I he cure for astigmatism is again

~':~t:111~~~~n:;~J;~:;~ with flat field and corrected for ~tigina'.

The Evolution of the Camera lens

~he . firs t step forward. f~om the convex lens wa:i the \Vollaston d em~us which was or1g111ally designed in 1812 10 give improved

ti~: ·~:~u~~~1t:t~=~is:sc.:;;· r~:'.:<t:~1aa1::Cr 1L~~~/:' .:.!':~r.;; ::'!~ .for snapsholS. The meniscus lens is still used for inexpensive

TllE SC IE,VC1:: OF PllOTOGRAPllY

The next ncp was in 1hc direction of chroma1ic aberration and took the form of a Jens with two components of different glasM:S, (Fig. 1). It was invcnled in England either by Dolland or Chester Moore-Hall (records arc u11eer1ain), but was first applied to early pho1ographie cameras by Chevalier in 1829. Lenses of 1his type, but cemented together, arc known as (lt;hromals and are still u&ed on the bencr snai»hot cameras. As wi1h the meniscus, a fairly small stop is needed.

Hi. I. Diagram 1ho.,..ing the hlllory of lens de•·dopment. c..~11Uy: Kodal. ,,,_,,,,., The year J840sawan ad••anec in lens design which was so remark­

able as to be almost anachronistic. A German lens designer, J osef Petival, worked ou1 a complete theory of the primary aberrations and computed a lens wliich reduced them to such an extent 1ha1 ir could be worked al an aperture or f f3·6.

Although most of the aberrations were cured, the lens sti ll retained considerable curvature of field and astigmatism. Pctival knew how to correct for these faults, but could not do so as he was limited to two kinds ofglass--crown and flint. In consequence, the lens gave good definition in the centre of the image, but definition fell off rapidly towards the edges. This defect was unimportant in

TUE CAMERA LENS

portrai ture. Lenses of this design arc still in \UC for purposes where only a narrow field is required, for example as bo1h taking and projecting lenses for amateur cine camcr:u and for very lo11g focus lenses (300-600 mm) for 35 mm. cameras.

For landscape work the achrornat continued until 1866 when a fur1hcr step was made. Certain aberralions such as coma, curvi­linear distortio11 and oblique chromatic abcrra1ion give precisely opposite cffecu according 10 whether the stop is in from of or behind the lens. The idea or using two identical achromat.s placed 1ymc1ric­ally about a central stop seems to have occurred lO two di:signcn simultaneously and independently, Dallmeyer in England and Stcinhcil i11 Germany. The Jens became known as the Rapid Rectilinear as ii permitted a stop of/ /8 lo be used. Since it was, in fact, two lcllSC$ in one, one oomponem could be unscrewed and the remaining lens used a.s one of twice the focal length and/ /number. If two components of diffcrem focal length were used, the photo­grapher had at his disposal lenses or 1hrcc different focal lengths. It was from th is practice that the term 'com,crtible' lens arose.

The Rapid Rectilinear lens offered little or 110 <:orrec tio11 for spherical aberration, curvature of field and not a great deal for astigmatism. In fac1 with only two kinds or glass it was possible to correct for astigmatism or for curvature of field, but not for both, so tha t it was impossible to obtain large-aperture lenses giving a sharp image over the whole field.

New vistas were opened, however, by the discovery at Jena in 1886 of new glasses with characteristics different from any previously known. This led to improvements on the Rapid Rectilinear lens by adding further components, and lenses of th is type, invoJ,,.jng six, eight or even ten components, made it possible to corrccl for both astigmatism a11d curvature of field. Such lenses became known as anastigmah.

The last fundamental and substantial advance was made in 1893 by H. D. Taylor, who calculated that three components only, suitably scpara1ed, arc necessary 10 correct all the primary aberrations. Disregarding traditional lines, he desig11ed a triplet of high performance and aperture. The le11s was named af1cr the manufacrnrer, T. Cooke, for whom Taylor worked.

h was the beginning of a new approach to Jens design and othcn, ~on the triplet design, appeared i11 rapid succession. In 1901, Al~u simplified the lens by cementing two of the componenlll, whi le Dr. Rudolph improved secondary aberrations by using a ~11:1entcd doublet in place or one of the componcnlll, 1hcrcby inventing the well-k11own Zeiss Tcssar le11s in 1904. Many modern

TUE SCIE.XCE 01' PJIOTOCR.~Pl/1'

lenses arc based on the triplet design, though some, such :u the Sonnarf/1·5, employ more components.

The discovery in 1934 of anothc1 set or new glasses-the rare­ear1h gla.ssc5-has led to still further improvements both in lenses of fairly simple structure and others of complex design.

LensAperture- f/numbers We have already seen (p. 32) that the brightness of an imagf' formed by a perfeclly transparent lens is proportional to:

(ltns diamdt• ) • facallmgth

But since the lens diameter is a lmost always smaller than the focal length, this function would be a fraction. To avoid this, the image brightncs.s produced by a lens is normally taken as the ratio between focal length and lens diameter, the value being termed the / /number. In th is case the image brightness is inversely propor· tional to thcj/numbcr squared. T hus if we compared two lenses, one of//2 and the other ofj/4, a little ari thmetic shows that the image of the //2. lens will be four times as bright than 1hat of the j/4 1cns.

The maximum aperture of a lens is, however, seldom used in practice. There arc a number of rca_sons for using a smaller o~n~ng such as control of exposure, incrcasmg depth of field and obtammg maximum lens performance which arc discussed later in this chapter. This reduction in the opening was origina lly obtai1\cd by inserting metal discs known as Waterhouse stops in the plane occupied by the normal lens stop. These were graded to give a ra tio of 2, and the term 'stop' is st ill commonly used to denote a change in exposure of this magnitude. However, the " 'a terhouse stop was replaced ~y a vadablc iris diaphragm which coul.d be adjusted in size against a scale of f /numhers calculated to g1.\·; a ratio of 2 (or one stop) between one number and the next. I wo scales of numbers were in use, as shown below:

British: J/2., ff2.·8, f /4, f/5'6, f /8, //11, etc. Continental : Jf2. ·2, JJ3·2, / /4·5, j/6·3, /J9, / / 12·5, CIC.

It can be seen that they diffCT by about a third ofa stop. However, since the last war the British scale has bcc11 adopted on an inter· national basis.

T iii:.' C.i .lfl:.RA LENS

The/ /number system is open to criticilm on the grounds that a Jens is not perfec1ly transparent and absorbs some of the light. In practice this is negligible as a total thicknC$.$ of 5 cm of glau absorbs onl)' 2·5 per cent of the visible light. A more serious l0$S occurs by reflection of light at the air-to-glass interfaces. For an uncoated lens, loss by rcflcc1ion is roughly 5 per cent at every air-to-glass interface, and is thus about 10 per cent for a single lens, but can be as high as 40 per cent for a complex lens having 10 interfaces . This loss of ligh1 is known as thcjlarefaclor. I t was to allow for losses from both causes that a new system of calibrating lenscs­known as T stops-was proposed, based on the actual transmission of the lens. However, 1hc coa1ing of lenses, which was started at about tht- same time, proved so effective in reducing the IO!IS of light by reflection that the T stop SY$tem proved an unnecessary refinement ($CC below).

T he f /number system holds good in practice for most general photography since the distance between the lens and film is roughly the same as the foct1l le11g th of the lens. However, when the object to be photographed is m a distance lcs.s than ten times the focal length, the lens- to-fi lm distance becomes significantlr greater than the foca l length. In this case the effective f /number is found by means of the following equation:

Lens Coating

This is based on the phenomenon of light inte1fcreocc already referred to on p. 36. The thickness of the layer is usually adjusted to give maximum tr-.1nsmission in the green-yellow region of the spectrun1. At first calcium lloridc was used for lens coating, but the layer was physically soft and was easily rubbed off. Other fluoridC$ arc now used which arc as durable as the glass itself.

Coated lenses have the desi rable characteristics of \•i rtual absence of fla re spou and ghost spot5 which arc due to unwanted surface reflections. It might be imagined that because more light is trans­mitted by the coated lens some decrease in exposure would be needed. l11is is not so in practice. Surface reflections cause light from 1hc brightest areas of the subject to be scattered over 1hc shadows or the image formed in the camera thus increasing their density (in the negative). Thus if exposure is based on the shadows,

Tf/E SCIENCE OF l'llOTOGR.~l'H1.

more e:i<posure would be needed 10 produce the same density because of the reduction of scattered light. The main effect of a coa1ed lens is to give increased contrast and 'cleaner' colours when using colour materials. All complex lenses used in photography are now coated.

Lens Angle If we return for a moment to the pinhole camera, it can be readily seen that the angle of view obtained will depend 011 the distance of the sensitive material from the pinhole (Fig. 2). The same prin­ciple holds good for camera lenses irrespective of their aperture.

·. f71 .. ---u :>< .... [71 """~_.. .. ----~

Fig . 2. The angle of a pinhole camera varies with the film di.1tan<:<:. situation appl ies to lcnsn.

For general still photography, the practice is to use a lens whose focal length is approximately equal to the diameter of a circle which will just fit the rectangle of the picture area of the camera. This gives an angle of 53 ° as the normal, though in practice it may vary between 45 ° to 60° with different cameras. It follows that if the definition is to be equally good over the whole picture area, the Jens must be corrected to cover this angle of view. But as we have seen, lens aberrations tend to increase with the obliquity of the light rays and few lenses fulfil this requirement at maximum aperture.

The 'standard' focal length of a cine camera lens is usually twice that for still phot0graphy and therefore gives an angle of about 26°. Such a lens would be considered a moderately long-focus lens for a still camera. However, many of the belier class miniaturt

T lfE C AMER A LENS

cameras and nearly all tedmical cameras arc designed to allow lenses of different focal length to be fitted according to the require· mcnts of photography. For portraiture, a lens of twice or three times the normal focal length may be used in order to obtain a pleasing perspective: lenses of even longer foca l length would be used for distant subjttu, such as big game, or io the press photo­graphy of sports events. On the other hand, for interiors or other subjects with res tricted working distance for .the camera, lenses of shorter focal length or a wider angle of view will be used. In both cases certain requirements must be met.

Long-Focus and Telephoto Lenses

In designing a long-focus Jens for a given picture size, it is posiiible to take into account the smaller angle of oblique rays and hence smaller amount of correction needed for aberrations which occur at the edges of the field. With very long focus lenses, relatively simple designs can be used. However, for miniatm e cameras the problem is more one of bulk: the long tube ofa 'straight' lens would tend to unbalance the camera. For this reason the majority of long focus lenses are of telephoto type which is based on a combination of a convergent and divergent lens, as shown in Fig. 3. A converging

! . . : i

E A c ti Fig. 3.0p1icaldiagramof tclcpho1olcns.

lens A would bri11g parallel light to a focus at B. Between A and B is inserted a divergent Jens which makes the focus recede to D. By projecting the boundary rays from D un1il they meet the bound­ary rays of the parallel beam at E, it is seen that the compact arrangement of back focus is equivalent to a lens of much longer foea l length placed at E.

Indeed one can go a stage further by moving A away from or towards C, making it posiiible to vary the focal length of the com-

Tlf£ SCIENCE OF l'lfOTOCR:IJ'Jfl"

bination. This is the principle of the 'zoom' lens used in eine photo­graphy for obtaining the effect of dollying to or from a subject.

For such a lens to be effective, the movement must be so adjusted that the focus falls accurately on the film at all focal leng1lu. More­over, thc//number of the lens must remain unchanged and aber­ra!io11s must be corrected a t all focal lengths. Such lenses arc there­fore very oomplicatcd (and hence expensive). A typical zoom lens fo1· a 16 mm. eine camera is shown in Fig. 4, the foca l length vary-

Fig.4. Diagra1nof\"ariablc f0<;us orzoomkm.

ing from 20 to 60 mm. at a oonstant aperture of// 2·8. T he movable components are shaded. Lenses of this type, more appropriately known as variable focus lenu:s, are now being applied to miniature still photography as a means of replacing a number of interchange­able lenses of different focal length.

Wide-Angle lenses

Lenses which take in an angle of 65 • or more are usually classified as wide-angle. It is here that the lens designer must reconsider the effect of aberration$. So that all rays strike the lcn$ as nearly notmally as possible, many wide-angle lenses tend to be strongly meniscus and arranged on each side of a small stop. The required standard of correction is often only obtained by limiting the maximum aperture to / /8 or evcn// 16, though in recent years lenses have appeared which give acceptable performance at f /2·8. One of the 'classic' wide-angle lenses is the Ross/ /5·6 which covers ananglcofabout 100".

Standard lenses for use with view cameras must also po$SCSS a much wider oovering power than normal to allow for val'ious camera movements, such as rising front and back tilt, sec I>· 70.

1\n alternative wide-angle lens design is the 'reversed telephoto' type. By reversing the lens, an effect equivalent to looking through

THE C;l.\f1'RA LENS " the wrong end of a tclC!'icope is obtained and so includes much more of the field of view.

f'is,. 5. Di3grnm of the/ /'f. 20 mm. Flcktogon wide.angle lens.

Convertible Lenses

T he mO<!e~n convertible lens (not to be confused with the early type ~cnuoned o_n p. 52) was developed to simplify the inter­changmg of lenses man eye-level rcOcx ca~cra by interchanging the fn;int component of the standard lens wi th ei ther a telephoto or w1dc-ang~e oomponent. T hough this simplifird the mechanical o:i1mruct1011 of the camera in leaving 1hc shutter and pre-sci lens drnph~agm un~hangcd, the interchangeable lens componcms arc · anylluog but simple and involve as many as six clements (Fig. 6).

.. TUE SCIENCE OF l'JIOTOGRAl'llT

Supplementarylens11 These arc thin lenses of either positive or negative clmractcr which can be placed immediately in front of a camera lens to reduce or increase the focal length. Positive lenses of this sort, usually referred to as close-up lenses, arc commonly used as a means of extending the near focusing range of hand cameras. T he lenses arc of meniscus design and their foca l length is norma lly expressed in d ioptres, namely, the reciprocal of the focal length in metres. A + I d ioptre lens thus has a foca l length of I metre. This method of c:<prcssing foca l length is very uscfol since the combined focal length of t v.'O

or more lenses can be found by adding positive values and subtract· ing negative values. Thus two + 2 lenses arc equivalent to a + 4 lens. Lenses of up to + JO dioptres may be used, though bc11crd~ lenses of this sort are usually achromats.

A point of intercs1 to 1he photographer is tha~ with the camera lc1u $Ct to focw at inlini1y, an object will be sharply focu$Cd al a distance equal to the focal length of the supplementary lens. Thus a +2 dioptre lens wi ll give a sharp image of an object i metre ( 19! in.) from thcsupplcmemarylens.

r.,. 7. l'Cl'itiv.: .... ppkmcntary knt used for clotc·up phocography.

One advantage of using supplementary lenses for close-up pho10-graphy is that the effective f /number of the camera lens is un· changed whereas by a normal extension of the lens, as we have seen, the effective f /number becomes greater (the relative aperture smaller).

Perspective Since (ignoring minor errors ca11$Cd b)' lens aberrat ions) the camera records a view exacdy as seen from a certain viewpoint-that of the camera lens-the perspective of the fi na l print must be correct, pro­vided the prints arc viewed correctly. Most of the apparent distor­tion produced by wide-angle and telephoto lenses is due to viewing d ie print a l 1hc wrong dista nce.

T/1£ CAMERA LEX~

It is.obvi?w from Fig. 11 (p. 33) 1ha1, irrcspcc1i\•c of focal length, 1hc prmt ~v1ll_reprcscnt the v1cw as the eye saw it, provided that a con1ac~ prmt is held at a d istance from the eye equal 10 the distance of the image from the lens. For mos1 subjects, this distance is very nca.rly equal to the foca~ lc~gth ~fthc lens. I fa print is enlarged by x diameters, then the v1ewmg d~tance of the print to subtend the s.a~cangle at the eye, and thus to give correct perspccti\'C, should be x umcs the focal length. Now the normal dis tance for viewing a hand-held print is about IOin., so that cameras of about IOin. focal lc11gth arc the ?nly ones which will gi\·C a (l111lat:I print showing correct pci:specuve. Such cameras are hardly ever used nowadays, so 1ha1 virtually all contact prinu gh•c distorted penpcctkc. Fo~tunatcly: the distortion is unnoticeable except when near objects arc included, and even then ihcy must be such that a dis· 1or1io11 in size is obvious-thus a part of the body near the camera will show up distortion if it is out of proportion wi th the rcu or the figure, but a bush or gate would not make it obvious since 1hey may have any dimension relative to ocher objeeu. ~rom th.is consider~ t ion of distortion of perspective, we can draw

an interesting conclusion. Since we shall view a print at about IOin. from the eye~ correct perspcc1ivc will result if we enlarge a prim ~rom a ncgah\'C ~y a factor o~ l~n divided by the focal length in inches. Thus a prmt from a m 1111ature camera of 2in. focal length should be enlarged five diameters. h will then be identical wi th a CO'.llact print from a ncga ti\'C taken from the same viewpoint in a l~m. focal length camera. But the angle of view of most ca meras is fairly cons1ant, about 50 °, which is approximately that subtended by 8 x 6in. print viewed at IOin. from the eye. One can therefore for· ~ula~e the rule tha t a print of th is size from the whole of the ncga· live g1\•cs about the correct penpcctive for hand-viewing. Exceptions ar.c, ofcounc, cameras with wide-angle or telephoto lcn5es, or indeed "'.1 th any lens whose angle of view is very different fro m 50°. Ob­viously ironly a portion or 1he nega tive is used, then the dimension mus t be reduced accordingly. Again, if prints arc exhibited on a

:~~1~h~1~~7o1:~ :e~~~!!:t~ceai:o:1;<'~;i:.ft., and the longer side

Converging Verticals

A photograph showing parallel horizontal lines converging as 1hey appea~ to do so 10 the eye, is completely acceptable as a true rcprc.­tcnta11on of the view. This is true whether 1he horizontal lines aTc

~:~~l:y ::;~:i:: ~'f ~:il~~:~{~~~-(:~~ ~;,0:i,~1~t~~~~.~~~ ~.'~~~c:; li11 .. 1howingcon,·e<gcnccoran:la1iv<:ly1rruo.llamount,as<><;Cunin

:h~~=J~ iu:;~;~~'. ~ ~~7-~~n~\'...~ ~~~:la=~~~,'~:~~ building i1 loppling bacltwimh, compan: Fi1. 8c and d.

fi~.ll<•od<l.Con,-,,"*""'"°''."'""l linod .... 1otll1in1 1i><<>n><,1i1u1uolly

""'"''"''bk

.~~,c~~~:~~°",=i:ls\O ~~~Ypa;~~\~~~;• ~~~·~.~~ ': photQ8raph ;. .-malty vi""·ed on the la·cl. Ir the pnnt " hdd >'<:rticallyiorncdittan<:<:abo>-.,e)..,l'"""l,thccon\..,<ICl>CCl>tt<>mes mon:acuptflble.TI>tnoccm<alsotobetomethi"llolthcprecon-

:•;;:.:;:;.: :::.::! ;:,,~ ,.;.~;:"'~~ :~~~~~~ ::~~~;~~ printford1c 1irnplc""""°ntha<thcparallclsiclctol •hcpnntdraw

;;~~s~r~;~~I~~;::~~~~c;TJ~~~hi~~·:~I!~ d1c lcm10beniocclTClati,·c to1hcpicturenn:aorthccamcra,so

THE CAJ/E/f,t LENS

.. ,0 includemon:olthctopandktsoftheforqroundint!M: ,_..-ded irruo.g<:. Ahemati,-.,ly the camcno can be tilted uP""'rdo and1hefocalpla11c1iltedso1ha1i1ranains\•tn.icalor1headjtt11-"'"'" may conoist ol a ppl)·ing something ol both ll\O\"CmCnto. Camera• ol 1ht. oor1 att commonly used ror atthi1cctural pho10-!!"'Phy and " ·i11 be dcalt with mon: fully in 1hc ne><t chap1cr, p. 69.

C l ~c l l of Conft11l on a nd Depth or focus

\pcrfcc1 lcn1 inpcrfcctfocuiwould'cndcrapoimonlhcobjcc1 asa pointonthcirnage. ~ut sin(c pcrfcc.tion is unauainablc, a point is rcl\dercdasa 1mallc1n;ularatta ofhght,Call<:<l1hctfrtl1efto•/..,;••· 11M:liuof 1hiscittlcwilldcpcndon1hcquali1yolthelcnoand1hc accuracyoffoculing.Ob>·iously,thcomalkrthecittlcofconfu<ion the bc1tcrtheddini1ionof1hcimag<:. Ho"'"'""'•atd1e110rrruo.lvicw­;n1 distanceoll0in.,acin:Jcofabout1-hin.indianu:1<:rapptanaia ptrfccdy sharp poin1, so that an image appcan to be in 1harpt:St focuoirapoinlisttn<lcrcdasanan:anotgn:a1cr1han1ht.dimcn· ""'1.Wi1h•ifi•·enleru,1hcuwill 1huobcaoertainlatitudcinl0c:u.­m1 within which 1l1e cittlc ol confusion is l ... than this minimum, nd tl1editta»cebctwcenth<:frontandttarlimini11crrnffi1hc •J>llteff -·

O.pthoffocu11hou!dnotbcconfusedwithd•f'llto/fi1Uwhicbii th>Cu1Sedlk\(r. l)epthoffocu<iscon<:<:rncdsoldywith1hcpcrmi1-'blc mo\'crnent of the scnsi\i\"C material in the camera, relative

to a fixed focal plane of the image. It thcttfottconccrnsa photo­Kr~ phcr only in regard to accuracyoffocu>ing wiih, for example , agroundgl..,.ocn:<:n.SinceillOiltpllOtOflraphcrsdonotpooscss c..mc"" of this oort, they ha,·c oo practical inter~• in depth of G.-uo,a11dha•·ctorclyonthecamerarruo.nufac:1urersforcmuring 11ta1th<:scnliti,·ematcrialispo11itionedwi1hin1!M:1olcr.lblcdcpth o!'x:us.

•·mm l'ig.9,itisclcarthai 1hcdep1bolfocuot.incruoed by d«:re;r.i.i"llthcapcrtun:.ConccptioruofdepthollOcusmaybcclari­fied byananalogy. lf1heconcofligh1fromlcnstofucuoitima(ined

fl1 . 9.U<p1holl0<0>(o)otfoll o i><'1u«:I•) ~-;<h l<n•>IQpp<d down

TllE SCIE.'ICE OF f>l/OTOC/Ul'ltr

I~ be: did, and the circle ol CJnfuoion :l ring ofyhin. in<ern.ol d1ame1er,1hcn1he~hoffoc.,.iogivenby1hed;.....nce1hrough which the nng .. .;n ohdc before jamming at <he limita. Lenses of diffcrcn< foca.l lcngduha,.., !he ... me depth of foe"""' <he ... me /numbc:r,oin«1hcangleof1heconc of ligh1 io the .. me

These conlidcnnioru ofcirele ofoonfusion and dcplh offocw apply101hcopticalimagcproduet:dbythclen1.Clcarly 1hcyabo app~y101hcimagcon thenega1i,..,,andlimilarly1od1epaperprin1,

!'~r~~ ~t~l~~~-~~ ~,: :,'::~t;;:'.;n1 ~':C::~1:,:i~1~c;.p:~.: ~,~~~;:~ ra thcrthanapoin1,so1hat1hcpcrmiuiblecirelcofconfu•ioninthe camera, and hence thedcpthofl<>cus,is reduced in pr<>portionto

: ~~~~;.~:!'!~'in ~.h:! ".,( ~~.~~'~:!:.;:=. but at 1hc "'me/number, then the depth of focus will bc: s.wlltr for<hc•urK>cal lengths. Toproduceprintiofcqua.loizcand tharpneoi 1hc dcp<h of foe"" will be directly proponiooal '°<he foc.o.llcngth.

DepOof f it ld

'_\'chavc1CCnt~lthc.-.:ii~ccrtainarnoun1ofla1i1udcinthepooi-1.on of <he $Cn11uvc ma1enal rclativc lO the plane of the optical imagcinthecamera (depthoffocu1). Thcrcitiimilarlyadegrceof la1i1udc in 1he pooi1ion of an object while .,;11 rclaining sharp focusonocrui<ivema<erial in a fixed pooi<ion in lhccamcra. This l:ui1udei<known asd'f'tlt•/falJ,andi1ofmuchgrca1crpractical importance tophotographen. I f an object Oioaccurately focuoed in the camera a< / (Fig. 10) the.-.: will liepooiti<:iru in front of, and bc:hindthcobjec1,which"·il111i!ll>einaccq>1ahlcfo<:us.~ limi11,.•illbc:thooc poaiti<:iru L1 and L,atwhicli a point on the objtf'1willbc:n:ndcrcdaoacirelcofThin.diamc1er.Thedistancc bc:1wcen theoc wt.idiary limiis ii ealkd the "l'A •ffal'·

Nowacirelcofyhin.diame1era1/wouldbc:fonnc<lbyacirelcof ri•X•{rin.diamcteratO(sttp.n). ThelimiisL,andL,an:there­forc 1t.o.e poinis which~ cona o_fligh< .. ~.•h doe le"' ao base, such 1hattheplaneofOwil\1nteroeptac1rcLcofd1amctcr r~l X •/rrin . As lhcobject 0 approaches lhe lcru, the dep<h of Held will rapidly

~1~·~t.~~ 1:.;::," 1~~ o:~r;;:.:~;1~ ~·~\~."~::,~:,~;~:· ~~·.;: in1i•CasM<lecr<:ascsandvincrcascs.

Moreover, itwillbcsccnfromt"ig. l01huby1<oppingdownlhC

~:'~<

:--~::<::n>:s

Lcru. we 1h<>ukl dccn:aoc th~ angle of lh~ cot1"' and hen«;,,,,,..,. •he di:pthoff><:ld.

WIM:n.,..,comp:o.-.:lenscoofdiffCKt11IOcalLcnstlu,wor1Unga11hc

falit~

T/lE SCIBNCA' OF f'llOTOCR.if'llf

Thedcpthoffieldforthccamcraimagc, and henccfor«1mact prints, is thus inV<:ncly proponional 10 the'"""'' of ti><: focal length.Enlarg;ngthtprintfn)mthtshort-focuscamcral<><>omparc prin1sofequalsizeclimlnatcsoneof1hcsefacton, butthtshon.· focuslenu 1illh.uanad"antagcininvcncratiotothcl0callength

A1pecial«1ndilionofdcpthoffieldariscswhenthclc1Ui1foc.uocd forobjectlatinfinity.Whilcthis givcsanimag.:a1theprindpalfocu1 of1hclens,<hercwil!beaccnaindistancefromthclcn1•"hercapoin1 1aura: will prod1>ce at the principal focus, a cin:le of rh in. diameter. This distance is 1ennt:d 1hc /typt<f«lll tfis14#U, and evtty objec< beyond1hiswillbein 1h.arpfocU>a1thcprincipalfocalplanc.From the argumentsad\'all<'cd in considering depth of field, it will be seen 1hat hyperfocal distance decrcasct. when a l<:n• is ilopped down, a11d is >hortcr wiih shorter focus lenses. Ira lem isfocuocd onthehyperfocalpoim,tknanyobjectfrominfini1ytoadistancc oflM/fthc hyperfocal distance will he in •harp focus, and this principleisutiliKdini"°"ptnsi"cfixcd·l'ocuscamens.

LensPtrformance

Almo1t1hc011lyadvanu.geoflcnsnofJa111crclati•..,apcrtu•ei1 thc brightcrimagc.Onthcothcrhan<l,theyg;velcsolatitudcindepthof fieldanddepthoffocut,andthceffectofleruaberratiombccomcs mueh morc markd as aperture incrca,... La.<ge-apcrtu,.., lenses whollyorpartiallycorrectcdforaberrationscanbemade,bulthey arcboth«1mplexandc~pcnlive. Wecanthercforcmakcdicrough generalisation that in payingahlghpriccforacamcrale111,""arc not buying a lcm g;ving higher image quality, bu! a lcru g;,~ng a kizAWimagc.Thitischi<:Hyofa<h'antagctothcphotographerwho wishcstotakephotograpl11under•·crypoorlightingconditions.A> a general rule, a .,..,ll-«11TCC1<:d ler11will giV<: ilS best peri:>nnanceat aboutZ11opsbelowiumaximumapcr1urc. Fur1hcrrcductionof the apcrtu,..,m.ay,indccdwoneniuprnformana:duetotheclTectof dilTraction,p.36.

Ha"i ngdo:cidedon1hcrelati"eapcrture,hc""tuircs alc1U"'hi.eh willg;~aoharpimage,rcason.ablyfra:fromdistortiorr,withade-

~~1~ "'.:~:u~dof~h;1~:t~~~~~t:~':!.t~~~";,"fn ~i:~~:~~ optical1ys•cm,butthepractical pbo1ographer can .....:i. hi1le~ by

f!::'..~'fc:,1::g~d=~~J7:y ~':';~a;o•=· J~l ::C ~i:.,·~~~::i object is not eaJily meuurcd. Numcrous test objects i...,.., been

TllE Cl.MEii.< Lli.l>'S

dC"o·iocdWilh1<'1JOfcqually1paccdblackandwhitclinesaidilTercn1 .aJcs. Whentl><3CarcsharplyfocUSttl,thc"" 'uUwJ>owtroflliclen1

=:~: 2""::E'E.2tr.~:; ~:!~·;~:;?-:~ ...,n trcorthccdg.:of 1heficld.Morno•·tr,forthcrrcsearchh..,ahown that len.cs giving high rcsolo ing po"·crfor black and while line> donotne«llllrilygi•·cthe1harpestrcsuh1I01'thehcstdilTcremi ft•

onofthelow«1ntnotdctailofadistant1ubject (Cbap. 18), Test bject• in "·hich 1heconln>t be1wttn light and dark is lowor gndu• tcd 3CC(lrding1oati .. .,.,., functionhaV<:bttnlllO<C1ucccsiful

and wercusalin 1cs1ingacrolcru<'>duringthcwar. Whilc1hccxpericnccdpracticalphotographcrcan.<00nforman

idea of the performance of his lcm by a i ubcomciou• otaiistical imes>mcnt of many photographs taken with il, there is no simple way ofmc»uring aboolutcly iu ability to gi•·c a abarpimage, and ctr• tainly nowayofexpressingitasasinglefigurc. fa<:n«1mparari,.., 1 .. uha,·c 1heirpi1falk. l'h0tographinglcmtcsteharu,orthcc1Jof '"''-sprint,.·illgi"c10meinfonnaiionaboullhclcru,bu1itwillnot

::::-~~ra':':'~~~~~ ~;,~i•~~~;r;i:-.~r imag.:, or the rendering of

The bes• ,..,.yofcomp.arinit l<:n,.. is under normal «1ndition1of ute. For example, thcd<:<ail inadiuantvi<:wwhH:h is ah•"&), •1'ilablc, a distant building taken from the office window perhaps,

can be photographed with the lelU under cxamination-finl IO

thatwmc ipccific detaili1intheccntrcofthcpicturcandthen10 that it i1 at the edge. At thciame timc «Jmparat ivephotographs "'lake" w.ing a lens which has been cOO.Cn as a permanent tandard.

ChapUr!I

THE CAMERA

T~~~~~~~~~·~~;~~;~~~;,;~ai~o~~~~~;E:~~~l; and lheM:nsitivity of the material IO low th~l o:posurc.of a u hour or.,_.,,...,...,nccasary. Th<:n:wasthU1no1>eed IOramccbanical shuutt:thclcnocouldheut1cappcdandcappcdbyha11d.Withthc arrivalor,...,,platcs,camcraotendcdtobecomernassi•·cttrnctur<S. Studio carncras "'Cr<: b;osed on 'whole-pla1c' oizc (Six 6i in.) or largcr,andafiddcamcraon'half· plate' (6j x4:1in.). llowe•·cr,l>y thee11doftheninete<:mh ccntury, thcK<><!akboxcam=in"cnlcd by Ocorgc t:...Stman "'"' a lttadr popular and a wide variety of

~=·c':'";:,8,!:.gr.!":. ~,:;~~~ ttudio cameras to miniature

The rnanufactun: of hand camerat, initiated by the Kodak • Company- ti!! among the most active producen-\>ccamc one of

the specialised induHri .. ofG<:nnany where it took the fO"m of p<e<:isio11cnginccring.lnl9'l~lhefirstLcicaappcarcd,aminia1urc camera I.rated on the UM: of35mm. "'idc pcriOr:a.tcd motion pielu,.,, film and giving a nq;a1i,·c 36 x 24 mm. It ptm't'd surprisingly popular a nd has,;,..,., become the ttandard si:oc for HnaH hand cam=1.Duringthcl930'ta numb,,rofminiaturcca1ne,...•1yttems' appeared. These indud<:<l a rangeof•ccC110Ties, len~, attach­ments andadaptorsdaigncdtoincrc....,thcvcrsatihtyofdocminia-

:~~n~ca7:.:r:z...";;;; ~:~:uc:'.,.. much f0< the prof-.ional and

Since the last ""'r, the German camcn. indU>tl')' has IO<md a powcrl'ul rival inJapRnandin<e<:en1 )..,a,..themarkctlo.asl>ccn inundated by an ever-incn:asing range of small cameras which almo.n defy classification.

Norhasthctechnieal camerahe<:nforgoucn. From the hands of 1hccabinct-makcr,i1hasi..o:.mc1hccona:rnof1hccnginccra nd can1akccqualplacewi1hothcrop1icalinstn.rmcnu..

T1chnlealand\li1weam.,u h iscon•..,n ient to.,art a sur•;eyofmodcrn cameras wilh the view cameraasi tcmbodiesaUtheb;iosicfcatur<SOfavc,.....tilecam.,,.,and ..,,·cra lfcawrn11-hi<:hdono1e~istinthe\•astmajorityofsrnallhand uo mcru. Current sizes range from SJ x2Jin, 10 10x8in. oo that lhc unallat1naybo:con.idercdpor1ablc,whi lco!1elargcstisratrictcd to studioutc.1l>cyarcintcndedto 1.akeplatC1orshcctfilm , though •malt.T~esmaybcU>Cdwi1hadaptontolakcn>llfilm.

~1~.~~~:i;}:~~~§~:.~h~~:i~I;~~§ >d 1hcouensiono( thcbcllows bctwccnthcbackandthclcns

panclisnormaJJr t"·ia: 1hcfocal lcngiho(a 'otandard'luu (double

~~~~%~~~~!:~~~~~~~

~,,. 5. i ....... ,.,;.; <•"'<"'w;<hoim~

~:\!:i:'.;~~~

~~~::~ <din=- --

l1ocpossibili1yofvarying1hcdiotanccbc1"·ccn1hcimagcplanc and tloclensaho ma.k"' thctc<:hnicalcamcn. a.n idul inttrumcnt forc~upsandphocomac:rng,.,,phy(magnifica1;on.of1hcobject) .

Wi1hanotcnsionequalto1"ice1hcfocallcng1hof1hcnonnal a.nglelcm,.amc-scalcimagtsar<:poaible.Ho...-.'''t'l''byusingalcns ofmuchshonttfocal lcng lh, 1hcsamco1cnsion tnak"' it p<miblc toobtainirnag<:1of<»n$idc,.,,blernagnification.

his001"•cnicnl,,..thcrlhanlogia l,todeal"·iththcovc1whclming rn.ojori1yofcamc=ontheb:uU1batthcya.rcdt:sig11cdt01akcroll film. I t might be more logica l to<kscribc lhcm ascamcn.1 "'hich do -makcuseofaground-glass1CttCnorcamc=in"·hi.chthcfilm plane ii fu<ed in relation 10 •he lens attpt for .-rnal FOcusing "'"''cmc:n1.Bu1pcrhapothcrraldiffercnccistlt.at1hcyarcdC>igncd to be used in !he hand Na com-cnicnt inttrumcnt for general pho1ography

Film loading

Filmsdco.ignedfurdaylightlo.o.dingfalli11101woclaucs,1hooc tpooledonaflangedOOl"C,.;tba muchgrcatttlcngihof<>p.>quc papcrforproitttionagainstligh1fogging,and1hoocloadedin

Tl/Ii SCIE!>'Cli 01' /'/IOTOGRAPlll"

CUM:Uco or cartridges. The (Ol'ln<'r, c:ommonly rcfcned I<> a• 'roll

~~~~~::=:~~p~~ ~~~~E ~£~7~;=7~f~~; thcca,,..,,,..Succasivef,..rncooffilmarebrouglnin1e>t,hc11nage

~:~::::1i~~::.=~=~~~~~!T::~ window and employ a toothed ,.·Mel which 'mcawres' che requ~m:I JcngthofncwfilmlOrcach(,..,nc, 1hus 1pttding upthc.operauon. A> the ....,<)f3~rnrn. pcrfcirated filrn doct nol call fora \'11uat fnmc

~:~~~~~~E~~~:r::;~s:~~~~7= i..~ The film is anachcd to the take.up 1pool, and after the camera back

::t~7c~~·~': ~~ ~t~co '=;'1c.:'1a1i: ~::.:n':'r:1~ fot 12, 20or 36framco bucdon a fori:>atof36 x 24~m. A frarnc

~f irf ~~&~i~~{~g~~Iti~[~f.~ thc1implcactionofa lc\'<r.

Sire and Format

~~::~~~~;~i~~5~;~gg~,g~f~; pcnnitcnlo.rged prin11 ·tobcmadefrom1maUncgat"'""•pan.lyon thcgroundsofi~ai.cdi-u.bili c 1·(anallcrncga11•upcrm1tthc camc.-atobc1maller)andpanly1hroughthcad''elllofcolourfilms whichgi,..,dittetc:olour1.-an1parcnciei (anallt.-anspa1encicsare "<tuallygooclandlcssupcrui~).Currentoizcorangcfromll x8 mm. (based on the use of 16mm. cio>e film) 10 6 x 6 cm. (ba>cd on chc use or 120 mil filrn or 70mm. wide pcrfora1M film). Cameras baled on "mm. pcrforalM film arc regarded as '1tandan:l'when they

~:;~da,:~~~ ~~~:..':!e~ .;,;.4,:~7"~~; :1;::-::: :;; x3 t;::nc~ '.

\\'hilciti1111uallymmidcrcd1huatte1angulo.rformat(ci1hcr hori•o1ualor,..,nico.l)"·ichan upcc:tratioof4 :3 g;,-.. 1hcmoo1 .aci<f1·ingsl1arcforapk1urea"'a,no11andanlhu"''CTbcenade>f>­l«i IOr ~ntt:tl purpote co.mcr.u. View cameras approach more ciooely1od>e ideal, and ""'tind1uchP1C:1 as 8f x6j in., 12x9cm. c1c. A great manyomallco.mcnu(i•..,•oquarepicturc,whilcthc >1andan:l(Ol'"1rtm. (36 x2~mm.)gi•U •3 :2aopcctratio, For a nrpti\'C which is tO be enlarged, lhc fonnat is not impor1ant. Indced, 1hcrci110mt:1hing1ob.o .. idforthcoquarcformatwhich nuikcofulleruteof thcCO\·cring"°"·era11tlangLeof 1hclcru.For nui ll crarupa,..,nci .. , i.o,...._..,,.,arc-ctangularformat ispn:rcr..blc toa oquarconeonthcgr<><mcbofcomi-i1ion.

The lens

llc.-.:againthC1eisconoitlerablc'"'i.o.cion. l n1hcchcapc1tol cameras, the lcn1 will ooruis1 of a oinslc mcniocus lcru of fixed • p<rtureandpcrmancn\lyoctallhchypcrfoal dil!ance (p.66). Thi1 i1 only rcasiblcforalJnall RJ>C1<urclen1oroneofvcry1hor1 focal leng1h mch as 1hM fitted 10 a camera gi"ing an l l x 8mm picture or narrow gnuge cine carncr:>I ~I on 8mm. width film. Jn 1hecaocof1hechup1n~pshot camera, photography is praclic~I on die l>a1i1 ofusingafilm whoac: tpced io a0011\ right for "'ell-lit cx1 erior \'ie"~ M" 1m~ll aperture anti fixed 1huttcr •peed. How. c••cr,•udiacamoraisus11ally 1ynchroniocdforflashbu lbs,andmay lh"" l>c uocd for 1n•Jl$hOt1 indoors without r«011roc to 'time' Cll poillll'CS.

If"" lkip by R number of in1ermediaic-priced cameras, we come

:::~11~~ ~;~f;1~;:~~g ~~~~ r:;::; ~~~;~rd~h~~:~n~ As dcpth of field dec:rcascs with largcraJ)C1tUres, 1hcamcrawill

~:::!r,:t";' r:::;•;:f,!';:')~'!1~ 11~:1;:,..<>,;;;.;~1 ':i:i7n:::.:i;: ~ ria_blc •f>ttd slnmcrofll>e diaphragm 1ype, p. 81. Some cam<"" 11.1h11 groupofTcrlirni1edchangcabili1yoff'ocalleng1hb)'mcansof C'>twcnlblclcnscs,p . .s9.

In the thin:! group, "·id1 pricaoften u<X<:ding £100, 1hc lcn• 11madcdeu.chablcf<l1ha1 other1ofdifTcrcn1 focal length may be fito<'t!. ln1hiscase1hc1huucrisuwally of1hclO<:alplanc1i·rc, p. 81, though in a nurnberofcamttU a diaphragm 1hutter is fined

nu: SCJE,\'CE OF f'H0TOC11APlll

immediately bchi"d the km. 1n;. places «rtain limi1.at.ioru""' the UM:oflemex1cnsiont{inphotomurography),butifiw:sthead•"a11-1.agc ofshuucr synchronisation at all opeedtwithelcctronicflash. Since many can>en' in this group embody pre-Kt k:m diaphragm -•ml,Mlditionallcnscstcndtobcverynpcnsi'·e.

Foc111ln1

We ha,·ealrcady1ttn 1hat focusillg"•ithavicwcamera i>dcmc visually by oh>ervillg the image on a ground ~I,..."'''''"'· This method iJ olwioualy not convenient for a hand camera, but the ... me effect is obtained byal)">tcm known M r-cffexfocusing. T his cantakevariouslOrms,butbasicall)·,it.,,..,.W111ofi....,ningahinged rnirrorbe1wttnthelcnsandthcim.o.gcplanc:ooutor-c-di~tthe

image to a ground-gia..settenat •tM:1opofthccamera (Fig&).

Attl1eopticaldisu.occofttM:ground-glasstcrtt1\i1idcnticalto 1bat oftl1e focal plane, an image "'hicb appcan oh.up on the r-clle~ tcrccn"·illbc•harpintllCncgativc.Thc .. tofroleuillgtllCshuucr ~n• cat*' the mirror to hinge upwards 10 the roof 10 leave 1he optical path dear and ~al off the viewing tcrccn. It hu the minor disadvantage that the imagcdisappcanfro1n 1hetcrccn. a ~noment l>ofore the exposure" made. Ra1her more scnous obJcctoons arc 111"' 1hc image isr-cvcn<:d lcfl to right and is 1hcrcforemisJcadi11g

intcying l(tlOllowasubjcc1_in motion and, withao.mallcamcra, :::::11toosmallforcnt1calfocusingandntt<banau.Uliacy

,\ comidcrable impro>·cmen1 in this •yotem ""M in1roduced in l9~bycrnbodyingapcmapriam andcy,..Je"'JOlhatthelJTOUnd-

=7~?;:r~~ r, .. :?.:O:~~h~~:~:~~~7i; ~!!" ::.:~~~~

"f11Crrca1adnntageof1hcrcllcxsystem;,1hatitcanbeusalto

;r~rE~lT2"~:.:::-:~~:7,5%~~!.~~c?~Ei~ rnaki ngll1cCl<post1rc.

1 .. :~·;,, ;:,1::7:~'!..v:~i:~~':,.~e '~'.::1e:~~;g,'t~~1~' :::~:~~\~~11~~ ::;~~~~"J,~;:~~:~·. 1\• the le"' 11only used for viewing it 11eed• only

~~~~~ .. {~a~Jc:S::k~~rE~:.:g~:~:.~;1 chcapm camenooffer acceptably al.arp images ror objcc,.7fec1 " more from the lens. ~·or •ub:jccts a1 c'-1- distances, a lo...tt­pmo·cr poi.iii, ... le._.,mct.imt:s known q a i-trai1 altllChment­•~•cncb th.- ncar d;.tanoenmge toabou1~f1.lnth;,cascthedil1ance ntus1bcmeMurcdwi1hr-ca.;oruoblccar-c andobjccuinthed;,1ancc '> •llbere~alightlyoutof lOcus..

With len>C1ofl"rg<:r:opcnur-c some means of(ocll$ing beco1ne:s

~~~~:.~bl~~.:?i1~:~S~~~;i;~~::~;~~~E~~i "·ultohort focus len .... Toovcroomc1his, a<er-cwthn:ad has been

1/IE $C/ENCt; Qf. PllOTO C/11',ll'//I"

adoptttl as the moo< sati>fact<><y '°"'hod ?r making the un.~11 adjurnncnu quickly and ao:omucly. Dcpcndi"!I" on the amount of 10.ward "'°""'°"nt nettlttl, the th"""d can boo either ll11c, moderate, ~en.use and, indtt:d, in oome cases takes the form of_ a helical thread. The J<m,,.rd """"mcnt, now accomplished by •hgh1ly less

:h~:~~=~:~: ':,~a~: ~~h:t,~:=~\~~'~,!~:.,;f,,:;t~;'.

~~~;.!:~~:~:~~::r~~l:~~~:r\~~~;::!7.~;~:·:fir~;fi~~1~~;~ cloM:rtlistanCCL

U•uolly thefocus.ingmo,·ement onlycxtend1 to a ncordista.ncc

r.!~,~~:o,;~1 ~~"c.:~~~U:~";:~ i:,;~';.i1:.:ti;,~:·~~e;; ,:; :!":"~:;~;~~.:;:.;..~; ~~~:-.;,.~c~~b:::::,~·~;:i:~

"""'' ~~i'!'.;!.7d::.:: :: 7,!j~ oc::~::.~:i~~ :,~~~:~:.:a~ :~~.<>;'~ :.;:=;:~~::/ ~~r. =::~:.::: :::;:"':

f1~~P:~;~f.~f§~f:~i~:~~~g~:~~~~ imothecameraandcouplcdu>thefocusirtgm<>V<:mcntofthelens

f"iz. 10. O.poh ,,f~tkl ><•I< odj•«n< •~• he dinot><o o<olo: of•'"""'" lo:n1

i\thirdtypcofrangefindcrusuallyfiucdtoC)'C-ic-..:lr<:flex ca""'nu • • an ><hem:ui'" to ground·gl:w foclllinr, tmploys two 1emi-cin::ular "-edga of glass mounted in a cin::ular hole in the ccntrcof1hcfocusing><~oothatthc1hincndofoncwcdgclics alongi.idc thcthickcndof1hcoihcr. Thepointwhcrcthctwo•ur­faccsc..,.. lics in theplancoff0<usi0 that whcrca line01"01hcr •hupcdgc i1outoffocus1heobjcctisdi1placC<laftcr1hest)'leofa 1pli1. imagcra1igcflndcrofthetypealroadyrcfcrredto.

·nicr:>ng<:findergivcsnodue1othccx1cmofdepchoffieldand thil isn liO tn1c for moot rcAex cameras except nt maximum aperture. To coinpr:»satc for this, the distance o.calc of the Jens normally licsalongsideadepthofficldindicamr (Fig.JO).l'roJ>erlyu1ed, thc depth of field indiatoran bca,·aluablcaid 1ofoeusi,.glincca1 1hc•mall apetturcs (j{8 orj/ ll } normallyuK<lforoutdoorph.oco­i'""PhY, 'lOncsoffocw" an boJclttted according 10 the kindof •u[Jjectbcing 1akrn.

The ror:,,,;,ig ocrccn of 1hc view umcr.a and IOr thM ma Her of 11! ~llcxc.amtnualooKf'"CSthc_pul'Jl':"Cofknowingwbatarcaofthe v1cw infrontofthccarncraw1llhemcludcdinthr:rcc:ordcdirnagc. l lowevcr, lOrnc rtsef'"ation mw1 be made in the ca.. ofa twin­lcns rcflvc camera, as the \•iewing !ens introduces p.>rallax r:rron ot dose di5tanccs. Compem.:uion for p.>rallax is normally included up to1l><:ncarcstf0<wingdistancc,butfor11illcloscrdillanccswith the UIC of a 1upplemcntary lcru, it \>Ccomcs ntcasary to make oomc

~~~~~~~i~.,:m.;'.:~~~i:::,:e:s ;..:~l=~-'~ design(<[

~~~~g .:;,1'.,.7!.h .:a':. ~~!t'·,!:. ::C~~ro;t~~-~ built into the top of the c.amcn body which dr:fin.et the pic•urc

Wc;~~nt:~~,7:;.a.,1:°pos":',~..:.~i~~;!":~ ::~~ fu~r;:~:::::;~~e"';"c:::!:~ncaa~~io!,~'ta:= v1a.·llndcrunccdC<l.SuchvicwfindeNoftcngivcparallucorrtttion O\'(r 1hc normal range of focusing. The limp!cr kind of viewfinder

~i~~Ji:£~~;'a~:o~~e:i o;; ~:~:.~c~~t~ ~~~~e;;

;;:.•hecamenoame<:h.anicaldC\"iuforopcningandci<>ling1he

Shuucn It.ave cvol,·cd along two lines: the /nu 1U11n which inmup11 thcligh1justinfrnnt,jwtbchindorb<:1,.«nthclcns c:omponcnu, a1~ the /ot:.t-pun.. shun'."' which int<:«<pu the ligh1 as n<:artod..,1en111iv<:materialaspoo.t>ble. f.:ochtypchasiuadvant­age1anddisadva111agc..

11•< '"".""".The ""'P"hot camera hu a very 1implc \)"I"' of 1hu11e.rrnwlHChprcssur<:onthercleas. buuoncomp«:SSC1a •pring 11n1il 11 llicks~ thin metal plate from one potition wanothcrjust b<:hindthelcns.Aholeinthc<:C'.llt...,of1hepla1cuncov.:nthelen> for about •'•Ott· The rcll>O'"'I ofprcssu1ef""1'1 the buuon C>IUSCS

11..,platcto<<:tumloitsoriginalpotitionbutacappingplatc prC<o..,nucxpo1u...,on thercturnjoumcy.

A more cla'-atc shum:r makes,,.., ofd"' rno,·.,mcnt ofa cin;ulac ""'1alplatcand;.kno,.·nasthe1tt1<>rshu11er.Tiicdurationof upoturccan b<:\'ariedeill .. rbych.anging tl"' angkof thc 1tttor or iuspttd ofrotation. Shn11crsof1his t)·pean:commonly used in cinccantcr'lU.

11.e moo• commonly usnl lens shuuer (if,.·c igoorc 1hc numerical supcriori1y ofsnaf'lhot cameras) is tha1 known as the tfiop/11'41p1 ork•/1Ju.11n.Thi3consi3tsof 1hr<:eor~,·cthinmctalbl.ades,inter­lca' "cd and capable of rotating round pivots round the lens. The mo1ivcpowcri1aspringwhich is pr<:-tcn1ioned,eithcrby aocparatc l<:vc r orby theactionofwindingon1hcfilm. l'r<:SSu...,onlhcrcleuc buttonallow1 thcspringtorota10rhcbladesotthc shu11cr so tha1 each in concert unoovc,... part of the lens. The •ctio11of1hc opring

~· :::'i:"s!'.:~11;i;,; .~~~- ~"::..'.;;,"~"C::l~~n!.J",i;i;";~;;' ~:~~~::: bchro .. ghtintoplayasade!a)"cdu1ionrelcasewiththcobj«tof •llowing1h.ephotographcrtofeatureinhi><l"·nphot<>sraphL ln anahtmau,...,andsi mplcr,..,rsion kno .. ·nas thc ...,,·Hlsltfa"', the firstpartof the1huuerrelcqcactiontcnsions ll.,. springand thc Co1t1inucdJ>Ja1Un:ofthcbunorithcn1ckatctthcblades.

Fot:lli-/J(MtsU11ns: These <:OTUUt bas.ically ofanopo.quc blind ,.·ich • •ran»'<:ncolit .. ·hich paS>CSacroosthcfilm,ao ncario thcfilmas ]'.>00tiblc.Arangc<>f1pttdsisobtaincdcithcrby•·arying the spttd oftravdof 1hcblind,orth<:"'idthofthc •l i1.Jnpra<:ticeth<: spttd ra nge io obtained in two ><ages. Slow spttds from 'l lo "If Ott.

~'."ob1aincdhyvaryingthc1imcduringwhicha sl ite<JUal.tothcfull "-'>dthofthcpiciu rearearcmaonsinfrou1of thcf11m.lnd111ca•cthc

Tiil!. sc11urcs OF f'llOTOGRAf'llr

a.,ion of the 1huncr ii 1imilar to 1ha1 of• diaphr.ogm 1huucr, in 1hat the whole of the film ii being upooc<I at the same time. T he r ... tcr •pttd r.onge from ~'- 10 n'~occ. ;, obtained ?rvaryini:the widtholthe1li1,whilethc..:tualumcoftravclr<:ma1111o:>nst•ntat around .J.,occ. Thus, although the dTocti•·e uposur<: in ·~of

~ ~~i::::ti.i.:r:!~'::;~:~=~t!'!:;';:~I= ofafa11-rnovingobject. l fthclmageofthcsubjttt,whichinthc

~~7ionis.':~~~!;>;;;~·i~~':" .... ~";:~;:i:~=. with the """""" dfoct when the image movc1 in the. o~tc dirtti.ion.Amon:compludilt0nionoocunwith1hcsl_1tlJIO\'tng vcrticallyt0ahoriiontalmovc1nemolthcirnagc. lnthlJea<ethc

:lJ: l:;~!:.i~.;.~! ~=;'oi:~:.\7:"00~.'~.,!";:: suchdist0niooswil!beamidcdifthcumcraiss"·unro•J"'llncdln 1hcdirtti.ionofthemovingobject•tthctimcofc:<posur<:.

Shutt1r £ftleltne7 Tue blades ofa diaphragm 1huttcr nC«11arily take time to open and eio.e, 00 that the shutter ii not fully open during the wholcoft~c sl1utter action. The accion ofa good modern 1hutter il•OO"·n in

Fig. ll,whichha1bccnobtainedbythe1Ueofanelectronic 1huttcr­"'"ingmachinc.

·1nc101alamountoflightlc11hroughby1hcshuttcr(r<:prcscnted

bv1hcattaA8CD)ilth111lcathan1hclightwhichwouldha,·cbcen pUscd lwi the opening and closing of the blades occupiM _no tum; (repracmed bythcrtt.tangleA8'C'D). Shuttcrcllicicncy1Sttp«:· JCntedbythcratioofthcl'onncrtothet.iucr,andforagood be<wttn-lensthuucrscta1~\occ.1houklbcabout85--90pcrccnt.

Thctime1altcntooptnanddoocthcbladcsisl:t.rgdyindcpcndcnl

e>l" thet01alcxpo1ur<: timc, 00 1ha1thcshor1crtheupo1ur<:,the

~'.~'u~'i:!:~;t"ie~~;."h~p:~(::r.<>pening and shutting. Thi>

Again, ifa leru illtopPCd down toa low apcr.tur<:, theshult<:r bladawill ull(:O\'er,his1mallcrttntral arc.adunng thcfint part of tbeiropening;openingand cloling a •topped down leru wiU th<rdOrcbc•mon:rapidopcration. F..fficicncythusincn:asctwith dccrcalingapcrtull:.

Tucreisa popular misconception that the timeiakoi iounco>-.:r .,,. ,,,,...,,apointonthc....Wtivcnutcrialbythcleadingandl'oll"""· in<i: cdgesofthedotin:t.focal·planc 1hutterisvirtuallygcro,and hcncc thatthcelfociencyofafocal-planc1hutterilpracticallylOO pcr cenl. Thilwouldbetruc:ifcheshuucr"..,...,inthesameplanca> thc film,orifithadtoin1c«:ept a'611Urayoflightfromthclcno. ln prac1ice,itluuofcou1sc,1obcdisplaccd10wardsthclens,and the cdgcoe>l"thc1loc1akcanapprcciablc1imc totra'·el acmsstheoon­''"'1!entbtamoflight fromthclc111to apoincon,hcfilm(fig.li).

Thcelfocicncyof•foca!·plane 1huuer isabout 1hcsamc a> thatofa good diaphragm 1huU<:r. /u with lens 1huttcrs, llopping down and ... inglongerupooutt timawillincttMeiuclf..:iency,tinceci1her adju,.mentwiUd«r<:ascthcratiobctwccnthcdiamcterofthecircle ofligbtintc«:<:pledandlhcwidthofthe llot.

UflC!INO;"t' Of' >QC;A~ Pt.A>ll ""'TUN

Fw. H.-....,. ol'x..i ~ 111,.,...._ To 1umup, we can say the IOa.l·plancohuucr olT<:r1ag<eatcr

•ptoed_rang-eandbcing"-.:ll·ttnlO\"«lfromthclcns,doesnotcreatc cornpl1C>.1ionli11changingfromonclcnstoanothcr. lt prcocnu1he

l1 th1rtanld11IC1n1era? 1t ilaqucotionthatmigh1..-ellbea>ke<lby1hccnquiringreader"'ho bu1hepurctiueofafairly<::<pensivccamerainmind.ltis,of

~~ki~':";~;~~~~~;.li~ir~::i.d ~~~; c:"....,,,. can apecttoembody all 1hefeaturesrequin:d at some 11mc0<anotherbyf>MP1c"'·homakc1*ofca~lfy<><1require nothing mon: than the ~family reoord photognph, <hen theincaperui,•elJlapshot camera could betheideaL forarchittct­ural an~ co~mcrcial display photography, tome kind of view camcra>1the1deaL The~rai<photographcrmightal.so chooiw: a view camera, but 1fhe•pecialioes m 1he photography of children or the i1~formal portrait, then a sm~ll renex camera might be the best choice. Mo-tfoll·Umeprofcssionalo have .. vcral cameras and inthi1 lies1herealaruwer1otheques1ion.

°"""' THE CHEMISTRY OF PHOTOGRAPHY

ll1m1nt1 1"d Comp011nds

11~ff0~.,·~::,t~o~;u~~a:~~;· .':!c~1'::1i',~~hi.::;'~~11,i\':~'::.~~:;,

TllE scu:xcE OF f'llOTOGR.H llr

hydroquinonc,h)·po,ctc. ;\lanyoftltcscc.o.nbe•plitl>ynormal chcmic.o.I h'"Catmcnt inio 11•-0or more different 1u"5tanca. Siker bromidc,IOrCl«lmplc,can\Jc:~ 1ogi•..,,;t...,..andbromi1>C. Hypomay\Jc:spli<upincowatcrand anhy<lrowihypo,andthaein tum can bc:decompoocd-watcrgivingtwogaocs, hydrogen and oxygcn,a11<lanhyd rou•hypog"·ingoodium,1ulphurando:oygcn. Howcv<:r,11point is lOOto reachedwhcrc1hccorutituent1can11otbe furthcrdividcdl>y anyordinarychemiulorphy•icalmeant-"'"" cannot oulxli\"idc sil\"tt, bromine, hy<lrog.::n, 0>1ygen, IOdium or tulphur; 1hae a rc Cl«lmpla of the hundred or so clemtn1al ou b­stanca of which <:\'ctything in naturc io compoocd. Wc call thac oubttanca tl~•ls, and the matcria!t made by chemical combina­tion of clcmcnu U>m/>D•ndl. Thus oilvcr bromide, hydroquinonc, gelatin, sodium sulphite and hypo att all c><amplOll of chemical compounds.

When clcmcnu combine they do oo in certain fi~cd proportions by weigh l,thuswa1cralway1con1airuoncpa.rthyweightofhydrogcn combined with eight paru of mcygcn 10 gi,•c nine paru of wa\<:r. Silver bromid<:coni.aint l06 parusilvcr a nd 80 parts bromine-­any a11emJ>1 t0 induce dilfcrcn1 proi-•iont to oombiJ>C .,·ill gi•-.: oil•-.:rbromldcofthiscom~itioriwithanunrcac1cdCJ<CC111ofOnc or1hco1hcrdcrru:nt. Sudiobocrvationsga,·eriscto the idea that elemcnll cons.iot of a mau of extremely minu<c partida of die cle­ment-cal led a/.o!!ll-and !hat oompou,.tls arc built up of agglo­mcratn of one or more uoms of each of the indi•idwil clements, such aqlomcrata being called -'«whs.111c rclati,..,,...,ightsof the atODV "'ere deduced from a study of the compooition ofa large numbcroroompoundo, and when related tO the lightClt clc""'"'• hydrogen, as unity, provided a listofwhat arecallctlatomUwti:~u. Thus hydrogen has an a tomic ""ight of one, oxygen 16, sodium Z3,sulphur3Z,andooon.

TI1e correctness of this conception ofa1oms and molecula has been fullyoonfinncd bylatcr.,..,..k,and we can l>OWmeaturc the ab$olu1c w~ight a nd si>c of 1he aioms of all clements, and their arrangement and di"ane<: apart wilhin the molecules.

Chemle.lNom1nelatur1 The chemis1hasgi,..,n1ymbob to the elements, H for hydrogen, Oforoxygen,Sfor oulphur,llr lOrbromin<:,Agfor silw..(.t..I.

1"/IK Cl/KMlSTRr OF PllOTOGRAPllT

a rgcn1um), Na for sodium (C•. natrium), K fo.r po1au.iu1n {.·fr. ka]ium), etc. A compound can 1hercforc be 1pcc1ficd by d1<: •)·m­boh IOr iu corutitucn< clcmcnts, with omaU nurru:1al sulf11CS sbowing thenum\Jc:roratomsoreachelcment inonemoleculcoftheoom­pound. 1llc~mposit~andformula<:ofoo11>C 1 ypicalpho10> gra phic chcm1cals arcg1vcn below:

,\nhytlroUJhypo (sod ium

thio<u lphatc) ;\nhydroUJsodium

•ulphitc

(AmpoJitW. ZaiomsHi·drogcn l atom O~i·gcn luomSih·cr l a tom Bromine ZatomsSodium ZatomsSulphur 3atoms0xygcn ZatomsSodium l atom Sulphur 3atoms 0xygcn

Fa"""'" H,O

Ag Br

~ .. ,so,

;\!any C')"Stalline compounds contain water combined with the 1nolecule, called water of cryotalli!.ation; thus crystalline hypo oontaini water combined in 1hc r-.u;o of fi•·c molttula of water to onc moleculcofanhr<lroufhypo,andit isthercforeformula1ctl&1:

!\"a,S,O,..'iH,O

ltismuchclearcrtotlilfercn1iatein thiswaythco~ygcn inth<: hypo molecule from that in the combined water. Altunati••c formulae for cry•talline hypo, such aJ Na ,H ,,S ,O., or even '.'\a,S,O., i.i,.,o ,, although correct, would be: le11 infomuti•·cand muchmorcconfming.

1"1/li $Clli1'"CE QI" P/IOTOGltAl'llY

Oflen !he equality sign is replaced by an arrow to indicate in whichdiree•io:>n<hcn:aclion norm.allyprocttds..Someruo:1ionsarc rcvcnible, 1hcdin:ctio:>ndepcn<lingonoondi1ionsoftempcnturc, rclati,·econttntra<i<>,.., etc. R.,,·cnibili1yisc:i<prufed by replacing the cqualily sign by double a""""S, thus: .... Chemical nomcn­da•urt <hus fulfils 1wo main funoiion1. h io ao useful <oachemis< u1honhRndi1 1oa11cnographer, a nd a tthe•:unetimeit eml>odie:s hisknowlcdgeofcornpound1 and1hcirrcac<ion1.

After this brief review of the fundamental • or chemiotry, we may now proceed 10 a e<:>n•idcra<ion oflloc chemit1ryofphot<>graphy, ,.·hcrcfurthcrexample:sofchemicalcquationswillbefound.

l~tf~:~~ft\~fi:'a~~~~:f::'ils-P..,.ir•pll;1is1hc namc given IOJ>IO«SSel in which pic<uret~ produe«lby1hcagencyoflig:h1.Toproducesuchpicturet"'Cmust employ a ma1ttialwhich is...,iliNto~ght, that is 1ouy,onetha t tu~1<1mcchcmicalorphy;icalchang<:whtnlightaeuoni1.Therc

areseotttof1uchma1crials : somcmaterialsfadcundcrligh1action, others darken; IOmt arc clurnged bcaU$<l <heir m.>leculeo are dc­oompo:...ed by light, others because •heir mol«ults condt>UC and J>Ol~,thatis,jointogethertofonnvcrylargcmolecults;oomc arcr<:ndcrcd more oolublc in<:<:rtain ool,·cn11,0tMrsl ... sotublc. Any one of thac: changts could be harnessed to tnakc a photo­gr~ph ic process. The pioneers, ho,.·cver, were k<:ki ng the mO"t eAicient proc ... 1oproduccrcallygoodimagts,and1heir at1cntion na1urallytumcdtoagroupor saltswhicharcouts\andingly.,osilir< toligh1-1he0111isof1hemc1al si lvcr.~lanytilvcr,,. luwilldarkcnon

""potun:toligh1,acharactcriS1icwith1hn:cmaincau1t1: ( l ) ~ I Olt oilvcrulis an: light in colour. (2) ~IOI! oih•cr salts a..., umtable, many of llocm decompoo.ing

undertuchmildlydisruptinglntlucnccsasirradiationby ligh1.

(3) Whcnsih-crsa!tsdcoompos<:,nw:1allicoih'<'r isusuallyformcd. Thcoil•"CTisproduad.inafinclydividcdstaltalldoho>. .. no""<>f

thcbrigh<lll<'tallicappcaranccchaBC.ttn.<icofpolishcdmasoivc oilvcr. l ti1,illfact,U1uallyjetblackincolour,and1hisa.ccountsfor thc:darkeningof&ilverul11.

Tbe SllurHalldes

Of a ll 1hc 1ilvcrsah1, threcwcrcfoundtobeou111andingly 1uitable forpho1ogr•phicpurpOM:S.Thesea...-1il""<hl~Tidr,1i/.,,.bra"'Uhand

Tl/Ii CHtiMISTRI" O~· PllOTOGRAPl/1 119

~2~==~~~~~~~~~~~~:\~~~~~~= 1>ns of_llocumcfam1ly~group ofclcmcnis)andarckno"·n by thc

~'!::ri'Jc'."i:~;::;· !i7~~·:~~!":.7~e ;:~,:",!7,.~;.'~~v:~: typc whichnecd no1concem ushcre. T hac: threcoih·e rha lidcsa...,

~i~il~:~·f t~~~:~~?0~g~g~~~:!:;·[~~~. l"'he1ih·erha!idcscanbeinadcbydi r«tchcmicalrcac1ion be·

t"""" metallic 11!'"":'" and a ~logcn, and indeed d1U was die method .ucdby lhgucrrcmprepannghitpla.ta, ll coeiu.iti.sedapolishcd l•-cr ~1c (--;:11~llyoil,-crrdoe>ppet,forcconomy) byapooingitto

·;:~:.:""~1~:00~:i;~i:.·;:!r:.,~~uc:,t.,:. •:;n,.~~;'" e!'t'.!~~ i1honc: ofh.alog<:n,oothatlhc:<=ctio:>nil: Ag X- AgX, ,..oo., X rcpr...,nu chlorine, bromine or iodine.

ThcructionundOT<;Xamina1ionhasonccuriou1ch.o.ractcris1ic. l r "l:oon1idcr1he oolubilityinwal0T of theinitial andfinalproducu, "l: find that lhrecofthem (&i lvernitr:>te, poo.aium bromide and pot:lS>iumnitr:atc)arefrttlyoolublcin";oter,whileort(,&ilverbro­mide, is virtually insoluble. The reaci;on, 1hetti0tt, isJOmCwh.o.t •pcctacularin1ha1onmixing1.,...,dur,colourlcaoolutioruapale )l:llow oolid it produced immcdiaccly. It caket th.e !Orm of myriads of minute crytlab " "hich immediatdy ....,..i11u or dot l<>gcther and senle tothebonomofthc,·esselasahcavypalcyclLowcuni(Fig.l). Thefonna1ionoraoolidfromoolutionin1hitwayitcallcdp.t<ipi1a-1iffand theoolidiuclfthepmipi1au.

The PhotogrtphieEmulsion

h iscasytopreparelight-semi1ivcsil,·0<halidcinbulkinthisway, bu1i1i1 ra1hcrmoredifficult1oan-angefori11e,·cn1prcad.,,.·cr 1he

F~. 2. SilV<T i>«>mMI< p.cdf'l1• 1ed in 1be

.......... (/efrl """ •boe<><<olrtl>tin

THH CllE .lf /STRY OF l'llOTOCRlll'llf

ourfacc ofa_su_ppor11uchasgl.us,filmorpaperforU1t:in makinga p1>o1ogr:apluc1mage. l t"-asknown1h.o.t 1 he~lationand1ettling oh prttipitatccouldbeprc-.'Cntcdbyth.epraenccofa..thiJ(th:n ;,, a complex,.;..,,,.,.. material usually of animal or vcge1able origin ,ucli a1 $1&reh.-s, gu.ms, glues, etc.), and "·hen &i~""r ha lide it JI"'" cipi ta1cdwithg<:latmdisool''Cdinthcalkalihahdc10luti<>r»cach cryllalor 1r11inof1ih'Cr halidcrcrn.airu 1ubttanliaUysepar:1 tcdfrom it1 ncighhou'1inperpetualsuspcnoion (Fig. 2).Sucha1uspcnsion can. be l~orcd, ~1nnd lcd and coated wi th li ttle or no seuling out of the grallls; 1teanmfactbc treatcd a lmos1iu though it wcrcahorno­gcneou1 liquid . Thc1u•pcruion of mi nute silver ha lidccrystalt in Rcla tin "'""called aphorox<ap/ri<tm11lsiff. This""" an unfortunate na me,aschctcrmtmidtiffisnormallyl'CICrvcdforaditpcnionofa "iquidinanothcrnon-mitcihlcliquid;Kircumplc,mi!korp,ar:affin cmubion. The ccnn !>ltowx<¥'ti&,,.,J>t>u;,,,, might have bttn "-" "''"'"''"•bu! the old name is nowlO 1r:aditional and IO uni,'Cr­sally used chat any ancmpll at eottCCtion ""uld be qui•c futile Fig.3).

THE SCIF.iVCE OF l'HOTOGRAl'HT

P'"'"cn11coagulation),bu1iuoolu1ionhao1hcuod"ulp«>pert)"ofbc­in1 liquid when warm, and octting to a jelly or pl when cool, a ch.aractcri>ticwhich is utilioc<linthcwuhingandeoa1ing olagcs. l< driad<>w111oahar<l,wmpact,uniformlaycrwhichonm:iurncnt with water<>• pr<><..,ing..,lutionomcrcly•"·dband doct t\Otdis· .oh·cordisin tcgratc.[n1his waythecmulsioncanbejlffUJJtJ,1hatis >ubmitted to oe<tain chemical ructions ,.·ithout in any """Y disturb­ing <hc pos.ition oftheimag<:. G<:L ... in can be har<leMdbytuch agcnts u formalin or chrome alum sou to rai>c i•1 mching tcm[ICrature in ""'"te<orp«><><»ingool..,ioru,and u ir<hisca1akiil;ue~no< •uffi.

cicntiniudf,italsocomainsminutcquantitiaofcx1rcmclyp<l""Cr• ful.oco>Sitisingag<:nU, whichgrcatlyincruocthl: light11entiti,·ityof gcla1incmubion1rela1i\•ctolha1ofcmubionsmadcinolhcrmedia.

Efft~t of l ight on Sliver H1lld1

There is, of counc, much more to the manufacture of a modem photographiccmut.ion•hanthcpTCCipitationoftilverhalidcina gdatin rnedium, and 1he whole process ,.·ill be dacribcd late<. For thep...,..,nt,ho>o"CYCr,""Cnttdnotcoimplytha1ancmul.tionoonoiiu C11CHti.allyofminutccryi1a1tof,.;J.-crhalideembcddedin gclatin. The light....,..,.;li\·e con11imcn1 is the ,.;i.-er halide, which dccompooes underlightactionintoiucons1itucntclrmcnu,mc1allic,.;Jw:r and the halog«n (chlorine, bromine or iodine). lfo,.·e,•er, as has been 1hown, th= comtit"cnu will, unless prc•"Cntcd, rc...,.,1nbi11c, "' that on shutting off the light the rcvenc action takeo place ~ud the original 1ih-e< halide is ,..,.formed. Taking silver bromide u an eumplc,1hel'C\'ersibl.crcactio11maybecxprcs>edbythcC<juation·

light AgB:a,r ,\g-'- Br.

lipurcsil•·.,,.halidc;.cxpotec11olightinanopcnvaoclsothatthl: libcn.tedhakogcn,bcingvolatilc,candiffU1Cinto1hl:air andbcloot, or ahcmati,·cly if,...,upooc oil•·cr halide mixed with '°me mbstan~c whichrcactsimmedia1clywithfrcchalog«n togivcaotablcproduc1, 1hcoomplctcrc...,.,mbinationofthcliberatcd 1ilv<:r andhalog<:ncan· notpossibly takc place,and1hcac1ionofl ig htrt1ult1inthc1>Crma· ncm formation of "'me metallic silver. Among the many ,·ir1ucs of g<:latinisitsabi!i1ytocomlhnercadilyw:ithornallquantiti'."of.frec halogcn,togi\'e1tableproduct1.Whcn in1imatclymixedwoth11Lvcr bromidc,forexample,a1inanonnal photographiccmulsion,itfunc• 1io111 u a /tr-* «UJ>IM rendering the action ofligh1 on ,.;1,..,r bro­mide p<l"Ill'lnt:nl. I f the action oflight on a photographic cmulsion

THE CllE.11/STRI" 01" f"llOTOG•A.1'111"

i1sulficiendyprolonged,.;1,..,r maybefonnedin1uchamounuthat thecmul.tiondarkensinoolour,givingaprin1-outclfcc1;butamix· <urcoftilvcrhalidcandgclatin aloncdocsnotgivca oali1factor}' pri nt-Out material becauoe,ahhoughgclalin may beancaccllcnt a«cptorfor ,·cry sm.all quantities or halogen, it bcco"'aoaturattd when th~ cmubi<>n is only olightly tinted . F<>r oa~isfactOJ'"y print-out effcctit11 thct-cforcneccosarytoi"°"'"poratcco1"'dcrablcquantities ofa ""'°"dhal.ogenaoceptor,.·iththcg<:latin1uchas,.;1,..,,.citn.te, .. hichl'Cllcu";'h thelibcn.1edchlorinetol0rmoilvcrchloride. In print-ou1methods,ligh1providathewholeof1hccncrgyn-ry w formtheoih-eroftheimagcandhcncelongcxpooun:tohighin·

!~:n•y3~ ~~;~~-Forturuo.tcly altemativ<: methods of image form·

D1v1 lopm1nt

One of the most striking advances 1ruo.dc by Fox Talliot ""'""the ol»crva1ion.that"·he~lighthadproducedonly a""Cakimageon~ photograplucpapcr,1t"-aspoaibletointnuifyitbytrutmcnt,.·tth a mixturcofga!licacidand,.;lvcrnitn.tc(-p.17).·nu.wuan 1ndica1ion 1ha1lightnccdno<dothcwholcoftbeWO<kinproducing an imageor,.;1,..,., but thataoubocquent chemical p«><><» ""OUld ft/¥ the initial feeble imag<: "·hich light forrru.

n...1.,_.1haobecnimprt1vc<ltoiltchanatem tincc l'<>1Tolbot'o daythatl ightperfornuancgligib!camountof1hework,1hechangc prod uc.:d by light being "'"MU tlo~t n<>chcmical <>r phyiical dif. fcrc:ncccanbc foundbctwccncxpoaed anduncxpoaed grai"'ofoih·cr hahdc,<>1hcrthan 1hcirrcacti.on 1o apho1ographicdc,·cl<>per. The r...,1\hataphotogra.phicdcvclopcristhemootscnsi,i\•elalfor]ight expoo urcofanemul.tionis\-cryfor1un.atc u far q thcphotogr.ophic prt>eaois ooncerntd, but not,..,ryhclpful"·henwctry tolind1he l\Xtu~of•hechangcin1begnint.\\"hen""C tr}'todi>CO\'Crbydircct

=~~:;:,~ f::: :c;=;,: ~\~~~.,C\~:.:{:~~?c; ~~'t!;

~~~f§i~~{f ::f ::~~~ ~~~l~~~~~~~ai~~l~:~i~:: A dew: loper io a compound or mixture of oompounds capahlc of

<Onvcrtingasilvcroah into me•allicoi h'Ct". In chemical parlance, 11».ttrialswhicharceapableoffonninga mei.a!fromitsoaltorosidc

::~.~=~ ~~~~eo;:i:;c;i;h~toaj~~~

~11 (•) (!) (<)

Fiz. •· l'horomkr<-s;:i"• .:0.;~.'T.t, "(,'),;d: .. \: ;7¥~c. <•J o.nM•1 - o.o.,

whichcanconvcr1mctal1intothciroxid .. or1alts .ThcrcarcocorC3 of~ucingagentJwhicharccopo.blcof=lucingsih..,,.bn>midcte>

mc1allicsih·tT,bu1compo.rativdyf"ewa«:satisf'actoryphotographic dC\-.:lopcrs. Thac ""ill, when corttetly compoo!>dcd with other in• g"Tedicnt1,rcducet<VCnunexpooed >ilvcrhalidcofaphotogn.phic cmul1ion 10 metallic 1i lvcr, given time, but the small amount of metallic1il ,·erformedby lightactionwill aixclcrate1he red uction ofr:xpooedgr.&insbyafac1orofbe1wtt:n tcna.ndahundred times. Hence 1heaction ofa deo.-,,loperfor a limited time differentiates bet1o-.:cneicpooed:onduriupooedgr:aint, thcl0rmcrbeingla~y

oon,·cr1ed1<>rno:tallic1il>·er, .. ·hiletl>claucrarc:substamiallyunaf· fttted .1l1c develop<:rmc,..lyamplifie1thcworkini1ia1edbylight, but the a1nplifica tion faccor is cnormou•-'11.>out one thoUla nd million timet. The actionofaguncanbcca kcn asaroughanalogy, liglu reprt:1encing1hceocrgyr<:quiredinpu1!ingthctriggcr,whilc chee,_..,..,...1ygre1ucrcnc<gy •upp~cd by chcaplosi\•cin propelling thcpro;ec1ilcioanalcigouo10chccnergyoupplicdby1hc~clopcr.

If lhc oiker process had ach'an•aga over othcn in ttlu;on co prin1-ou\aspects,thcim roduc1ionofdcvelopm<'n\raiocstheadvan· 1agebyafac1orofone choUS11ndmillion,anditseemsunlikcly tha1 inthencarfu1ure the1>"n<m<H.1nt p<Jooi tion ol'oilverinphotography willbescriouslycballengcd.

ln eitpo1ingalilmorpbtcin checamcn.,different1>"mtta:ivc diffcremcxpo1urcs,depcndingonthcin1cnoitinoflightrcAioctcd from different ponio"' of the view. Oiffettnt parts of the film thcttfotthavcdiffcttnl]>roportionsofthecrya1alsorgraimttndcred dcvcloP"blc. After developing, and removing unchanged silver halidegn.i11sbydissolving thcminhypo,we arelcf1withdiff=n t population dcnsili~ ol the minute gr:airu of black met~Wc oilvcr, which pmducc the hght and dark toncoofthc nepti•~ lf•r· 4).

C1tttp1n 7

EMULSION MAKING AND COATING

p~~~?~:~lf :;~:1~~£f~·~~~=~1:~~£;~ •pcdficdcharactcr1'0CS,lhecmubion-makcrgoaintohisdarkroom and makaocoreo ofcmubiont,trying thccffcctofallsortsofdiffcr­c~ t physical tn:atments an~ chemical additions. Fina.Uy he selects by tn.al and error those oonduio"' which gi'~ him an emulsion of the desiredpho<ographicquali1y.lfowork isallthemon:diffieultu<hc

~7c~rt~~1

~E;~~1:~7m~~~~~;:~~s:~~~:t~ makerilna1urallyable1oarrivca1 adesiredreoultafterfarrcwcr u pttimenttth.anatyroin ll.., art, andthea[>pJicationof$1:ientific· allytrainedmind<c1;cn10hi1-and·missmcthodohasreouhcdinthc vcryhightta!Klardol1>rescnr-<laymatcrial1. But1hefactisdiacln <he emu~sion-ma~ing bra nch of pho1ographic ocier><:e, che 1heorUt lags behind in h1' endeavour lo explam rcsu h1 obtained by the P"."'t ician.Only ,·ery occuiona ll yoan1hc theoris lforgcahead to pomt1hc'""Y tohc11cr pho1ograph1ccml'l1io1t1.

Th1 0bJ1ctt orEmulsionMaki111

~~:: ~;~~~~:i;~~ ::::;.,~'~::ini~ls;~~~·:~;~~ °[~;;:~~%:~~; :~ :~~~Ide~!;..:~;:•~•:::~.:::::::;::=;~·~~~;,~; they would all be 1ub11antially unaffected hclow a critical ""POii""'' and all rernlcred dcvcloP"ble above that ""POii""'· The Ol)J'ltrast "'~uldhcnurlyinfinitc~udcxpoouttlatitudcvcrysmall,andaneg. ativewou ld bedividcd1n1oa...,asof nodensi1yandatta.<ofma><i·

~~~;3~:it::.::%j~;;~~~~::~~:: "

Tl/£ SCllf.,\'Cf: OF f'l/OTOGll,<P//r

of1henumbcrofgr:a.insspontaneouslydevclopoblc,and1hcipttd

§£.rI~t~~~~~I~~re~:; !;L~·;~:::~~::i~?:;,2~?E:~~"~~~r:~!~~:l~z~r,:i~~~:~.~

~~~~~f g~~~~~;§gf ~~~f gf~ ~uce ti.., maximum ;cm;iti,·ity n·hich each sn.in i1 capable ol siv1ng.

Thcoompooi1ion of the grains, 1ha1 ;,, the choiceofhati<k, de­pend1onthc•ypeofcmulsionbeingma<k.Silverchloridciouoedl0r slow developnw:nl popen of the "Velo~' type a"d for slow lantern plain. A mi .. ture of s.i lv<:r chloride and bromide ii wt:d IOr warm

~°:,,~~(=~-i~'::'m~fi ~;,~=~':~.i:r\:r!;.; t:nc~1~;~:; s;,~\~'.~~;r.~~~==-ri~:::·~~tir.~n:;:a;7:,:~rt~~~ tx1i::

Gn lnllzeDl1tribution

Grainlil~distrii>ution,upon".·hid>thepotcntialcha""r'cristiciof

~~;;~~:~~·::'Z?~.'~~~~;:.~.:s:t;,: :: gi•"<:nlothcpttcipitalionundcroafdiglnconditionsofs.ikcrhali.dc inlJ"lalin (as prcviou•tydcscribed). Tocmuregoodreproducibility fl'OQI batch 10 batch, 1hisandall 1ul.>fe<!ucmoptrations m'1St be ~o:doutundcr1hcn1o>tp«:circchcmicalandph)'s.icalconditiono. 1\10h-crnitratcoolutionof1pcci6edvolunw:,coru;<:n1ration an<i tcm­pcntureioadd~ roaoolutionofalk.t.li halidcandgclatinundcr co~urotled <:01>~1!ions of>•irring, the alkali hali<le almn1t invariably hc1ngin chcm•cal cxa:ssovcrthcsil....,r nitrate. The oi lvcr nitrate oolution rnay hc added rapidly, or slowly at a prcscrihcd rate thrnugli a jct, or in rcvecal portions at controlled intervals, and arn­rno1•ia may or m~y n01 bcadd.,.l to tho 1ilvcr nitra10

£.tfUL$10,Y .11.l//;/,YG A.,YD CO,<T/XC

Immediately after emub.ification the grains of s.ih..,r halide may bc•oo...,,., lltobcrcoolV<:dbyopticalmicroocopeo,andthccrnulsion i1U1ually •·eryinrcnsi1i,..,. h is1~givcna1pccifiedhea1treat­men1 known u ,;,._;,,,, usuallyoflhc order of:50-60 rninutos at " ' C.Ouringthi11rco.1ment1hcgrainogn.duallyinc........,inlizcan<i dccrearcinnumhcr(Fig. l. ) .

(o) !•) fif. l.Em"11;,,,,grain1(x2,m).(•) /\h'1omul~f><ook>n,(•)•f«r fun,;poninc

Tl..,,..,aoonforthischangc,kno"'nuOm1. .. U•if>t'ti~1.i1that 1hc waini a,..,,Jightlyoolublcinthcexa:ssalltaliha!idcandintheam­rnonia, ifprcrcnt. ltiifoundthat thcoolubilityofu1,..,mclysmall ~ryotals varict ""ilh Cl")"ltal s.izc, the unallcr onell being oomcwhat -oolublc.Thcy1hcreforepassintooolutionandcrystallircouton the largC>"Cl")"ltals. Small grain size distribution, and hcnoc high Ct>n trast,;. U1uaUy obtained by rapid emuls.ifiauion and 1hon ripening;largcgrains.izcdispcrs.itybyjctOl"muhiplcemubifi.cation wnhrnore in1cr..eripening.Sincccon1inuo:dripcninggivcshiBher ••·cr;igegrain s.i•c, it foll"""' that it is morcdifficuh 10 makc&.1 •muJi.ioni ofhighcontnut,orolow,fi™1graincmuls.ionioflowcon-

:~~t~~'"~~~~~~~l:~~ ~;:~:~·:~ ~;~~; ~~d~ti!~~. 1:~~~~ many emulsion formulae ("-' in jct addition or multiple oilver addi-1ion), 1hc two pr~ procccd 1imu ha ncously. Although increasc

THF. SC/li.l'CE 01' l'/IOTOGll,.l'llY

in ripening incrc<lSCS avttage grain ""nsitivity and a""rage grain liu:, ii canno1 ineru.c the •peed of an cmubion inddini1dy. for oome featOR at ptt:SCRI unkno.n1, gn.ins bf:gin co i.:.c ..,,,..;,;,~1y when 11>ey.,,..,..,.iac;ertain lize.

Conclitionlnt the 6raln1 Ha,·ing adju!!cd lhe gram SI~< <lu.t11l>u1....., 10 the required value, thati>,havingobtainedthc/'<Jk"li/l/charac1cNtics,i1i>wiual to mdis<d1cdesirctlcharacteri'1icsby1ui1ablyadjusting1urfaceand exocrna!condition>oftltcgrains.Atthccndof thcripcningpcriod •hcgelntin inwhichthcgrainsar<:1u•p1mdcdcomainocxccssalka li halidc, alkalinitratc (formedasaby·productfromlilvcrnitratcand alkalihalidc) andfrcqucntlyammonia. Theprcoc1iceofalkalihal ­idesin thcconcentratiooswhichnormallyobtain hatadcp<ai.ing cffecton 1pecd,1houghlowconccn<nnion1•r<:desirablc. The alkali ni1ra1ei1 p..,..min1uchconccntra1ionl1hatitwouldcn.•cthc cmub.ionby cry.callilingoutwhencoatcdanddriMonfi!m or glu., and the continued ~ofammoniawould tend to a slow con­tinuance of ripening. These may all be r<:ll>O\"M by adding more gelatin. onting thccmuhioon to a >1ifl" gel,thredding1hroush a pcr­fonued 1ilvcrorscainlcssstccl plate •<> (i•-e-4/uofup to fin. diamcccr (Fig.2) and""'hingchcmin a n>COl1baskc1orcanv.., bag immcl"loedinrunningwatcr.

The prcscnccofthctolublc oal ts is n<1<todcle1criou1 "·hen the cmubion i11o bcco1nedonpaptt,asthcymign1cfromthccmubion inw1hcp.t. pcrbasc.Mor-co--cr,1bchighn1 1pecd1arcnotncccsoary orpapcn,00 1ha1manyJ>al>t'cmulPons--apccia!!y chLoridcemul· .ion-rc un,..,.,toed.Afterripcning,1hey are n>e<dy docto<edand coat.cl.

'\q11i•-c cmulsiom, which a r<: a lmoo1 allcoa1td on glaso...,. film, i...,-cwbc washed,andafterthcconccntr.Uionof tolublc 11al11has r<:achcda1umcicmlyLo ... · ligurc, thcthr<:ds a rc rcmdttdandgi•-c1> 1occondhca1 trc.atmcnc, kno"'R as di:tsriM or'!fltT·•i,,,,.i~:,Urnn and 1'"mpcn.mrcsbcing ofthcumeordcrasin rheripcninglreatmcnt. fhe~ildtolvcnu for sih·cr ti.:'lidc ha\"C bttn r<:moved during the '".uh1ng_procc11,ooaftc,...ripcnmghao littleornocffectongrain1i•c d111ribu1oon.ltha1,howc"c1,amootmarkedeffecton1ho'°n1itivity ?f1hc8rain1.0ncofthc virtues ofgelatinasamcdiumforo1nulsiom 111heprc1oncc ofminutcquanti1iesoftomewha 1un1tablcorganic

~£E6E.~:~!.~£ .. ~~h~u}~1

:i1

~E;i~\~! (~:J:~.: >ft_hcgn.iruarc appar<:ndythecamcof1hcgn:at!yincrcax<llpced

•~h af1c<-1i pcning imparts (p. 113). The prcstnccof..,lphur-cot'I·

:~n~~~n~~·~:.~~~'=:'i,~:~~c1:;,~ :.':~=: ~~~;~

~~~~~~~~~~i~i~~~~~

n11; SClfiNC£ 01' l'llOTO<;RAl'llr

OptimalcharactcriSliaatercachcd.Anti-ba(1trialp«:ioen-ativuarc added, ilnd the cmubion ;. immediately chilled lo P"''''"" further ch.o.ngc a nd11orulinarcfrigeratedroom.

Doetorin1

Samplcsofeachcmub.ionbatcharchand-co;oteda11dtcsled1ocn­•urclhatthcpho<og"'phicproperticsarca1 d1cy 1houldbc. When !lteapprovedl>atchisrcquircdforooating,i1i1n1eltedandcenain oolutions,k nown .utlotwrs,addcd tofacilitaiecoati ng a nd to modify dtephyslcalandoemitome11icproperlicsof1hcfin.alfilm. Aleohol, forex.amplc,m;iybcaddcdtorcduccfrothformcdduringcoa<ing, glyarin to make the dried Layer more pli.able, 1urface acti,.., agents 1uch.uuponinor1)'Tlthct;c1urface-tensionrc"""·entom.ake•hc dried film morcaslly wcucd by pnxesiingtolutions and reduce thcriokolairbelll0rm.ation,hardeningagenu1uchuehromcalwn and formalin to d"""""'" th~ oolubility and ,,...,lJing in "-arm oolu­tions, oensitioing dycs to cnend tl>e 11a1ural ra11gc oloolour oensl­tivityolthccmulsiontogi,..,.,IM<~licorfHw~iecmulsionl,

asmalladdit.ionohlbliha!ideto gi,..,reduct.ionoffogandbcner keepingprnpertics,goldsaltstngiveincrcasedsensiti'•ity,andotltcr addition•maybe madcfor 1pecialpurpo1t:1.

Commercial cmub.io11-making mcthn<b may differ bctw•cn dif­fcrc11t mu1ufacturcrs, and cach manufacturcr u1C1 ecr1ain proc<:M<:S oraddition•whicharcguardcd a> tradcfeelclS,bu1cmub.ionmak­inggcncrally follnw•1hchroadpattcmwhich hasbttndcscribcd.

~1J1 [~y fll.~HS£ DEVICE <)ON$

n,.J. t'iln\Qrpop<r.,,.,ingmach;,...(n; ... )

CDIUna

l"he Mtl4<tJ,,,,.11Jsionispipedtoacoa1ingmachincinliquidform and applied to the ..,,,,.,, or b<U~ ""hich may he ol the glaa, film ot ,.,,.,.

film- a nd papcr-coatingmachincsarcvcryo.imilarin principle, and n<:assarilydiffcr markedly from plate-coating machines, be­~auoc oltbcdiffcrcncc in flexibility and con1i11ui1y of the Nu 10 he coa ted ( •"ig.$).Film$andpapcnarccoatcdinrollsuptofourfttt in wid1handtW<1thousand ftttinleng1hby~ing1hcbaoeundcr a rollerwhic hjuotdipsinto liquidemul1ion in a 1h1llow trough, though1everala lterna1i,·cmc1hodsofapplyinga uniform layer of hquidtoonc 1idcofthcbaoeha,·cbccnuoedin1hcindiu1ry. l'rom lhc coa1i11gunit,1hccoa1cdpapcrorfilmis lcdovcrchilleddrums, or through a rM// lx>x containing rcfrig~rated air, whereupon the

liquid cmub.ion •<:<> maz<I. Tbc material i1 !hen automati.cally loopcdimofcotooruabou<ibxftctdttpl>ytwoftt1widc,L>ypcriod· iexllyhookingupthccoa1cdba>con""OO<lcnpoln. Thcpoles,.;th 1hcir dcpcrMlcnt festoons are trarakrrM to "'°"·ly moving endless chai"1"·hichpa>0down.adryingtrack1<>me thr«hundredftctin lcngth.Mc1iculou!Jydcanairofcontrolledtrn1pcralun:andhumid·

~ ~::! \~':.:!:~.:r:n!~:;'~~di~~he:;.:.cni!';,'g "~~:!'!~:!tiy lij~ ~~~~and 1poolcd, priortoslittingandcho)>ping tothcr1•quired

l'l•te..u>ali»g machines are buih on different lina. The plates to bcooa1c<l,h11ttcdeml1ocnd<manc>ull<:Qband,arcfcdon10the

l1lc amount oliblvcr halide per unit atta o(fiJm (coating """ighl) ii

;_:~~:~~ac~=;:..~~~n=:~:;:~f:~ 1Cribo:I fo<- thc product coated. Th;.;,. adj~tcd to.the opiimum, that il, mthatYalucat whichfurthcri~mooa.nng " 'eight p~ d llCCI no further imp="Cment on charactcnHict-mdced """'"""" ooa1ingwcightintroducesadvcn<:propcrtia.

\!any modem fan ncgati,·e materials are doublc-coaocd, the film fint r«:eivin.gaco.atingofa"°""Cr tTilulsion before theexircmcl_y rapid cmulsion io appl>ed. When the expooun: ii ouch that the max1-mum denoity of the npid top coat io reachcd, 1hc "°""Cr undc"'?"1

~:~ti·=~~;~~:=~~:=~.~:~~;~ ?-led product, a double-coated matcnal "'OUld (ivc dcgrtt:i of •n•<n><:rbl.ack-which"'<>Uldmen:ly""!uin:longcrprintingume

to ~hl, a..:ia;::a:i:~:::...,a:n~pctl where the M:noitivityolthe etnu lsiorii1dividcdintothr«separatelaycn,ooatinirmayinvolvc

::r~r::i::t t:~ ;;.~!:' ~~:~r~':.ng other things, expl~ins the much

fv.10.u...1>1e ..... ••nc Clli>C-)

Super C011ting

lfac:oatedanddricdcmul.i.ionis 1ub~ted 1<1olight111rfaccabrasion, uforaamplebyrubbingligh1ly"·i ththcbad.ofthefingernail,1he grains affected become dcvo::lopable, and on dt;..•elopmen• 1he track of the abrading body""°""' u a blad: line. Sonnal h.o.ndLingof papcrorfilmproducu,espccia!lytheformer,"'OUldgi•·enumcro111 muksof th is typc. Fortu,,ately 1hi1i1mcrclya1urfaccdefect,and can l>e rcmov<:<1 fmm a developed and fiud fit".' or paper by vigor· ou1rubbingw1thw<:\COUonwool.Suchal>ruionmarkJcouldnotl>e produe<:d if1he 1urfaccwercfrccfromsilverhalidcgrains.Acwr<I · ingly moo< film and paper producu are 1upcrcoated ";th a thin lai·crofgcla tin, applied on th<: Kt but undried emulsion coating. Platu arc 1eldom 1upcr•co11ted-therigid ityofth<:1upportallow•

CElATIN SIJP(R COAT

t~r~~~SITlvt: LOWER

SENSITMCQrJJNG

DYEDAH11 -HALO BACKING

1:.1/ U LS/0,V MA KI.VG A.VD COAT/ ,VC

handlilljl[which doa not subjcct the emulli.on surface tO\uchdanger, andhoriwnt.0.lcoatingmaya llow 1ZightKttlingofgrainsfrom the surfacelal<=r.

h rJta andSubltratumtaatlng

AgeL:uincmulsionwi!ladhercP.lisfac:torilytopapcrbasc,butootto untttaled film or sluo. Ne\..,rth<l'* photognphic papu base is usuallycoa1ed "·ith ala)·crofbarium1ulphate,suspcndedingelatin, p<ioo" loemuls;onc:oa1ing._llarium 1ulphaic, or"'-t.fo«,orNryU. (although this la>t term 11 """"' commonly apphed to barium h\..!r"Ollide) isa.n incn, inter>Klywhiiccompoundwhich formsa.n ideal rd'lcc1ing surface and "·hich can l>e modified tog;,.., mane or glossy elTecu. •'orocna.inproducu,hQ....,,,.,., .,mu~iscoatcd

di,..,.,tontopapcr. Emulsioncoa1ed ontoun1rca•ed rlutorfilmwo•dd tcndtopccl

off wh<:ndry,orfrill offwhcnwet.Th ilisprobably duc!o\hctcnd­ency of theemulsion 101"·d l andoontrac1whcnwetanddry, and thc•uddcndi/Tettmia.linthilrcspcctl>etwccn1hempportand thc rcarlaycrofemullioninooin•etv.·i thit""'akensanybondl>etwccn the two.Adhcoiontc>gla11 i1Stturcdbyha"ing incontactwith1h<: gla11 a layer ofhard<:ned gelatin which 11'°"~ lcu tendency to vary in dimeruioni. Thi1 i1 ac hi.,..edbycoa1ing1hcgla .. wi1hanextre­mely thin ouhstratum of gelatin hardened wich chrome-alu m, or C\'en of dU'Omc-..lum tolution a lone, which harden• the under layer of the subseq uently applied emulsion.

Adhcoionofe mulsio111ofi1m ~ilStturcd by a different m<:thod Film~ ii made from derivatives of~cllulosc-the comple.ic organic ma1<:rialwhiehformsthcmuetur.tl partor 1kele1onoflll0$tplant life, andis metindaily lifcin 1hcformofp;ipcror cotton. l fcc!lulooc " trcated wilh acids a ccllulolc sall may be formed, but a• ouch

~~i;41:1:,•;i::e:.e~i~: ~~~~;"..:~:~~i:,~.,a~'::lu~= a1er fonncdbytrca1ingccllulooewi1hni1rieacid,forexample,il ccllu"-c nitrat<:• which, suitabl y plasticised, was used for many ycar.asfilmbasc. HowC\..,r, itha1pdually becn 1upcncdedby uf<ryor-JI- baK, which consisuof ccllui.:.c ac:e!ate and/or oilier <lowlyrcla1edcstcro.Adhesiontofilmbascmaybeob1ainedci thcr br_ctching the bate with tolvcn11 to give a roughened surface to

;;!~;~E~°'==:~:::.::ndingon

THE sc11;.xci; OF l'HOTOCR ,t l'llr

Cclluloocatc'1 a n: llllublcincenainorganicool......,11,11.1Chu a«tonc,whichcanmU.withwatcr,hutifwatcriladdtd1oaccllu. Loocatcroolutionina«tonc,thcatcr ilprccipitattd.Similar!ygcla· tin di1pcrscs in warm water, but with the addition of acetone it pn:cipitates.Hoh·cvcr,iti•possiblc,withd ifficulty,looblainaJOlu· tionofa •mallquantityof both ccllulo.cestcr andgclatininamix· turcof acctonc andM•a1cr.Suchaoolutionisvcryut11lablcao thc i.ligh test •·ariation in oompo*tionofthcool•-cnu (IOo .:xamplc, by evapon.tion)n:•1d11in1hcp«cipitationofcithcrgclatinoruUulo.c estcr. l fa•"'1'thinla)·croftu<:haJOlutioniscoa1tdontofilmbasc and all.,,.-td to dry, thcorganicoolv"1lt being more volatile will cv;11pon11cfim,dcpo*tingoellulo.cestcrwhichruttu,..!!yadhcrcoto thcba>e.Atalatcr 1tagc inthc dryingprocas,anintimatcmixtun: ofgclatinandes1crisdcpo*lcdandlhcresidualwa1crwhichfinally

;:~~t";dll~:;.~ a ourfacc dcpo$it rich in gelatin, 10 which cmul-

M1111-cul'll1c:kin1 of Fllm1 On drying the..,, emulsion la)~, the.-.: is a shrinkage to abou1 one· tenth ofthcorigirutl volume. This1hrinl:age ilalmootcn1in:lyuni· "-·-1,butthctcndcneytodcc..,...inarcau .. -clluin 1hi<;knenca.....,curlofthema1ttial, .. ith1hccmul"'°"lidcinward. On film base thccurliomootmarktd; paper, bo:<:auteofiu Lowe dimcru.ional 11.ability, oho"'1 much len 1cndcncy 1ocurl,whilcglass plaiesohowadcgrttofcurlwhich,thoughmcaourablc,ioncgligible fornorma l purpooa.

Onfilmb3.$<thil1roublci1ooun1crac1tdbygi••inglhercar 1idcan equallcndcncytocurli11thcoppo*lcdin:ction.Thcrur 1ideofroll filmand1hcctfilmi1oubt.cd (mbstn.tumooatcd)andcoatcdwitha layer of gdatin mmpan.blc in 1hickneis with 1hc cmubion ooa t. Gclatl11 backing """ld ha"cdisood•Oln<ages if applied to cine and miniaturcfilm,oo•hercarsidcisgi"uiatcndcncy toll•rinkby tn:atmcnt with ool.-cnt mU.tun:t. x.ray film nccdo no anticurl •n:at­mcnt , li ncc both 1ideo of1hc basc ha...., an cmullion ooating.

Antlhalohcklng

lfapoin t oourccofliglnitfoc:wcdonloaphotographicplalcorfi lm, the pho1ogrnphic image extends beyond the confineoof•hc optical i~bccauxlightiooc:aucrcdlidcwai-iin11><;ctnuisi:"",.duc•o inter-reflection bctwttn the gn.ins ollikcr halide. Th" sideways ""'""'of light in emulsions it called imodU."" and iu lignif.cancc;., ditcusscdmon:fullyinCha.p1crl8.Sorncof1hclight""hich~

EMULSIQ.N MAJ. INC ,t,\'D CO A Tl.\'C

thtoUgh1hecmubionwi.llbc teattcrcd inalldin:ctionsinto thcfitm

~~·~·~~~~~~ :·~~~ 1ne;;~.,~ ~17."(;;;' ;;)• :::t;:1:~ :;~~~ hl rnorpl.a'.ctobcaboort.cdby1hcblackbackingpapcrorda rk11idc. Lightm1kmgthcblack 1urfaccof1hcfilmorplatcatmon:than thc c_ri ticalan!(lcwillbctotallyrcncctcdandwillagain 1trikc1hccmu l· "'""• form1'.'g • ring or ha lo a'?" nd the original image-hence the

::.:at:';.ro;.~~.\~c~T~~=ll~t~17h~~~~::!'~;;:m~~ oo tha• hala toon" more nouccablc on glau platco (Fig. 13). On thin 6lmo, che ringsarc oo1111allthatthcywual!yfuoo:wich1hcinitial 1C&1tcrtdimage.

Tiiiing Photoa:r111hic Mate~lal1 Thconlywayoftestingthcphotographicquality of ama1erialisto upoeeanddcvclopit,whercon it is des1roycdfor foturewc. Thc disadvantall"'ofrnanufac1uringmatcrials"·hichcanbetestcdonly bydestructivemethod>is <hatit isimpoOlibleto guara nttethequal­ity of any sample manufactured. One can ~ly take every p«:­eaution, test frequent 1amples and assumc that ir they arc sat;,.f~ toty, then •he material betwttn samplcl will also be a:oo<'· !tu utual prac1iceintheindustryto tei.t samplcoofemulsion bdOrc oo.>Li"ll' and sa.mpla from each roll immediately after emulsion applica tion, a nd again when dry. The euminatioo includes tesu ofphysic.>lchanicteristia as""'U "' '"""ofpliooorraphM:l>chav;.,.,r and tat1 formechanic.>ldefect1. lffault• are tuspcctcd,thcnmorc frequent tett1arc made until defective material is located and rc-

~=de::~z;~i\~!,~:~~:~:;~ ~~!';';;i:~:'T..~t;:;:t~c~~~~

£ "' (/LS I O.'i M ... ~· 1xc AND COATl,\'C

atcomesincon tac<wi<h1helinishcdproduct-istatednot only chernically,butforitopho<ographicaction ;andnotonlyiuimrrM"· diatcphotographicac1ion,butbyexhaUSli•..,incu1Nuion1esttforiu probo.l~e furn re action. Impurities may be pracn• in amoun11 "·hM:h arcunde1ec1ablcbychen1icalanal)'sisandyctexcnapowt'rful photographic action.

Dlmcultle1otManutacture

The difficultia of manufacturing photographic material• arc ••cry grcal , and arc increased by the conditioruofillumination--ortcn totalda rkncsa--undcrwhichmootoftheopera1ion1 havctobcrcr­formcd. Photographic reproducibility does not Imply merely chemi­c.>l idcntity fmtn batch to batch, but ph}'$ical idcntityof•n a>tonish­ingly hiji:h order. The tnhanced 1<:m.itivity to light of modem mater­ial• hN unforlunatcly, but not unnatura.lly, invoh'ed incrca>ed 1<:nloitivityto<>1hcrconditiom1ocha1moi1turc,hca1,prQfUuand ll'KCI ofimpuritieL TIIC last named arc moot troubJt10me, apccially "·hcnooncentrated in1pccks,oin<:1'moot impurities produce black or white 1pot1. lmpuritit:s which would be unde1ceu.bk if uniformly distribu1ed00<rldruinaproductif00t>«n1Tatrdin1pecks,ointe thc aru a ffected by a 1pcck may be ,..,ry much grea ter than dtca,..,aof 1heimpurity,andthrceoffoursuch 1pottperhalfpla1ewouldrcn­derama1erialquitcunusable, lmpuritiain1peckformateu>ually eilherwater-borneorai r-l>ome,and 1<>clal>ora1eprecautio111have tob<:takentofoherthcset""""mmoditi"'. Evenaftcrmottcarcful air foltr:uion, dust may be introduced by orc"" 'i"""• and for this rc;uonlheyaredra&edfromheadtofoo1inlintlessclothing. l ndC<'<I, thcgencral11andardofdcanlinessismochhigher,forex.amplc,than isnecasaryorusualinthefoodindustry,and i1morccornpor.1blc with1hat inal>iochemiea!laboratoryoran opcn.ti na; 1hea1re.

°""". THE PHOTOGRAPHIC LATENT IMAGE

TIH: PH~TOGR.Al'll lC latent imagt: praenu a fasciMtin.g and intr>eate probltm which ti... punted chemiw and physi­ciou l0r 1hcpast hundred yca'1, and ahhough wc ha,-.:ama.sed

qui1calocofinforma1ionabou•t~ •ubj...:1,ccrtainaspecuarelikcly topUJ:•lcchanistsandphysicisuforalong"·hileto<»mc.

In 1hc first place let us OU.le "·hat,..., mean by latent image. The 1uspmsion ofo.il,.,,. halide grains in gelatin "•hich ooiutilut<S the photognphicc~ul1ionison!y,<cryolowlyreduttdtomc.talliciih'<'r

byalkalincoolut101llofcc1ainaminoorhydroxydcm-at"·esofbcn­•cnc, that it, t.y pl>Otognphic deo.·clopcrs. E~J>Oll""' to a minu1c amoumofliglncautc1achangci111hecryitalswru.ttbythcvclocil.Y ofreductio111osil''Crit incrcal'edbytcntoonehundredum ... lt u thio change which if called 1hc lalcn! image. The minimum amount ofsuchchar1ge(ortl.'ccri1icalupo1ure)necessarr 10init!atc1hc tkvcloprncntofagramwill ,·arywi1hthcde\•elopmgsolu11onand time and lemper::uure of development, so duu reference 10 fol'"/ ;,,.agt ha1 liulc meani!ljj unles1 it i.1 awumed lo i1~ply the develop­~~l~\~:.~.a grain under some arbnranly 11andard1"'d developmen t

lth.,alre•dyl>ttnpoiutcdout(p.93)thatthcmootdclicatc""' oflatentimagcformationknowna1prcscnti1thcactionofadcvd­opcr,10thatitilimpoosiblctoim'Cltig•tcdircctlyore''Cntode!ect thccha11gcinducedbyminimallightu.poourc,b~anyothcrchcm­;ca1 or physical means. Ne..·crthdc• we can obu.m a grea1 deal nF indirect.,..idcncc.Th"'ll"'chanl"inthcsih·crhalidc•i"tcm"'hich we tcnn l:ucm image could ccmcci, ... Wy be a pl>ysical one (u For example a Local dilturi...ncc 01' a general change in the ery11a! stn.ic:turc ol sih·cr halide), or it may be a chemical one (ao for ex­amplc d..:ompooilion in10 sitvcr and brorninc)~thcre;, no othcr reasonable ahunative. l,et ..,. Ke how these ,,,,., ahcrnati,,..,._ phylical or chemical chanae-are •upportcd by ttru.in aopect• of thcbchaviourofthclattntimagc.

Tl/£ PllOTOGll.-tPl//C LATl!NT /M-tGI!

lnthefi'1tplacc,silvcr!)t<)midcillcM10lublc1han si lvcrcl>loridc, andif thclattcriltrcated"·ithasolutionofpot:usiumbrornidc,silvcr bromidcandpot.:usiumchlorideareproduced-notl..,rcxamplcof ""'1U""""'f>M1;.,, (JU p. 89). NC>Wan exf><*<lsi!Vffchloridc cmubion mayth1>1bcoonvcr1ed intosil\•crbromidcwhich ;,otm capoiblc of image·"·;.., development (indefd a latcn1 image ha. been producedonthallouobromidc!which .. .,..d,...'Clopcdaftcrcon•'C•­bonto•.il\'Crbromidc). ·1n;..,..idci>cepoinu1oachcmiealchangcu ancxplanationoflatmt imagc,sina: ltJttnu..-tunlikclythata chanl"inphysical1latec:ouldwrvi•'C1UO::hdisruptingreactiol'll.

Again, if an exf'O'Cd emulsion ii fixed in hypo, the latent image """"imin1hegelatin,andilcapablcof.....callcd~~ (p. L31 ) ina.i.....clopcreontaining.olublesi]\•crglts..lnthiscaxthc wholcofthcsil•'CrhalideWbcenremovcd,andwi1hi1ofc:ou,...,any physicalchanscin1he1y>tcm,yc1thela1entimal"•~'i'·cs.Ot>e must conclude that latent image fOrmation ii a chemical change whichWgi•'CnriJcloaforcign1ub&tanoeinsoluWeinhypo.Since prolongcdact>onofliglu ruuluin theforma1ionof1ufficicnt meu.l­liciil•'CTtobedctcetablechcmicallyandoonlirmedby•uchmcthods ao X-raycl)"ta llography (p.292),i tilreasonabletoassumcthatthc latcntimagesimilarlyoonsi1t1ofmetallicsil•'CTbutinqu.antiticstoo •mall to be detected. Tl..,chcrnical beh.a"iourof1hela1cn1 image 1uppon11his••icw. Forexample,trcatmcntofanexf><*<lcmul•i<>11 with rcascn" capable ofoxidisillll and thu• dcstroyin.g meu.llic si h·rr (forexamp!c,halogcn.,acidpennanganuc,acidd1chromatc, etc.) will dcotroy a latent image. T realmcnt of an .emulsio.n with certain reducing.age'."• (forcxamplc,.sodiu~arscmtc) which arc capablcofreducmgsilverhalidetornctallic11lvcrc.an,undcr"°"­trollcd conditior11, give a dcvelopablc ]~ten~ image. M~rc con­finna1ory cvidcncccould be ci tcd, all pointmg rn thc samc d1,....,ti<>n, oothatit ;, ooweo111idcrcdwcllatal>lilhcd that the latent image coru.ilto of metallic silver. 11te deeompooition of1i lver halide by light is i.:rmed ~sis, and 1he sih•er IO fOrmcd ~7/i& Ii/ct>'.

Oe .. lopmentCutrH lfthcd......,lopmcnt ofanemulsionisol.crvcdu11dcrahlghpo...-cr microocopc it will be Ken th.at while u.nexf'O'Cd grainJ 1how no change, the ~lopment of e>Cf'O'Cd grams btginJ at one or moo-c poinuonthe•urfaceolthegrain,and1pttadlthercfrom1hrough thegrain(•·lg.I). bpoouretolightth..,.tt:<:mstoha•'CIOn':""1ttr•ain~

-.uon the grain at which de-.•clopmcn1;, in11i.a1cd. Since u.pooure

Fi8.l.Elr<t'°"rnit""'*ph ( ><2}.000) -m&<k•-....._..,.,..~....,.o.dtb< <1rhin:s""'"'"''""b<ornid<'°P">'i<k'hemo1<rioolioo"tb<-.,lop<d'"""'·

,..,._,11.11.n;.,, "'-<1J.Moo--.., ~·-.i.L>J.

10 ligh< also forms me1allicsil,-.:r, ii isc,·ident 1ha11he photolytic silver is concemr.utd at certain tp«l<s on th<: grain ourface, and con61Truuionol"1hisclcductionisaffurtltdbyprolongingligln...:1ion un1il1hc 1i l\Uip<ebarcdctcc1ablcbythcclcctn>nmicrooop<:,or e.·t:nthcoptia tmiercocope.

This immcdia1cly .. u us a poocr. Light is doub1l ... aboo<bcd 1hroughou11hcwholeoftht:grain,,.·hythcnisthepho1olytie1ih·c• notformc<l;ua uniformcloudthro11ghoutthegrain~Hownndwhy do1heatomsofpho1oly1ic sil,·ertravelthroughthcgraintomcc1a1 cenainrendetvowontliegra_in1urface?Perhapsthefiritquesl":nlo beans"-.:rulis- rcthescpomuonthcgrain1urfaa:al""hieh1>lvc< collccu merely random plact:1 on a uniiOrm surface ,..hen: oiker •tOn>S, mu1uallyam"aettdbyoomemcchanism,happ<:ncdU)meet, orarethcyinoomcwaydiffercntfrorn1ht:rcmainderofthccry11al priortolight cxpoourc?Th.atisto tay,doccrtainst1Uiliuirys~kl upon which<hepho1oly1iclil•·crioenCQuragetltocollectcxi110n1hcgrain• ofnormalemul1ion, priortoexpoourc?

THE f'llOTQ<;tUPlllC t,ITEXT I.II.JG~·

Sheppard'1Cl•HlcalWork

Toob1aincvidcncconthilpoint,le1usrcvicwbricfiythcisolationby Sheppa.nl in l92$ol"tbe .. nsitUingcomtiluenUofgelatin. It i..d bcc" k"°"·n, oincetheintroductionofgelatinin tocmuloionmaking by 0.. Maddo.i<, in 1871, !hat cmuloiom made in tbismc<lium i..d enhanced scnoiiivity (pp. 92,99). SIM:ppanl was able lOlhow by mootlaboriouiandpainstaking""Mklhatthc..,nsi1isingprop<:nia w~redueto/4bi/1 (tha1 i.,u1mable,rear.1i,·c)1ulphu<compouudoin concentrationnowkno,.·ntobcofthcortlernfonepa.nlabilcoulphuT p<:rmi lli onpa.rtsofdrygelatin. He •howedthatthetypcof 1ulpt11.r CQmpoundwhichhei10latc<lformtd 'additionproducts' .. ·ith •ilvcr bromide, which easily d<:<>ompoocd on ""'nning, to gi,.., oilvcr iUI• ~ide (Ag.SJ. ThcrcfOrchc pootulatcd 1ha1 thcunC>Cpoocdgrain CQntainooniuourfa.:.:Rnoitivity1prcbC<lnois1ingofoi l•-.:r oulphidc formed during emulsion manufacture by adsorp1ion and decompo-1ilion of 1hc !abilc tulphu< e<:>mpound of gcla1in. Suhscqucnt work inthiscountryha1givenquantitativc 1upport1o th ioquali1ativc pictu<c.Sht:ppanlwen1onto 1ugge1tthatduringC>Cpo1urcthe phntoly1iclilverCQnccntra1cdiuclfon1heoilversu!phidespeel<s,a poo1Ulatc ,.·hW:h has becomcknownasth<:<-ml""iff.s/>tt* hJJ­IAo.iis.Thiswasasfa<uaclM:mistwatablc tog<>.Tltcougga1ion ofa1uliablcmcchanilml"orthemoblli1yo(lilv<:rthroughthcgrain 10as1oconccn1rattonoil•'C1"1ulphidc1p<ebc:ompletclyhafficdhim.

Th1Strudur1ofthlo\tam

l'IM:11agcwa1tet forthccntryol"tht:theorct;.:..lphylicil1,butbcforc "-.:COlloiderhiscontribution,,.-.:mU11leamoomcthingabou1 his ""Orkontht:11ruc1urcofthca1om.lngcner;1lhchaddctcrmincdthat atomsCQnsistofaccntr.al, pooi1ivclychargtd nocleusroun.dwhich l'C\'ol\'eindiffercntorbitsanuml>erofclcctroru.,1hatis,par1icles ;~~~e:it negati'-e charge and with much smaller ma.. _than the

Sinceatomsarcelcc1riallyneutral, 1hepooitivccharg.:: of the noclcusequah1he101alchargconthedectroru.As'''"P"D'hrough the chemical elemenu ;,, ordcrofincn:asingatomic ..,..,;ght, IO !he maosandpooilivccharg<:oftlM:nocleuiincrccasc,wL1hO>tTaponding incrcasc inthenumbcrofeltc1romrcvolvingroundi1.Theyarc1hus limilar tosolar 1ysmruofincreuingeomplexi1y,bu11o•i1honc,..,ry oign ificant diffe<encc; whereas each orbit in the oolar 1y11em is occupied hy one plane<, the cle<:1ron orbi1 will accommodate up lo a

Tl/I; SCIJ;lfCE UI' P/IOTOCRAf'llT

prcscril>ed n•unho:rofelectronshcfbreit iscompletW,andancw ou<cr orbi tis formW.Thedectronsintheusuallyincomplc1couto;r ortn1arethoocwhicl1functionincho:mica.I reactions, byhorrowing clectronsfromoihcratomstocompletctheorbit,orpartingwiththe dtt:trnns of1he outer orbit, hcl>Oe lhe ou tcrd..::trono are ca.lied H1t-.y electrons (p. 124), Thooc elcmmu with a a>mplcu:d outer 0<bitncithe1 par1 with norborrowclec1rons,andtherd'on:undC'l'l!:o nochcmica.l1ta<'\Oo<u what"'"Cr-thcyarethtinengast:t.

Tht outo:rorbi1 of tho: brom;nc a1om will acwmmodalccight clcc­or<>ns, bu1con1alnso11lysc,·en,1'hto.ilvcraiomcolllain1onlyono: clectroniniuinco1nplctcoutcrorbit. The total positive charge on the nucleus plu11hc inm:r cleclr<>n orbiu oftM bromine atom is 1hcreforo:..,,.·en, a nd1h.o.tofthc sil'"Cr a tomonc.Whenbromino:and ,;1,..,,.atomscomcintoa>niact,1hebn:>mine atombo>l'TOWSd1<spare clectronfromthco.il'-cratomandthusincomplt1ionofthtoutcr orbiu of both atoms i1 avoided . The .... ulting modified atoms ar<: tailed ioru, and ha\"C vcrydiffo:rcnt properties from the original atonu./\ lorcovcrthcions are nolongcro:lectricallyneutral;that from bromine has an utra electron, and therefore has a unit ncga· tivechargc,thatfrom sil\-crha.slootancltttronandti.-fo...,hasa unitpositivecharge.Thetwoionsthcreforcattracloneanothcrand tog<:thcrfonn1hencu1ralcompoun.dsih·tTbromide (Fig.2).

~® ~i.~@ < .... l ... ) , ........ )

Tl/A' PllQTQC/tAPll/C LAT/i;.NT t.'IACf;

\\'hcnoaltsouchu1ilvcrbro.mide,oodiumchloride(comrnon1a!t) llnd sodium carbonate (w;uhong ood a) MCJ>Toduced in lhcsolid .ia tc, the ions which compose 1he sal t arrange thcmsckco into a gromctricpa.ttcmwhichmo.nifatsiloelfinthccr, .. tallineformof ihcoal<- ln oolutionin"'atcr,i.o...·.,.,..,,., ,he iontA<s«i.tuoothatmoot, ifnotall,oC themarep<aCfltasindependtn1ch.argcdpa.rticlco.

1l>e crys1al struclurt ofa ma1erial is termed the rry.i.U /Mli«. 1l>e cryttalla<ticcofo.il•·crchloride ando.ilverbn:>mido:ilcubteinform, eao:hbromidcandoil>'cr;onbcingarransedahcmatclyand0«upy­ing the comer of a 1iny cubc-<>r more ao:cura1cly doc common oomcrofo:ighttinycubco.ThU1"'"Cry1iJ,,erionio surroundcdby ,;,. bromideions,andoimilarlycvcrybromidcionby•;,.,;1verions ffig. 3).

] liiso.impkaccountofatoman.dcrys1al"ruc\ure"""nca:oaarybe­forc,."CcouldOOR$iderfurthcrworkon1helatcntimat1;<.Thcreioonc

:~:n ";:.~e;y;:ic~ t:~:i:~l :~~fiu":i~~~-:;~::: adn~ti;!";'.'. inwhich,asthcirnamaimply, the current i•carricd by a flow of cleclronsand ofio111...,pttti,·cly. They are easily di.,inguishcd in <hat the former is virtu.ally independent of temperature, whereas ~icconductivityotaSC1a1 tcmperatu«:1app<0achingthatoC liquid •ir.Thisio not ourprislng-one would U.pttl 1ht ions, which are very large compared with cle<:tt0n$, 10 become immobile at such Low temperature.

Sil"erhromidc in 1he dark ha> only a very lowconduc1ivityai uormal tcmpcrnturco and none"' liquid air 1cmperaturei, The

TllE; SCU.'NCE; QF f'HQTQCllM'/11'

c11rrcntisthcrcforccuricd1hro11ghthccrys1albyiotu,andsincc 1hcrearecmlytwoiotuprcscn1,itmustbe<:3rriedbysilvcriotuor bromide ions or hoth. By an ingenious CJ1perimcnt proof was ob1ai ned1ha11hewholcof1hecurTcntwao carricdby i.i lverion1, and moreover tha1 they needed no energ)' to drag them out of their pooitionsinthe1il...,rlauicc. The currcnt-carrying1ilvcrions"·e..., 1hercforcno1 partofthei.ilvcrbromidecrystal lattice. ltwucon­eludcd that at norrruoJ tempe.,.tures a small equilibrium proportion of the sih'Cr iotu is OUl oC pocition a nd wandering 1hrough lhc crysu.1-thcir rclati•..,ly 1mall size ,,.....,]d allow them to do this. Somewoulddropbackin1opooili.on>•"M:attdby0thcrsilv<Tions, whilcfrcshsilvtTiotuwouldbejcrkedoutofpocitionbylhcthcrmal energy of the crys1.>.l, Always there ,...,.,.ld be a 1mall proponion of silver ions wandering in interttitial pooitions--the i•1ns1iria/ 1ih·er ions. Thus it has been concluded that the conductivity of sih·er bromide in the dark i•ionic, thecurrcn1 l><ing earritd hy inter­slitiolsil,•erions( f ig.4).

:; ... ~.~::'7",;,;·~ d•tkat (o)..,, n,.ll<m· p<ralur<, (•) i.,.. 1<m·

P'"'""·-~,;,;~

Asooonasthecrystalofsilverbromideisillumin.ated,howevcr,therc isanimmtdiateinercaseinconductivity,whichpcnil11prac1ically unahercd atliquidairtempemturc. Thisconduciivity induced by lislu, tcrmedplrt!W<1'"du<ra"''• i1 1hcn:fon:elutronic. (Fis. ~). What it the oourocof thccltttronsrcleased bylisht acuon? h must I>< eitherthesilvcrionor the bromide ion. Thcttrong probability, amounting almc.t to ccrt.:Unty when one oonsidcn t~e energies in­vol\'Cd, is that tbe dtttroou come from the nc:gau""ly charg.,.t bromide iotu {which thus revert to elemental bromine) n.1her than

01'ii1 01·~,'

± fO,.l. 10.«Mld ... ,;,.;,, ol>ilv<•btoMi<l<;,,<h< 1•~· .. (•) ....,...J ..,_ '"•

~:~~: ~= ~ ~\ . h<rki!J

(O > l•l

fromthcinncrorbiuofthc1ilV1:rion1olca•'l: •lilw:rreliducwilha •W.J>Olitivcchargc.

When the photoo:>nduc1a~ df«.t was compo.rcd with the photo­graphic dfttt, ii .. .,.. found that the ultimat~ M:nsi1ivity of lilnr halidetolightwasofabou11hep,....vaJucfor1hetwocffccts;morc­°'.,,.,.• the•pcc""lscnlitivity (ll1a•il,oensitivi1ytodifrertntcoloun) was tound to be idm1icaJ in the two cases. It therefor<: oeemcd to bc mc.tprob;ihlethat thctwocfrttts"'Cn:rclatcd, • nd1hatphotocon· ductan.cc mayplay 1<>mcpartlntheformationoflhcpho1og"'phic laten t om.age.

Tll1 Bum11·MottTlllOl'J

In 1938, Pn:ilesoor N. F. Mou and Dr. R. W. Gurney of Bristol Uni•'enitycxamincdthc,..,.idcncc10farrevicwcd,andpropoundcd & theory wlii.ch hash.ad• ITIOll prolOund effect upon work and

~~~~1~~1l~if~~~1~]~~ ""lremerapidity,beingdcllcctcdatthcsurfaccbyboundarycfrttu, •nd thcy1>a,.,,1hrttpoaiblefatet:

·r111; $Clt.,•'<..t. 01' l'llQTOGRAl'/11'

( I ) Theyeancon1biriewi1hlil•·cr ion1undcreonditions"·l..,tt thcfonnat;o.1of11orn1ofmctallicJih•crispooaiblc-atintcriorde­fectointhccrystal,IOoenrnplc.

(2) They ean ttrotnbinc wilh bromine aionu fonncd by the rjec1ionofclect<0n1frornbromidcion1.

(3) They may be: 1rapped in a pon;o.•ofthe('r}·stalof'°"-cr n>erg)' lcvcld>anlhcratoftbccrystaJ

Thcopcck<ofJil•-cr•ulphidclOrrnMonthegnr.ino"'faccduring emulsion-makins may be expected to OD1111i1u tc cffici<:n1 dtttron 1rapo,and1hcrnobilityofanclectl'Ollisoohi1htha1manyof1hoM: n:kascd by l'l;ht '""OUld irnmediatcly come intn contac1 with •he Jih"ttsulphidcop:eks.Such1pcck<,ha•·ingac<juircdnncnrmon: elcctrons, ""OUld be nqi:ativcly cha ... cd, and we know from the e:icpcrimcnts ptC\·iouoly deo<:ribcd !hat undcl the iuflucna: of the rauhingclec1ricfidd,clec1rolysisof 1heJil>-crhalidcwilltakcplacc, thccul'TCntbeingcarriedbyintcntilialJilvcrioru.ThcJ>Ol.i1ively cha'll«l intcntitial Ji[,..,,. ioru an: thuo attnctcd by the trapped cleetl'ON and n>O\·c 10 lhc1ilvcr 1u!phi<k spccko, when: they an: neutralised tn sivc metallic lil•-.:r. thUI forrning bigger and bcucr 1rapol0r moreclec1rons(Fig.6).

The Gurney.~lou 1hcory thus g;, ... us a logical mcoohanism, each

l'i6.6.Th< '°""'•'"'"°''"''"' ;m•g<, a<«><~ inr kl lh< Gun><y· Mo«•h«>ry. i.s;1,..ror<nnkk 1,.;,, •ho~·inr ><mi1M1,- op«k o(sil•<r >ulphkl..,.,,u.f.a:,ondin<"";,;,1 ,;1'"''"''"{r<ptct<n«<lby<i«leo). 1. l)chottl<•,..rl<ctront(n'J><<>­"""t<d by mi""' Mo!no) fmm bro< mkle ;.,,.., Tho,.,.,;,;,-;'¥ opr<k «•p0dr<trooo,h<comeo_.,ti..,ly

::;;.~7liE~'~'.:?'2 '""';"""'"""'' .. '"*"'~~n<;l•U•ho

~':"~;.;:::;i,,;:: .:~·= inooinomo;1ialpo;ioit ..... 1<1nWn...;o

:::=.;~=::::~ ..... 11;., .;1 ........... ..,;.;..;,, >p«k.~hit:kJ.,..-br<......oulli·

.;,,...1, ... ~,':."::.:··t1evooio,..

Tl/£ l'/IOTOGRAl'lllC l.~TEJ.'T IMAGE

"cpofwhichean be indcpc,,dcntlyoupporlcd, c~plaining theco':'­ccnu·aiion ofpho1ol)·liclil>-.:r ai 1hcM:ruitivity 1pcck<on thc gn.m •urfacc. ~·romothcrevidcnccitiskno1o·nthatthcphotol)·ticbrominc can i.o'.l«lwandcr1hroughthcgn.in by1hc1ransfttofancltttron frorn an adjaccnl brom;dc;o., toa bromine atom {oo that their

~::7:~=·:"/~":::Cli;~::::~= "~~"';i.'.~.·~:;:; imagelOrmationisthU1po11ulaitt!tooocurintwo stars-:-•primary rapidclcctl'Oll;.,Stage,folLo>."CdbyaJeCOndaryolol<-croon•cJtage which may continue afitt illumina1ion has ccaM:d. This theory iiouppo<•ttl by iu reuonablce>1planation of<>1httcharactcristics ofpholOgraphicmatcrialo,Juch~<e<:iprocityfailure1o·hichwillbc

conoidcredbttt. 11>cpubliauionof'lhcGurney-~lntttheoory irnmtt!iatclyltt!toa

numbttofcritical pho1ographice>1pcrimcnu by othcTworkcrsin which l>quid air tcmpcralUreo 1o·cn: ...W 10 diffcn:ntiatc bctwttn elcctronicandionic1taget,and 1hctW<Mtage (elec1ronicandionic) nature of latent imageforrnal;o.l hasl>eenJOfullyconfirmcd that certain ••pccU of the Ourney-~1<>11 theory a~ not completely ptisfactoryandmuehfurthcrworkandncwthcor•cs ha•·cl>eenpl'O' pounded. They are not 1o·ithout ~ic<;tiono and ha•·• no1 been universally accepted.

Latentlmag1 Dl1trlbution

Ae«>rdingto 1hcG11rncy-1'.lo1t1hcorya11elcc1ro11relc~by light actionmovcsthroughoutthccry11alwithex1remerap1d1 ty. [1can­notnormallymcweoutsidcthccrystalboundary,b111bounccsabou1 insidclloccrystaluntilitistrappcdinarcgionoflowcrcncrgy lC\·cl than the rat, During ill tra•-.:b, tloeeleclron doubtl.,.. makes !he acqua intanccofnume<OUS inlerslitial 1itvcrion1, bute<>mbmauon eannott~keplacetoforn:othclugcr ii lvcratnmowingtothclim!ttt! confin .. oftl..,lil•·~rhahdelatlioc. ·n,.,electron trapo-thcrcg•ons

:::-.:.~:~;:~~~;;;.~=:~.~~.~~~'7i:'e~:~~~ crystal.Such 1rapsrnayoocuron1hemrfaccorinsidctl>ecrystal,and whcnanclec1ronistrappcdtl>crcin,thctcilU1uaUyoulflcient sp;occ forittocombincwithaJ>Ol.i•iveJilvcrion attracttt!totheclccu·nn.

~~J&~~;~~§~~:~i~

TllE SCIAWCE OF PllQT0(;"Arlfr

ad'°""'lopnu:nt centre in the nonnal way, bu< ifthelaient image •puk is in the intcrioroltt..:Cl) ....... L, it will not be rt.ached bya nonnal~lopcr.andcannotinitiatedC"..,lopn>wt.

Thitstoryofo1ernalandin1ernalla1cntimageioonewhich1hc reader iocntided torttei,·cwithoccpticism. Hcitenti!led to ask, '!low do you i.wall this? bit mcrcgucn-work,or have you any cvidem:c abou11hcpo1itionofinconceivably•rnall 1pecbofsilveron cry11al•whicharcthcrruch·colcosthana1housand1hofamillimc1re in diameter?' The answer is that we have oo mud> evidence thal it would l>edifficu ltto contrivc anyahcrna tivccxplana1iontofi tthc factl.

ln1hcfirstplacc,1hco'·e,.,,.,helmingcvidencc1hatthelatemimagc «H"i•t1ofsilvermetalhaobcenvicwedpreviowly.The,,...i1iMofthc la1e111irnag<::ingrain<hasbecncs1ablithedbyoperimcnt1,ofwhich

•h~~l!:~::C ~:\:~ a full exposure, •uc:h u would gi•·e a high dcnl.i1yonde-...,lopment.Ifinsteadofde-...,loping 1hcgrains,theyare 1tta1ed with an oi<idising agent (for example, acid dich.romaic which willdcsu-oymeu.llicsil,.,,,.) andthen1'-oughlywu hed, 1hclatcnt imagc it dcstrnyed,andthegrainsarcasundc-...,lopablc u thcy"""' priortocxpo1urc.Onthcothc<hand,ane~uWonooueueddiffcn from an unexpcwd emulsion in lts bchavlQUr to a ~C"..,lopcr con­taining a propor1ion ofhypoor othc< J1rong 1ilvcr hahde ool,..,nt. An uncx1N»Cd emulsion mettlydissolvcogradually,whilc thccxpooed and treated emul•ion may aficr a while produce a considerable den1i1y. ·ll1isi•takenascvidenccof1heexi1tenceoflatentimage

:1:!: ~~ ~~~:~::;:(vT:t:.1:~1~~:.!:t~.~~:;~r,.':~~~ ~~~ ~~

comeunprotcctedwhentheoutcrlayenohilvcrhalidearedisoolved in a ool,·ent developer, and then they initia1e development of 11>c remaimkr of ihc silver halide grain.

~lorcrdinedcxpcrimcnucanindicatell>cproportionsofgrainsin anemulsion"'·hichformintcrnalonly,cx1emalonly,andbothin· 1crnal and utemal latent image. Moreover, methodt of manufac· turcha,..,bccndn;.edtoproduceemulsionswhichundcrnorm:al opo1urcc:onditionsgi,.., ·an)·thingfrom1ntalutemalto10talin­temal laient image.

~·rom the point of view ofnonnal photographic procedure, _inter• nal la lent image is a wa&\e of effort and any mean< w~.ich will en·

~=e ~~tr:":ff::~. ';t f~r~::~o~':~: ::.c: t~~: ::::k=n~~~ ri per1ing process (m p. l42)i1ooelfcctivein incrt:uingemulllon •1>eed.

TH E PllOTOGRAPlllC LATEXT /MA C E

The distribution ol 1he la1cnt image-1ha1 io, the rcla1iv" amounu &;.rmcdonthe •urfaceandin1hein1eriorofthe1rain- willdcpcnd not o~lyon 1hepropcnicsof<heemulsion bu< allO on thcinteru.ity and ume ofopooutt. Undcronlinary upo1utt condi1ions, the electron< arc rclcasM_ at a moderate ralc •uch that they will tend to betrappedprefettnt1a llyat1bemo<ceflicientsi1eo,on1hc •urfacc. ,.·hcre1heywillberapidlyneutralis<:dto form moreefficient1raps, The ge_n...,.l 1endency will lh~refonbe towards •urface la lent image fonna11011. Vc~highinte,,.,uaacungfor shortpcriodswill,onthc otherh•11d,g1vemetothe suddenformationofadenoccloudof clcctrons in1hecryital.All1heavailable 1rappingti1e1,00thon the •urface and in the interior_, will immedialely be "'.'turaied. High inteiu i1ics thereforeg1ver1SC Ina higher proportoonofintcmal image.

LatutSult· lm131

8ygi,i ng a oeriel ofincrealingcocpo1u,..,.toportio"'ofa1ensiti\"C material, one ~n dc1crmine the oposurc which prodl.1C1'1 a just pcrccptibledcrattyabovelOgonnonnaldC"·elopment.hmigh11hen be deduccd1ha1cxpo1urcolcosthanthisvaluchavefailedt<>c•usc any change in 1he oilvcr halide grain. Thi> deduc1ion would be wrong. Con•iderable prolongalion of dcvelopme111 (for ex.omple, dtodoprr1t•l t<>fin•li!J) or the use of more ac1ive developing agents would ll1ow a J>erceptible density difference at a [o,.·er exposure \•alw;. There are more refined methods which ohow that expoourcs unde1ccta bleC""enbyavigorousdircctdevelopmenthaveneverthe­lC11 ta...ed a changeinthegrain.lt;,on 1uchcvidcncc u thi1 that the poo!ulatehasbttnmadcthatthelatentimage•peckmustrcach • ccrta1n sizcbeforcitfunctionsuadevelopmen1ccntrc,butit;, elcar1hatthelimitingW:e (that;,,[jmili"fnumberofoih•cra1omo) ""ill depend upon the dcgrtt and vil!"ur of dcvdopmcm. A latent i"""e •peck which ;, I<><> small 10 act u a <1n..iopmcn< "Ct\ltt ;, called a lW-/..u.iimagc,anda >ub-imagcforoncconditionof ~tmaybcaftt!llatentirnagcfor""'"'vigorou1dn..,1op-

Experimenuon1ub-La1ent image formation have1hown 1ha1 in its ,·ery carly11agco-probablywhenitcxist1a1a1ing!eatornofsilver - ititunotable.ltisliablctoemitanelcctronandrcvcrt10uilvcrion. O nllocotherhandi1;,ancflicicntelc.:trontrap,andthettfottalter· na1ively may grow rapidlyduringexposurctoforma0table1ub-irnagt

THE SCIEXCE OF Pl/OTOGltAPllY

of perhapo three or f"our lilvtt atom., which on continuo:d <:><p<lllur<: ~Q•ufficimtlylargetobcafuU latent imag<'. The proportion of •ub-la1cntimagcfonned,li.kc1hat ofintcmalirnaga,willdcpc:ndn<>1 onlyonthcpropc:rti .. ofthccmullion,butaboon<:><p<111ur<:condi­tions. The >amc arguments can be advanced l'or both. Under <::<J>O­Jurcoonditi.onf which occur in p,.,.crice, thecl«tro<1$ar<:libcr~ted at ~modc,.,. lc,.,.1cand1hc>po:alatwhich oilvcr ion1canln0''Cthrough thccry1talittuch that 1hcycan,.,.pidlyncu1rali"° ' "'Ppedcl«trons 10 gi~c a rnorc efficient trap. Once latent image formation star ts, thcr<:foTO,itwillpr0«<dra pidlytogivcfulllatc11timugc1pcclu,and comparativclyfew oub-latentimagc opcclu.llighintensit)'<:l< p<lllUre willgivcahighe<mcent...,tionnfel«trono,"'hichsaturatealltrap­ping titct, and bcfntt: the si!'·cr ion1 can catc the oongQlion by ncutralioing1hc1rappcdd""'tro1U, thc\Jrief<:><p<111m-ci10\'tt.Many cLcctroraar<:1o·a>ted10faruphotographiccfl"ic~isconctmedby i)(ing repelled from the >atur.ued oitct and ttlCOmbining with h«>­minc atonu to rc-l'orm bromidc ions, 1hus undoing thc ".,,.-k of cxpoour<:.Sintttheapoi1un:timcat•..,ryhighintcnoitiesisusually 1oolnief<0allowoevcralcyclcooftrappingandncutralisationatonc oi tc,highintcnsitywillgi....,ahighproportionofoub-irnagetocattc.....i throughout the grain .

Ti...econsl<krationool 1ub-La<en1 lmagc and latent image dis­tribution may be applied tocxplain a numbcr ofotherwisein<:><· plicablcphcnomcnawhichwc ohall.,udyinChapter 15.

Ckop111 9

DEVELOPERS

!;:::d rr.:;.~;i:ti:!'; ~.:c:~q:':~h:~c:~~~ti=:.~::; ::~~~:~S~fi:~~~~:oL~!:.7iEi~:;:~~n~::~?. taming oa rbon a rc known to date-far more than the 1ocal •um of all

;,~;;~%:;:~!~~2:::r~~~!:~~£~:··:~~·:~~:~~ pou11d1 inallanimala11dvcgc1ablclifc .

... ~: .~~~"'..:,~~:~ (~~~;;i;i:1"!,::r:~ .. ;;::~ 1«malncled....,.pcctfultocxpon.enuof1hi1bn.110ho(..:icnce,but, t1tleoapa n , 1hed..Uficationi>a•'tt)'1Crvie1'ableo11c.

¥S~E~1~~~~ff.£~~J~~"~ ~;\;~~~~:~::1~.'::1';;1~~~:;n.~ ~\::~~d=~ c: carbonatomt inring (orheugon ) fonnatinn,withahydrogcniuom • Uaohcd 1o caohcarbon,and it1fonnulai1!hcreforeC,H •.

'"

va1111c1 It wasrecogni..:d thal differentelemenll had differem combining powers, long bef0<e the dtctron theory afforded a 111tisfactory uplanation. Thiscombiningpower wq .. Jled o./t119.To1.t.ke1ypi""l uamplcs, it - found tha1 !he •-akney Qt hydrogen (H) is one; oxygen (0 ), 1wo; .. rbon (C), l"our;•nd sulphur (S),iil<; though under different circumstanee> certain demenu may uhibit different •·alencia. Structural formulae were cvolvul which illustrated the mannerinwhichthe atomsofa molecu!eare linked1ogether. The actualpo$itiontof 1hedcments ina11ructuralformulahavc no1ig­nif>can"", provided thu thc linkaga:, or 0<11tn<yho1Ulsconnect the elemcnu colT'CCtly; th"' fe>nnalin, CH,0 may be rep~nted:

H"' c=o H '

H-C-H II 0

, H H-C~O

Allarecorrcct ;ine;u;h,hydrogenhuonevalencybond,ox)·gen two, and carbon four.

T ypical.,ructural formulacare:

H- -0-or H - H 0 - 0 H - 0-H H)·drogen 0 - ;\lolecule of ~lolecule e>f Wuer

Atom O~;:::• H ydrogen ga1 Oxyge n gais

H

H_J__H

h

H

o, s4o

/o/ ~o

H

Methane Sulphuric Sodium Acid Thiooulphatc

(H ypo)

H H I I

H-C-C--0-H I I H H

Thcvalencyb<>ndm;tyberq.ruented byadot,ormcrelyby the juuaposition of 1yml>ols, thm cthyl alcol>o! (abo>'<=) may a!IO be rcpttM:nted a1 CH,.CH,.OH or C , H ,OH.

(<) ,., "' (~

Phenol l·I H

H ' N

,H H 'c/ 'N/ ' H

I I u- c ,,.c"t-H :-.'H, H-C,-C~C-H N l l.Cll 1

II I 0 II I 0 H-C C-H H-C C-H ' c'~· 'c&

h ,',

"' U) ~) ,., ~lethylaniline

Anuml,.,.of<lni<'•lirrsofben~enecanhcohtainedl.>yreplaein!flhe h)·drogen•l.>ygruupsofelemenue>f-..;l;ichthe~JdYO.<Jlgroup (-0 H,

"' O l l) and1he4111iH group ( :-./ iJ or-Nl'I Jarc1ypical. Tims,

by ...,placingoncolthchydrogent (anyono-<heyareallequiv;alcnt ) br• h)"dr<>Iylrroup,..-.:obtaintMcompoundl'/tmoi,orcarbolic ..:id,whosc structuralformulais(&) ,but,.·hichis diagramma1ie&lly

~:;.:-:( ~,!~:;.:.;:i:;~~~~·;i~~t~~u:: :a~enri:;"~";;;~(;f, (/),or C , H , NH ,.

ll~ 1"HE $CH'NCE OF f'llQTOGRAf'/lr

lt isaltopootiblctosubltituteoneorbotholthehydrogcna~of thc aminogroup{-NH J 1>yagroupof•tonu,togivcan16.mt.1tJ ,,,,.;,,,, ,,..,,. Thus ,. . .,_ ma~ .•cpl.ace a hydrogen ~ a methyl group (-Cll,), to gi•·~ mcthylamhnc (z), (A), or c.11 ,.NH.Cl-lr

0 OOH 00" o" 0 lf.plr~··- /'yr«~Qftthi~

,,.. l's••·•"'i•opltt...H

(Quinol) (Cate<:hol) (1-'yroga\lol)

NH. CH, NH. CH,. 00011 Nil, N(C,1-I J 0

0 0 0 0 Nl'I, NH,

J~i~~::~J~~~;t~.::::::~:::·?~~ ..

l\'ccdlcoo to uy, many a ucmp11 h:a'" been made to dt:tcrminc the coocnt;..I ch:arac1critticdistinguishing a sucasful dc>·cloping agent fromothcrttducingagcnu..Som.,ttducingagciu1arcmorcac1ivc in'°"'ingcle<:trontonto oih1'riontth:anothen, aproperty,.·hich canbcaccura1elymeasuttdandiskno>.-n aJrdwlifff'Olt'llWor r""1rf'Olt'llW.Whcnthcttdoxpmcmialistoonq;a1i,•e,allt.il•·cr gniin1arcttduccd,CJ<~andunapotcdalikc;"'·hcnitistoo po1iti"c, the<c io diff>euhy in reducing cv~n the el!potcd grairu. It w:lllhousht1ha1a rcducingagcnt,...,uldl>eaplt0togr.>[>hicdcvel­opcr ifi11rcdOl! poten tial fcllwidtin certain limi11, hut thi• hypo­thcaishadtobcabandoncda,thorcw .. norigidcorrcla1ioubct"""'n dcvclopingactionandttdoxpotential. l'robahlyd<ebotda•ifica­ti<:lnofa devclopcristhegcncralisation thatmootdC\lclopcrsha"c the gmcnil formula X-{f~?J:-X, whel1' Xis either hydroxyl,

amino,or111bl1itutcdamino,and•itzcroorawholenumbtr. When • it :u:ro, the compounds arc HO.OH (h)'drot1cn pcroKidc), HO. 1'1-1 1 (hydroxylaminc), or ~ll,.NH 1 (hydr:a:rirw:) , all or .. ·hich may actaJdci.·clopcn, •ndall thcfonnulacofdC\1'lopcrtlistt:dabovc will be found to fi t in with the g<:ncr.>l IOnnula, the - {C- q-

bting part of d>C btnzcnc ring. Thcrc a"', ho"'"'"'· ccrt•in co.;,. pou1><llwhichac1udC\·clopenbutdonotconformtothi1gcncrali­.. tiou,10th.oot""cannotyctprcdict"'i11tahoolUlcccrtainty"'hethcr aparticularorganicrt:ducingagcntwill ac1 ua dcvclopcr.

OthlrCan1tltu1nllof10n1laplngSolutlon A•,.•tlt.-atcr.-Whenthchydroxylgroupi•attachcdtoabtnxc11cring, aJin manydcvclopingag<:n11,itimpar11amildlyacidicch.oorac1c1 to the compound,"' th.oot it f.....,,, sal11 with alka lis. In the presence ofalblia•uchucauolic.,,..;la.JOdiumc;ubon.otc (pu11'w&Jhinr .OOa),borax,c1c.,oorncof1hedC\'clopingagcn1iscon'"'tt:din10i11 .OOiumoah,inwhich thehydrogcn atomoftheh)-dro:tylgroupis rcplaccdbytodium.'ll>CSealkalisal11ofthedcvclopin1agmtarc much more active than the origin.ol ag<:n11; thw "·hilc a plain aqw:ouooolutionofadci.-.:LopingagcnlmayparliallydC\..,lopa normallyupoeedmatcrialinOC\lc-ralhouri,1heprcacnccofa nalllli wiU ttducc dC\'~lopmcnt time 10 a few minu!CI. £.·en dC\..,lopcn wltich donotCOntainahydroxylgroupdcvcloprnorcr:apidlyin alka!incoolution.Anallt.alisuchnc;u101ictoda,todiumcarbonatc, bor:u,ammonia,ctc.,isthc,..,forcancsscntialcorutitucntofncarly aUdcvclopcro,andisknownuthca.:uk<at.,;andonthenaturcand

TH€ SC/ENCK 01' PllOTOGltAl'llY

conttntralion o( 1hc acttlcratot, among oihcr thi~, dtp<:nds 1tM: onorgyofaOO"elopcr.

Rtsu..U....-Whtn 1he sodium salt of a dev.:loping agwt reduces sil\<r:r bromide, the reaction may hc n.ther cruddy tt:pracntcd:

Gc11crally,chemicalttactiomatt slo""·eddownbytheprescnceof thcendproducuofattactioninaccordanccwithwhati1knownao thcLaw •/ M4U A<l;.,,.,andiftheconccntn.tionofendproducuis high.1l>ettac1ionrnayhcstoppcdor'!'..,nrev<:rted.lnphotogn.phic dc-.·clopmcnt, the µroence ofono: olthc cnd producu, alb.Ii bro­midc,thusreurds1hcdev.:lopingaction. lndecd,whcnadev.:lopcr iluhaustcd,itisnotoomuchth.o.tthedC'..,lopingagcnuhavcbecn used up~ that thc.o:lulion hasttascd to function owing to 1hc n:urding effect cl'1hc accumulated alb.Ii bromide. In the complete ablcnccofoolublcbro<nide,...,,.,..,..,.,.,manyd'!'·clopingoolutionsarc tooac•i•"etohccontrollablcandunuposcdl"'i111an::likely1ohc reduced, with CO""'<!Ucnt chcmic;ol fog. This CXC<Sli\"e activity is climinaled by the addition ofa small concentration of potassium i;;;;;;~,:1,~hichisequivalcntt<>oodiumbromidc)whichfunctionsas

Ofrcccntyea'1othcrrestraincnha\'ccomcintowc,andattsold as dt<~l•I'<' i~sortmlifouanu. In general they attorganic com· poundsofaringllructurc,containingnitrogcn,andthcyreactwi1h 1il•·cr oalll ti> Wrm a very iruolublc compltx.

/'1rJcW/iD<.-Analk.o.lineooluticmofadc:vcloping"l"nlrapidly a~lltMygcnfromthcair, andthis,inturn,Midisesthedev.:lop­ingagcmwiththcformationofiruolublcbmwnMid.ationproducts.. A"""'clopcrcon>istingonlyofdc\•ok>pingagetit•ndalkali,.""ithor withoutbromidc,wouldthushcunsatisfaclOt}'inusc,sinccn"""'ld bccomcwokanddiswlooredonapooutt in adishorunk. ltis, thcrcl0n::',commonprattia:1oindudcinadc-."elopcra!'tH<Uii«, whi.ch hindcrsn>tidation, and pl"C\"Cl>U the formation of coloured 0Kida1ion products. Sodium oulphi te functioou admin.bly in this capacity.

lti1frequentlyaplaincdinphotographicli1en.1utt1hatoodium 1ulphitchasagrcatcraffinityforatmofphcric0>tygenthanthedcv­cloping agent, and therefore rcacu pttfcrcmially with dissolved oxygcnintheoolution, lcavingthcdcvclopingngcntsuhstan tially

t~~~;~;:~;~~~~~~~t;~~~~~~~~! onoKid.ationadC'"elopingag<:ntproducco o~'·el)'""'"Cr.i l dilfcttm producu corresponding to prosrDli,·e oxidation ':'"'8"' hc!""ttn tl>eunchangcd d.,.,·clopingagcnta11d the brown l"CIJDOUS

final product. "l11ccarliero~ida1ion prod'"'"• on.a:1hey arc forin"'!,

:~1~:1~~:.1.y ~~l~~,~.:a11~~i~ .. ;,· ~~;~i-:'i::~ ~=·~~~i;:;:~~~~:~ ~~:;~~~ t::::,~el~!i,v:~!~n~l~~;;'~";:,~1:.~~;r::i:::: ~~:P~:~~~1; '"1;r'~~:::~,"~:~~~7t~i', i~~~ i~~::;~~~1 :ir:~~::I"'"· because d'!'·­

~"':r~.id;.::; ~"':,7: !::;:.i::,:. wuh~~=::~btin. '."· ooluLM. aud <k1.•elopmcnt ol an unharde11cd emulsion in a ~•'"l

~=~•~.:i::-;u1:!'n:;;~:gc::u=. :~~~~n':~~:;.::~~

~~~~~~;;;r-3~~~s~~2 s~~~ ~~:·~~~;E uuffl along ,..;,h the liker image. Such dy~ imagco arc ui:w in m~kin~ colour photograph> (p. 264-). lf sulplutc wctt prcocnt m <_he usualcu11ccn lration ,de,·clopcrox1dationproductsw<:iu ldttact"'"h

:'i:::c!";~~c;~::~~n~o :~~·~;~.,;·i~!~,-~:~,";u~\:~:c~~~'~; :"~~~:;; al><cmorinlowconccn1ration.

A normal dc.•cloper thuo coruists of an organic <kor/o,i~l "I."!'

=~:~~:~~~~~:;z;;sEt:::i~~F~·~:";~

'Chtmltal'111d' Ph11ital' Dev1 lopmt nt

:: ~~1::c:·~,lt,~;',,";;'~;~~~l~;::~~~~;~~'d:~~;~~~~·!:'.~l:,'~~:;.:~I~

'" ;;:~.~~~~~~~~l:::-~~~;;:~:~E3~ ~:f,!:~~~;;r~:~~;.~~~~aj~~;:c~::"a::':!'~~~ Jtw/61'"""'·

,h;;1~,~·ro~~~~~~::."':·1::~:t:1.:::~:t:t17.~ ~:;~~:;:·~l~ .• ~:i::: m1dc c11·11a\laui<:c,and1hclaucrthcrcductionofoilvcrio110 i;1..,h1-1 ion. l'""'l'hy•icald_c,·elr>pmcnttakcsplacewhcnau e:<IJO'Cdm~ lcrial

~«~~~i:··b:;;::~!~:~;~~·~~~~~~~~'.-~~;~"~~;;-~~~~~1~;~;'. ~:~,·;'~a~t~;1!,:~~~ •,~::,:~°'~~~ ~~;::.:,'!::hC::~~.1,:!::e~~l: ,..,,.,,., ;, 1hatoodium1u!phi•eisamildtolvcntfor oilvcrl>romidc, f0rmin11 a ocnnplc:< acid ion coritaini"ll oih·cr,"' th•l a focr the material I"" 1..,.,n immersed in d"'-.olopcr l'or a littlf. ,.·hilc, .,..ch

:;;:;: ,:~:~ i: ~~;~~~-:::.:~~:z7~;~·:~ =~:.::!::; ;::;::~ >oris (~8 +-) ca11not bei_ncrcaoed (o..-ing 1othclimi1 .. ,up by the sol u1.Jilu yof •ilvcrllrom1dc), thccomplcx 1il•'tt-COllla ining •ulphi1c 1on1 may thc.m .. h·cs_bercd<i<:cd,orthC}'llla)"aCt a1a onor<:rcady tourcc1ha11 .,h-crhRhdcforrcp\cn ishingthcsolu1ion'"id11ih·crions af1cr1 hcyha''Cl,.,.,n rcmovcdl>yrcduction

ro:;:·1~'.:;~,~;:1~~·~~o!';';a~io~~::::d~~~;~1:,~·~~~,',',~ :::(~~;,~~::•::~: probahl y ,-,,acha 1ufl i<:ien tlyhighconccmra tion r<)undc~chdevclop­' "ill'"' i" 1oact as a sih•crhalidc0<>h·cnt,0<> that it \\'ould bc di!lkuh

;h~::,=~~i~:::,.";~~~~g";!~. exclude ohc 1iwibility of 0<>mc

C1mp1ct 111• l ooH Gf1IH- Fil1rnenlll')' '"wth

~~~_::,1~'~!~:·r" :..::r;;~r:~e;::; 1!..7i'n d::C~ .~

~2~~~{~~~~~~~W::i~:j,~~§¥:

b<:am im tead ofa light beam, much finer resolution of detail io obtained and, ao a res ult, much greater U>Cful magnifica1ion. It was1henfound 1ha11hc •pongymau wa1 reallyinmanycases a tanglcoffilamc•ns( Fig.Z)

0.Hlaprn.ntHect•

r,.., ad Tt..,...a/1Ur.-Thc 1ima and tcmpcnuurcs of dei.·dop­mcnt l'e<:Qmm<:n<l"<l by manufacturcn for their products rcpresem a com pn>rni!.C. As development time io incn:a>Cd, more and more grainsarcdcvclopcd.Sp«dandcontrastincrcascunti l1hey roach JJlll.ximum'•alues,a f'tcrwhicharitc infogcauses\hcmtode<:n:a•c. Grainineosandfogir>e:~, thcla uer slowlyatfi .,. 1 ,la1cratan increuingra1c.Thcrccommcndc<l1im<: 1hercl0rcrcprcsc:ntsde-·d­opmcnt toUSJ ll1an the maximum•pttd and contrast obtainal>Jc, but wapointwhcrcfogandgra.ini,,..arcflillrcaoonablylow. furthcr devclopmcn1 ,.·ill CCT!ainly give more opttd (u>ually twi~, and no1 infrcqucntl~fourtimco,.. mucb), butat1hccxpcn..,ofhighcoruraot, fogandgram.U11dcrnonnalCJtpo$urecondiU<>nsthcreaimmendcd

,,..,.1.0..:1 .... mic­

'"'"" "'d<V<l<>pcd .. , •. ., , ,..;., .i.., .. ·101 rllam<O I UY form

(><i~.000).

de-.'Cl<>pn>cntshouldbcadhcrcd10,bu1ifundcr..exJIOSUreis1u>pec­tcd,abou1twicc 1hc1pccdcanb<: oh1•inedbyprolongillJld'"'-.:'°P'" m<:nt-;otechniquc"'hichhasrccmllybccntcrmcdJtrr/ot-wlkl fitudil.J.

The temperature oft he devdopcr a fr...:11 the rate of1hc d"mical rcactiomoccur.ingd1"ingdcvclopment, 10 tha to'·cralimitcdrangc differences in temperature can bl: compcnsatod b)' adju>1ing the dlO\·clopmcnt •im<:. fig. l shows a t)'pla.J Time-Temperature Chart.

U=flll ~·) f~ .... ·o: D£•1~0~UfN1'<•-) <(I

1V.S.TiM<-1<mp<"''""'cloan<Xdev<IQpn><nt 1r.on ..... tho i<><~ ..... line r ... cq~W.U.:n•

<1< ... lopm<ft•<ondi1;..n,).

BtomitU t"ffuls.-Onc of the produet1ofdcvdopmcn1 ioa lka\j bromidc,itoclfamtroinuofdevdopment (p. 126).Anareawhithhas ~iwd a heavy '""poourewillon dlO\•clopmcnt form a local high con­ccntr.o.tion ofalbli bromidc which will tmd 10 rcs<rict !a1cr 1tagcsof d~-.:lopmcntll0l only in1hcarca ofhig~dctui tybut,bydiffuoion,in ad,JaCCn t arcas. lf1hclllmis,"cttical(as1nd,..,-.:lopinrtankl).1hchirh

E:~~~;~~~t~~~~~s:r~:~:~;~:o:~; "lt,..,amc"'' or ' •op hau',andcanbernitigated or avoided during

1"111': SC IENC E Of l'llOTO GllAl' //I"

00"elopn><n1 by appropriate agi1a1ion. Uniform agitation, u f"o1 ex.ample 1he -if- rotation of film in a •piral holder, may cauoc bromide 11ttaks in 1hc wake ol high cknoitia. Some ouch 1tthnK}U< ua ,·igorou1 pcriodicshakcislOundtobemootdTttti•·c.

fA:; Eff«U.- While it i; not difficuh co elimillalc atcnb•·c bn:>­mi<le dTtttt, •uch as bromidc strcaks. i1 is morc dilficuh 10cJimina1c 1hcmO\'Ct•rnallatta•. llowc\"cr,ouchdTtttsarcoftcnbencficialand

arc .ornni....,.cncouragedrathotrthan1upprnsttl.for"""mplc,they "illsharpenthcboundarybctwttnthcarcu olhighaOOlowapo­•urc. Thctliffusionolbromidc from the high clcns.ity "·illcauoca dtttC.O.lle in the adjacent lower dcNity, ~nd diffusjon of f,.,.h de'o'clopcrfrom1hclow..-dcn>.ity"·illcau>eanincn:a>cinthcadja­Ccn t higher density (h g.4). The lines at 1hc bonier an: called Ma<kit linr,. Ae1 interesting >pccial case ofmicrobrornidc cfftt"

10 0 10-01iuNCl "'""' UNUl( O#'IM•.;c

~'I· ~. ll<p..ndon<e ol cl<n~ly on in"'J< Ii« (U><rhud df«:1). C-i.1_, ,...,, -' ma-.,,,_.,,,,,.;, Tlvory, 19411. Jo.lo 1111?"•-'S.... /o<.

<ICC"""·hcnsmaJJ,adjacentareasrcui•"e•"Crydilftorentul""u...S, uinn>(Jlaic rolour,,..,.,....,.orphoto-mechanicalwork. Ifthcareu .,.., •uffoc;..nlly small !he <:dgc cffttl may cxtcnd O'VC1 1hc ,.·hole am. Th..,. fi1. $•how. =ponding macro and micro dcno.itieo for thcoamc"'P<""""· Thisdcpcoclcnccofdcns.ityon thc s.iuof sn>allan:asisca!lcdthcE•ni-Jrff«I.

D1velop1rR1pleni1hm1nt

Thcc:om1><»itionof ade•·olopcrchangawi1huM:. l>loJt of thec:oe1-1t i1ucnuaregradually u>ed up and thcn:roredttrcaoc iu c:onccn tra• 1ion. The al kali is gradually changW to .odium bromide and the 1ulphi1c forrningoul phonates with theoKidioeddC\•cloJ"''• \ml doc c:onccmra1ionof•ulphitcandalbliarcoohigh tloatC\"Cn after prolongl!dU1C,thepcrccntagcch.aogcisomall. Dttrcaocinoulphi1c aOO albli (.od ium carbona1c, borax, etc.) is thcn:forc an almoot nq:li!Pl>lefactorin thccxh.aU>tion ola dC\-dopcr. A nl<)tt:p<>tcn1 factoris thccxh.austionolthcdC\·cJopingagcnuducto0>1idationand con•-cnion to•ulphon.atc,butinmootd"'"lopcrothc charac1cris1ia olcxhaus1ionarcduealmootcntirclytothcinctt;ucinconccntration of thcbromidcions insolulioo,..-hichocriowlyn:tardod"'"Clopm<:n1.

A"othcrfaclor is thcdttn:ase in ..t~-ord.,.clopcrduring.,.. o..-ing toiu rcmO\"alwhcndC\"Clopcdfilmis pa.ne<Lontothcfixing bath.Wccouldbringbackthcc:onccntralionofdC\'clopcrsbytop­l'ing up afte< uoc with a solution ronoaining a calcula ll!d high« <onccr1tra1ion or 1hccxhausted ingredients. The bromideinctta..,, howC\·cr, prcscnl5adifliculty. lti11crcascsa1 1ucha nuc lha\O\"Cn if1ucharcpleni1hing>0lutionromaincd ubro1nidc,thec:onccntra· lion..-ouldnotbebroughthadto1l1coriginal andi t iJncccn;lry to throw .>.K·ay a calcula ted por1ion of the uoed d.,,.,Lopcr before toppi,.gupwithabromide-frcercplcnishcTbrin(pbadthebromidc contt111ra1ion 10 the origin.al. Such n:plcnishmcm i1 practicable onlyinplanupn:ioenin1fairlylargcq1 ... ntitica. ll o..'C\u,limitl!d n:plcnishmcm canbcba>Cdontoppingupwithattplcniihcruntila volu me ofrq>lcnishcrequalto1hcorigi1 ... Jvolumcoltl>ede-."Clopcr hul1ttnaddcd.

ltntrnll'roctssln&

lly a modification of proa:aing, called ""'""" /!"><usi..,, a normal cmuloioncanbernadctogi\"e a positi•·ci11S!cadofan~ati•-crauh. In principle, 1hcmethodio•·crysimple. l rnmetliatclyafterdcvelop-

Tl/£ SCIEXCE QJ" /'llOTOGRAl'llY

1Mn1, the density (that it, 1heamoun1ofsil.,..,rhalideconvened t0 mcG>.llic sih·er) a t a11y point will incrcaoc pr<>graoi.,..,ly with !he oxposurcrettit"Ctlatthepoim.Ther"""i"i"l.•ilver halidewill1heu­fOrcd«Tt11.><progrC$1i\·elywi1hcxposure0andformapo1m1ialpooitive image.Thepo>iti,·ci•rcaliie<lbydiuolvingthe•ih·crimagcinacid pcrmangana1c(p.218),exposingthcresidual1ikcrhalidctolight, anddt:>·doping(Fig.6).

T hcl'C'·ersal proca1 isU>Cd mainly inproccosingnarrowgaug<: ( l6rnm., 8mm., 9-3mm,) black-and-white film, ar><l abo in proca­sing col01J r film. In all thesc, the final pooitive is in the form ora transpa,..,ncy. :\number ofpoinu in the proceso """worthy of con1id.,.,.1ion

l n1hcflrstpla.,.,,ffthctranspattneyistobc1atisfactoryi1mu>1 h~>"cclear highligh,., that is to .._y, the original exposure m..St be 1uchthatthehighlightcxl"*'"P"'"'icallythc1<.'/Nkofthesilvcr halidein thela}·er;thenqca1ivedcnsi1y1houk11hettiore,..,achl)_. With 110<mal ncg:ui,.., materiah. the camen. upo1urc 10 ..,..ch D_, (Fig. I.I, Chap. 13) ;. oome 300 timn greattt than 1he minimum cxposurelogi\..,agood nqativc (Fig. lt,Chap. 13). :>.:o•vforcamc,.., work, a ret..,.....1mattti.al3001ima slower than oorraponding ntga­ti,.., maceri.als could not be 1olcnuo:d, and 1hc dilflcuhy is M"en:omc byaCQmbinationortwodevices. lnthcfirstplacc,aool,..,m suchao alllmoniaorhypoisincludcdin1henegati\..,de•·olopcr. Thi1hdps 1ogi,·cclearhighligh"bydiuokingS<;1mcof thcuceusilvcrhalide which would otherwise be sullocqucntly dc•·elopcd to 1i l•·er. Secondly, 1hinco;otings of emulsions a,.., used, such 1ha1the 10<1.I cxpoou,..,Lo.titudeislinlcrnorc1han thatrtquimi10accommodatc a normal,;"". {log. E ]·~).The minimum exposure which !hen placesthe•Mrlo""Satthcfoolofthccha,..,cteristiccu"'c•hoplaces

thehi1 hligh,.a11heshouldcr,MatD .... Atypical11cga1i.,..,and corr<:Sponding poo.iti,.., chanc1eris1ic cun.·e is shown in •"ig. 7. Normally thcpoo.i1i,·c cur.•eis1101quilca mirrorimagcof thencga• tivccuf\·ebe<:auseof1hei11chuionof thesil,·crhalide oolvcrui111hc developer.

l1"·illbe..,.,n,therefore,1ha1thccxposurelatitudcorare•·croal em111.1ionis ,·crysmall,in wmradi11inc1;onfrom1ha1of ancgati •·c emulsion (p. 172).Anyo:xpoiurclustban that in "'hich 1hehish­ligh,.o:xpooctbcwholcof1bc silverhalidcwillgivcdc"schishlightt, "·hilca")"thingg,..,,.,ter"'illgi•..,blcaehed-outhigh!ightt,lackinrin dctail. l'herearc,t.o..·c·•cr,mcansofgivinralimitNlatitudewhich arc a 1>1~iN by manufactu,..,., processing their 1ub-Ha11tlard ot eolou rlilms,andthcuis,ofcou,...,actttain tolcran.,., 001hato:xpo-1urn o(halfor double 1hecotrcc:texpoou,.., (one Hop either way) otill gi,·c tolerable r.,u1 ... Olwio..sly, therdoTC, there can be"" fac1orof1a fetyincxposingrc•·ersalmatcria1s

ll shouldbeernphasitcddtat thcacidp<nnanganatC S<;>hnionmust bcfrccfromoolublchalida(p.218),othc.-..·isc1hcsilvcror 1hc1"'8a-1i•"imagcispar1lyrccon•·er1o:dtooilvcrhalide,which1upcriml"*'" a""akncgati,..,i1Jla8CM't:t 1l>clinalpoo.i1it..,.Afttt1he sil•-erh.u bttn disoolvcd,ar><lwuho:da"-.y,1hercsidualsil,..,rhalidt:hasbccn madccxt,..,mcly inocnsi1i,.., by 1hc acid l)':nmnpnaic, and it ;,,....,.1 t0imm<!,... thcfilmfor ashorttimeina10lutionofoodium1ulphi1c,

o'° •o ... {-

Tiii: SCH'XCli Qf' /' l/ QTQ(;ll,J/' /lf

whj,,hrac1aitioatMK£idualhalide. Flnally,ahh<>ugh thcto:Ond d~lopn><ont>hould<hcorc<icall)·oon•·onallttSidualoil~h.al;,Jcto n><otallj,,li]ver,thcrcDoftenaraiduumofoil•"<:"T"halidc (usually iod;,Jc) whichcscapco1hcaucntionofbothdC\°<'lopmCntt,andoon­""'1UCn1l)•itisoommonprae1ice1ofU<thcl'C\..,_1 filml..,forcfinal """lhing.

Th1SolnntT,.nd1rM1thod

Thcresidual sil•·er halide remaining afte<de-·cloping thencga1iv1' rnay be used 10 !Orm a positive in a Jaycr<tdj1tu•I 10 ili<:..,miti•·c rna1crial,rathcrthaninthe""1Uitiv1'la)·criuclf.Againthemcthodis ••cry simple in prineipk. An emulsion-coe.u:d paper 0. <:><posed, tr1:a1W .. ·i1hade--.:Jopcroontainingasol\-.:nt (usuallyhypo),anda pitceofblankpapcrimmcdia1clyoquccgccdl11tooontacl";'hthc cmuWon ourface. The first ac•ion of the: •oh-mt d.,.-.,Jopcr is to d"'-dopthencg;ui•-.:inthc..,no.itl•·ela)-.:r.Tii<:raiduallil~llalidc is '""'' dilooh-.:d to form a physical dC\-.,lopcr. Some of the oilvcr­laden <IC\·elopcr diffwcs imo 1he blank papcr, wMre i1 depoliu n><o1allieoil•-.:rinproportiontotheraiduallil••erh.alidein<heeor­retpondingpartof the>C1Uiti•-.:ma1erial.Tha1i1tc>oay,aposi1i•-.: ~1"er i~age it formed in th~ _blank f"'per. ~1uch of the~,;,.., 1mage>Sdcposi1Win1he..,nlll""Cma1crial,so ohatthcncgat"·eis 1poilcd, and only 1he one positi•-.: prim is obtainable from one CJ<.pos1ue,a1hough theprintcan.ofoo11rse,becopied.

·rt.cdcp<-.itionofap<-.i•i•-.:lrnagein1hcr-.:cei•'inrpapcritfacili­'"'cdand1heimageoolour impun-.:dbyi"°""pora'inrin1hepapcr a 1ninu1camountof,...,,.linel)"di•·idcdrnctallie1il'-.:rorlih"<:"T"1ul· p11Kk,solinkd'"1 lli<:whi1e...,..oftMpapcr it hardly impaired. "flleparlicletactasnucleiuf'O""-hichthen><otal!j,,1il•=i1 readily <1cp::isi1cdby1M1il•=-ladcnde-·1'lopcr.Theincll!Sionof1h1'sol•-.:n1, hypo,inthenudca1cdrecei•·ingpapcr,andof thcdcw:lopingagen" 1n 1he ncga1i•-.:e1nul•ion affOTd• funhcr impro''""'"""· 11 is then "'''"'""'Y'"""et !he,,egali\"C\\"i<l•asoh11iouoon10tir1i11gonlyalka!j •11<lsulphi1c,bl:fore "JuCCgceinginoomac1wi1hlhcr«civingpaper

Thc1<>h•e no-tran1fer 1ne1hodha•bl:c11appliedi11""·er•lform110 documentcopying,andabo tocamcracxpooutcs.whereithasthe ad•1'n1•gcofgivinga,·inuall)"drypositi•·ef"'per print tcnoeoondt afler'""J,..Utc.Atpccia!lyd...;gned<:an><ora- lli<:l'olaroid-Land ca mera- ..:0111aim1hescnsitisedpapcrinrolll0nn,andal;oarollof 1hett<civi"gpapertowhj,,han:a1tacheda1in1cr.""ltplas1iepods oontainingthede.-cloper in viscouslOnn. After<:><pooure. the two rollsa...,wi1hdrawni,.oontactthroughtwopn:Muttrollcri"·hieh buri1 the pod, and opuad the <IC\..:lopcr C\'enly b<-twcen 1he two papcn.Af1eroi~1y~lli<:pOOti•-.:paperbearingagoodquality

1'1•-.:r image can be >Cpar.ot.d. dry enough for imm..Jiuc uoc ( ~"ig. 8).

THE MECHANISM OF OEVELOPMEHT

Tlif. }"UNCTION oflilvtr 1pocb i11 apooicd grai"' io fairly

::,~ ~:~f-~~~~~~{.~~ ;:'" .:~:f::'er~~~ dtpo<i1edon 10 1heoril1inaloih-.:r1po:ch, .. ·hichca.wcsthcm 1oact :uCV1011bia:crand,,_.,tfficitn1ca1alysu.Onccsu.ned,the reduc­tionol"agn.inio1husa,_.u/y1ir,orselr-aoccler.uing.What io noo. cenain i1 the way in which tit.·cr runc•ions as an aca:lcrator. The firsthypothcsiswhM:h"'&$gencrallyaccept..<!K>rle"cnildecad.,.,..,.. ,.._, d•i<oal 0<1u:"1/.A1Hu tltttry, bu1 t<> undentand •his, "" must considerinoomedc1ailthephenomtnaofoolul;;lityand•ui:iena<u-

Solub1llt1 and Sup11u.tur1tlon

Ahhougl~ l~lC slh'er halides a re dauo:d amoug tl~e ;,.,oJMblt cum· pouncb, l\ JS doubtful whethera11ything is trul y msoluble in any solvcn1, Q:rtai nly1il,•erhalidcsarcnot truly insol ublcinwater-1he minuleq1,.n1ilicsofoilverhalidct whichwilldiosoh·ccan beac<:u· r:uely measured by phytioo-chemical me1hods, and fta,·e the follow­ing valuei for 90lubi1i1 y in diotilled w~1cr at O' C

silver iodide 3xllt" grams/Jim: o.ih·crh1Qmide 8·4 x 10~ o.ilvcr chloride 8·9xl0_.

The diaoked sih-er halide;, con1plctcly diaociatcd imo indc­f"'ndcnl ~i l•'tt and halide ions, but we .J:-ld n':'t visualioc" .. tura­tcd rolu11on of o.il,..,r halidc q oomethmg 1tauc. Th~l cncrgy keepotheions (andaJ901hcwatcrmolccula) inoontinuousmotion. Sih"ttandhalidcionsmult bc:continuallycollidingboth,.·ithcach otherand"·iththecl')'ltaltoftGlidsikerhalMta.Sinco:rncuurcmem "'°""thatdiAociationofdioooh·cdsil,-erhalidci:ioompletc,"-.:know 1hatwhensil,'ttandhalidcionscollidein1<:1lution, thercisnooom-

'~

Tiil!. AIF.Cll,t/;IS.\/ 01' DEV£l0 f'.\/f;,\"T

binationtogiV(:a1il•'tthalidemolccule-o<iftherci1,thcmol"'°ulc irnmcdi.otcly rcdiaoci.otes. But "·hen ci1hcr sil•-.:r or halide iom col!idewit~Cl')'ltabofoi!\"erhalidc, therc;.aotrongteodcncytofit into place in the cryotal lanicc. At the ume time, 1urfau ions arc goingimo 1<:1lu1ion,and .. -.:nuoyimaginethecomingaodgoingof ioNon the1urfaccofo.il•..,rhalidelllnUhe1 likebcain a hi,.., near aclovcrfield.Satun.1ionisreachcdwhco1hen.1eatwhichion1join 1hec11....,.lourf.occjustbalancctthcr:atcalwhichthcygointo..,lu­tion.

lf"-.:co11siderauni1attaofdidsih-.:rhalide,then.1eatwhich ionsgointotGlutionwiUbec:onotantatanygiventemper:aturc. The n.1cat whlchsi~•..,r halide'-l"ormnlon thclanicewilldependon the numberofcoll<sionotheionsm..kconthe t urf..cc,,.·hichintnm def"'nd•on1hcconccntracionoftheionsin90lution. This,.,.Jucio callcd1hc Hlidii/ilyptoJw1and"'i!!,ofcounc,diffi:rin••olucforcach haJide. It;. u.ual in ph)'lic;o.chcmical f'onnulary 10 denote che ..,....,.1r.1iM•ofioNin90lu1ionbyM[u.arcbncke.,roundthcsymbol !Or1heion,901hat1hcJOlubilityproduc"of 1ilvcrha!idaatl8"C

Sv..11 • (,\g -+) x [l · J - O-!JxU>- 1•

s(...,•I - 11\if+J x 1n..-1 - ~·lxlO-" Si-.a1 - {Ag +] x 1c1-1 - l ·OxlO-"

!f1hc90hnioninwhiehthcsilverhali<lei1di$$0h·edalreadycon· 1aiNoomeof 1hehalideor1ilverion•(ifitco"'aim90medissol\·ed al kalihalidcor1il\'crnilrotc,for""arnplc),1hcnle»oilverhalidewill dissolvebcforctl1ctolubilityproducti1reachcd.Thetolubilityofa romJ><>und i1 1hu. rep~ by the pretence of a common ion. (T his cca..,,tobetrueof1il•·crhalidciftheconeentration ofthecornmon ioniiouff><ientlyhigh,llltolublcoomplcx<loubleul"arcformed. n1u1 . 11,..,. l>romidc may be fiuJ in l tr<lllg pota1$ium bromide oolution.)

Thw .. ·henachc1nicali11hro"·noutof90lu1ion,ci1herhyJ>"C­cipi1ationorbycoolin1awannoatunucdtolution1<:1:utodecrcase oolubi liJy, a precipita1ei1 formcd con tin...,..,ly uncil the product of concentra1ionofthciomfallstothc•"alucofthc90lubili1yproduct. llu1cheinici.olappean.1i«of aJOlidfromdu1ionwu.allyrequirei the p..esena! of90me poin1 or poinu to act"' nuclei upon ""hich ayotals may !Orm. In ordinary oolutiono microocopic 1pechofdw1 orpro1ubenmeetonrough,."allsof thc,..,..lproyide1uchnuclci,90 1hat1~dnonnallynodilf><uhyinoUtainlnga1<:1lidprttipitate.

Tl/Ii 5C//i,\"CE o~· 1'1101 OGllAf'/fl"

lfo ... ~_..,r, by oordully excluding nuclei, io ;. pouible 10 dcby pre­cipital..,.> uto1it 1he;u,_.m olma1crial in oolu1;on itgrc.o1er 1Non 1hc ~onnaloolubili1y.F.,..~mplc,ifahntronce"1ratttfool~tionol4Jf#

"me1iculowlyfihettdand cardullycoolcdwi1houtagua1ion ina ch<:mi.callyckan«f1lui•·,,...,l,cl).,1"lli>a1ioo11~ybef»"'"Cnt«f,and

as1<JH,.dl•••1tJtolutior1ol>1aincd.Violc1uagi1a1ion,orthcinlroduc. 1ioo1ofacryotalofhypotoact3'lnudeus."·ould""uhininotan1an­rous forrnatiou of a ma"" of imcrl,,.,king Cry1tals " 'hiclo would immoclia1clyrnakc1l1cco111C1>Uofthc''"""'locrni«:>lid.

Th• Oi tw• ld ·lh&1 Theor10TDewelopm ent "l11is throry tugges1cd that the reaction lx:1we<:11 1il•·e< ions aud dC\-.,Jopcrionstak .. placcU.H/1<1;,,..,thatis.docfrtt,diuol•·cd•il '·cr ionsarcreducttfbydc-.-clopcr,a11d110tthetil•·crioo>1whichfonn pa<toftheoil•-.,rhalidclattice.lnitsfavour"·c1houldmcntionthat a largcmnnbttofapparcntly,./u/...,actiontfuo,.., been shown in rcali1yu...,ac:tioollbet""ttnionsintolution. l fthc<eaction1oot< place1•M•l-,.,..,mightcxpcclthatthemc1alli.c1il••cr"""ldbo: dcpooi1cdi,.afinccloudbctwccngrain•,butit"""ll,.."'latcd1ha1 the metallktil•·cr formsa1uptcrsaturatnl tolutionofoil'""'· "·hkh will pttfcrcntially dcpoo.it on the nucleioloil....,r n1etal IOrmcd by •~po1urcol1hcgrain•.Whcrcd1ercarenocx1l<ltoedgr:ainsandlhcrc­

forcnotil•·cr11uclci,thcrcac1ion<loesootprocttd,l-aUKoikeri> 1101 dcpo•iu:<J from solution; where oilvcr •1-ks arc formed l>y cx-11o•o<1rc, they grow and reduction proccnl•. T hc1hrory ex1>lain• the bcginuiugofdcvclopme11tatd<scre1cpoinuo11cx1JO$C<lgra im, hut the pos1ula1~m that dc....,Jopmcn t is l"'"'e111cd by 1hc i11abilit}' of melallicsih·cr to nucleate in an uncxp°"'cd c.11ul1ion i1diOicuh 10 uphold, Ho"'"'"'• it has no"" gi•·cn place 10 more >ati•factocy cxplana1ioo,.oftl1emcchanismofdC\·clnpmcnt.

A Par•llt lPttb lt m l nordcr101hrowlij(h<0rt<hcn-hani•to1ol<IC\•clnp11M:m1hcrcduc­tioo1bytlc>.'Cloi>enofoikcrioruin'°lution (1luo1 is..tolublcoil•·e< >al1t) ja.bttnttudicd.Thttt1hcttactionilttT1ai"ll·b•:o"·a:noilvcr ions in<0hnion anddiosoh..,ddC\"'°11C1"imu.andcan oakcpl~cconl)' "'IM:n 1l1e iom collide. Although C".'crycollioion may not"""]\ in cl1emicalrcac1ion (ar>daccording<ocalcul:nionJ«1not ).i-ctafixcd pr0]10ttionwilldnoo.andanythingwhichincrca5<"J!hc1>robabi!ity nfcolli•i<>,,.,•ucha•riocin 1cmpcra1urcorin<:"'""'inconccn1ration willincrea>elhcr~tcoflhechconica l rcac<ion. Thcratcofoolli1ion1rnd

hcnc~ ohe ra1c of reaction will dead)" 11< propnroinnal 10 the COil·

"/HE .<fli.CllANISM Of" Dl.' l •EtO,Mli.•T

cen<n.1iot1olthc...,aaanis--1husiftl1econttntn.tionoloo1ercagent ildoublcd,thechanceofcol!i&ion;.abodoublcd. Thisreactioo1ilgreaolyaccelcr:a1edby1h<!preocnccof~dl

.;1,·cr; 1Notis,by<heprcocr><:eofaclou.dolmctalli.coil'""'i>anicles'° tmalllhat theywill passthroughfiherpapcr,andrcmaininpcrm­a11cn1•uspension.h-mslikclythatthemccha11ioniofthilcatalysis i11hcu.meas1l1emechanismofthedc,•clopmcmofc~~graino whichoomainoimilarparticlcoofmetallicsih·crcm1hcturfacc. By rolvi11g thcformcrprol>lem, ,.·eshould ha,•cgood evidence on the rncchanilmofdc,·clopment.

ll'hcnpar1iclesofmeta ll icsikcrarc1uspcndedinarolu1ionwn­l3ini11g 1ih·crions,oomeofthepositi•..,lycharged.;1vcrionsadhercto 1he1urfaecofthc1ilvcr(givingitapo1i1i'"cl.argc) bytomcphyoic.a.I orchcmicalmcchanismwhichgocounde<d1egc11er:al1crmMl«"PI;,,,,. Similarly,cofloidal1il'"'P"'<iclesinadC\-cloptcr.olutioo1may.W.od dC\..,1operioo,.. lfbolhionsarcpraent,asi11thccasc"""rccon. sidcring,ho1hions"·illbca.i-hcd,andoinectl1ead"'rbedionsa"' usuaUyquitc1nob;leond1esurfaa:ol1l1eadsorbo:r,...,ac1;onbctwccn the1....,willbcfaeilita1cd,tol0rmmetallkoilvcronthcturfaa:ofthe oil,•crpani.cleL"rh<:aculcnuionindica1es1ba1reac1ionbct"·ccn indcpe1odcmioollinoolutionhasla'"'Jcly1i'"''l>laceloa...,actioo>in ,.-h;.,h 1hc1il•·erparticlcoareinvolvcd,1hat is,1orcactionbctwttn a<horbcdionsonthcturfaccoftl1eoilvcrparticles.Anincrcascinlloc rateo1·...,actionistobecxpcctcd,sincetheehanecofcolli1ionh;u be<:ninercascd,andadoorbcdionshcldtothesurfaceha\'eamuch grcatcrehanceofrcactionthancasuallywllidingions. l nother wortl1,1hechanccoffmi tfule<>lli.;onsilgrca1er.

l"heonlypointlcftundecidedisthemechanisnoofdoclrat,.ferof theclec11·tmfrom1hedeveloperion 1otheoih·erion. l•comact lx:­twttnadtorbcddC\·elopcrandoih·criononecesoary,orcana11clec<­ronbctransferrcdfromdcvclopertooikcrionthroughmcta!li<ooil ­"CT, thc la1tcraetingasa tiny<:<>nductor? "n1ccvidcnoc-ms10 faYO<tr1hcb11crhypo1hcois..lthasbttn1i.o..·n1ha1i1;.~,,o

rcduccoih...-bromidcwhcn itsonlycontaet ""i1hdC\'Clopcrion ii 1h.,,...ha.;i-..,,..,·irc,butlhildocsl>OIJlfO'"'ha1clec1rontn.rufe< throughoil=ilneccaarily1h<:open.ti•·cmcchanisminphoto-1"'phic devclopment.

Comidcr:ation olt!1eadsorption ofoilver io•ll tO meoalli.coil•"r throwsromcligh1on1h<!pr<>blcrn. l namasaofmetallicoih·cr,iti> koown1hat1he,..,Jcncyelec1rons(p.ll3)arcnot<:Onfincd,uehloiu °"'<11ilvcrnuclcuo,butarcshared,andfrtttomo•·e1hroughoutthc t,...., hl11hisat1ribu tewhichgi•·tssilveri11clectrical~nduc1inl! propcrtia. \\'hcn 1ilvcrionsarcadoorh0<l,thcadrorbedt1lv<:rnucb

TllH SC//i}{CJ; 01' 1'1101"0CHAl'/f/"

=~:npl~~t~:t t~ :\~~m':'i"J"~j,~~~la~:~; ~~~:a~ ~~~

=~~~~?'·E~~!2}=:~~h~!~~h;,;:~~ 2:~:~:71;~:;:~E:~:JiE~c~~~~=~ ft:~,.:.. "!;~h:~,:1;;.~i!1~'.':' ..:,.gi~-C.,~,.~;:;:.,~r~: compkment of clcctroru.

}"or 1his rcu::m ;, would ottm 1h.a1 rcac1ion bc1..-ttn a tlc<i..:loper iona 11d,.;h·trionadoorbed 10,.;lvcrcallU.kcplaccanywhcuon 1h<: 1urfau:of1hcoil•·cr,,..t.al-llpart1of1hc1urfaccu.:ttjui'"'lcn1, and1hcoriginal,,..1allic1il>~thuoacuasaconduc1or.

l.ctuoapply1hc rcault1of1hcoee>cperimc1111 10th<:d .... ..,lop,,..ntof oilvcr bromide. \\"c ha,·cocm that e>cix-'ng a gr:ain •<> ~ght producei

;=:!r':.~~::;,:'!::ri:~~ •. ·~~~~~::::~."~-~:~l~~~~/~ adoorbed 10 the un<knidcof1hcoilver1peck, gi,fog 11 a l""'"'·e

;3~; ;,~~:~;~~I~ 1

~l::;ir:~~:s:~~i:~~;~~~:~:~l:~~~ bromide,andthcncg:>tivclychue:c<lbromidc101"m"'lut10nwillbc adSOTL>c<io11tothc1urfaccofthc 1i h·cr 1peck,butl.,..strongly than

~f;:~E~i.~;~~rr}2~~l~;~~~~i~~ :;~~:~~~:;~ =:ci~~;;;;;ili~ i:;;:k~~~~~e" ~:::·~;"!.~,~:~k~ ~':'.: ::, :1!:~;:,~~:,~:'a~;':;,~~1~1:~t=~~~.' f:;.~~~·.~~k;';.: :::;!";,:tu~~~-~~~~i;:.is ~~':~~c~:~::i~~'~:;:~ casinlyadoorbcd,d,.formcrbciugreducedandth<:.laner"'udit<:d, ,.,dun 1hcmauoloil,-.,rg,..,..'1a1 thce>cpcnxofthc .. h·tr1<>noin ~h<:

§j~'.g:·:ei£~~g,f:~:;;~~ u f=hromidciont.

TllE .<IECllAX/SM Qf" QEVliL-01'.llEXT

Thi• picturcrcpr...,nt1aprobablcmcchanUmforthcfintt1agcool d'"'"lopmcntofagrain. lf11,.mcchanismconlinuathroughoutthc d,..,-.,kipmcm,one"·ouldcxpcc11hc1ilvci t<>bcproduccdinthc<:<>m­pact Cl)'1tallinc form, The formation of loose filamo:ntary gnino ,. . .., originall)'e>cplainc<ion1hch)"pOd...,;,tha11hcformationofrelati>..:ly la~sih·erato1D1intheconfined1paa:bct,.·ttnthclatentimagc

opcck and !he sih-.,r ~Jidc grain would tend I<> pwh out the speck a nd its accumulated .. J,•er in ril>l>on form. Tl>U e>cpbnation ti not ··cry,..1isfac1ory, apcciallyin ,.;,..,.· of1hefact 1hat ~lament fonna­tionisno1 un.comn><>11"·hmsil''trilfor"""1ur><krmtirelydilfcrent chemiollcondi1"""'-ndi1i.onstowhich1hee>cplanationcouldnot •ppl)·.Amore,..1Wactooyupl1m1uioncouldbc<>frc,..,d<>nthcfol. lowing Ii""''

The•tudyol1hcrc<iuc1ionoloil,~ioruindution,catal~·scdlty colloidaloih·..-,shcn.'Sthat"''hetheroil"erionsord......,loperionswere p«:fercmiallyadsorbed 1othc sih·erpaniclcadcpcndson 1hepani­culard .... -.,kipingagen1. h alsodcpcn.dson rela1ivccooccntr.o.tions andoohcrcond itioru. 1\11hcbcginningofdc,·ekip,,..nt,thc •pcckis likelytobcchargedwi1h•hct....,...,easilya'"ailablcintcntitial1ih·er ioru,andd,..,..,lopmcntislikclytobcdtt"'i<<Ol.Asd .... -..lopmentpn>­ceed1, and the mass or.;1,•cr growt, dc,·clopcr ions will be more e.uili· adoorbcd, and in1c,.,ti1ial ion• 1.,.. caoily. Al the''""" time p/l,J•i<Dldc,•elopmcm will play a grcale< par\, for the''"'"'"' al­feadygi,·cn. h is not unreaoor1nl>lc therefore !OJ>O<tulatc in IOmc casaachangc-overduringthcdevclopmcntofa grain,frommainly <iwmi<al de,·clopmcr1t of ix-'tivdy charged 1ih·cr speck> (that is, withsih..:r ions adoorbed), 10 "'"inly pltysir1>l development of the larger sih·..- maw:s 11egati,..,ly charged by prcfcrc11tial ad>0rp!ion ofde,·elopcriom.

Ae<ording to thi1 hypothco.is lhc later 11agco of dc,·ekiprnent wouldbcchar'3c1criocdby1hcanractionof1K10iti>"oih·crionsfrom oolution1onegati•·elychorgcdoil"ermaw:s.Now1o·henaconductor 1uch as1ilvcr i1charged,1hcchargci1conccn1n.tedat any1harp protru1ion1,001hatitU1othcoepoin111l""1heoih·..-ioruwouldbc prcfcrcn1iallyal\l"aCted,givinga1>eedlc-likcprotubcraru:c1o·hich wouldrapidlygrowin1oalilarncn1. Alilamcnthuala~1urfacc IOriuvolumc,gi,•ingplcntyof~tunhyford,..,-.,k>pcrionstobc

adsorbed al<>ng iu 1idca. This rcou!u in a ,..,,.,. nrong negati,.., chargeooncmtra1eda1th<:1lp,andhmcca'"Cl)'1trongann.ction IOrpoo.i1;.·esih•erionsfromdu1ionatthcendofthcfilamcnt.O.. oontacttheoilvtri.onsareirnrncdiatelyncu1raliscd1ogi'·cmoetallic 1il,·..-,andtherd0rcarapidlilammtarygrowth.

FIXATION AND STABILISATION

A~!i~~~~~~1!~~:~~:~~~~~In!:~~EsEE~~ WQUlddarkcn and opoil the photognph. The image may he rendcr<:<l pemuonemor.fiu'byo11cof1womclhods:a!lrcsidualsi1'·ersah1may be complcicly removed leaving only the meu.UW: silver ima8"""""" proccss calledjixatio.t-<>r 1hc1i l...,r halidQ rnaybec:onvencd into oomco1her c:olourle11compoundwhichisnol light >en•i tive (that i" •hcimagem;oybcs/Milisd).

The Cheml•tl'Jof f lotlon

l tl1aSalreadybttnpoin1<:d out th.at ..,hcnoi1'·erhalidcisimrncncd inwatcronlyaminutcquanti1ydisoolveo--«> little1 ha1for allprac-1icalpurfX*'Osil'·':rhalida arc1ta1<:<lmbcinsolubleinwatu.Tha1 "'hichdoc:idissoh'<:is0001plt:1clydissocia1cdimoindcpenden1pooi­'i'""lva- and neg::u;,.., halide ions. The amount oloolublcsilvcr halidei•1uchth.a11hccoruxn1nuionof1ih•eriruumuhiplicdbythe C01>cem r~lim> of halide ion• is a comtam, called tlte 1~dili!Jl J!r~,J.,1, l'his •-:aria, ofcou....,, '"ith thedilfcn:nt h.o.~dcs, decreasing in value from chloride, through brom~ to 1hc much more inoolublc iodidc(p. MO). Anyaddi1ion a ff<:<:tingthcconccn1r:uionofci1hcr 1i lvcrorha lideion1 will affect the total amount <>fl.ilverhalide in oolution. Tbus the ll:pnos>ion ofoolubili1 y of silver halide by the - iM ff«I has alll:ady bttn nMcd (p. HI). Con'~)", the additionofanything.,·hich,,;11"""-•heconccntn.tionofcithcr o.i lv.;ror halide iortlwill result in more of the solid silver hlide diuolvinguntilthcoolubilityproductisagainreached.

Nowoodiumthiooulphatc (originallycalledoodiumhyposulphitc, andotil!knowntopho1ogn.pherta$1'J/O)ccru.inl)":n terfcreo,.·i1h theconctntrationof sikerioo11in asolu1ion. l tsslruCturalfonnulad shownonp. 124. l n oolu1ionitdU..Cia1aintoliO<liumand1hioou l• phatcions,thclattcrha,;ngadoublcntptiv"ch.arge,thus:

N"a,S,0,~2Na- + (S,O J-

l'IX ATI QN ..tXV STABILISAT/Q.\"

The: thiooulpbatc;.,., isa rather rcac:tive<>ncand wiU combine:

~·~)~:~·~::i.·~::~~ :~~~= '~~·~.::.~~~·r.:;;~~~.:~:'.~ ol the 1ulph.u~ .,oms is not exerting iu full valency, and it is on ohis Spllll: combinmg power that the ll:ac"""Y of 1hiosulphata depends.

~~~~~~¥l§~~~~~~~~~~ duringthcla01hundrcdycanall:rcrna rkablcfor thci rnumhcrand ,.,.rie•y.

ti~1~~~l~;~t";:~1a':'"~,~7n ~ .'.,~;~::::~:"d'~~~l;:';l:'.:

:E:~;,~~~~S~:~$~S~:~ solu~ cx1" .. 1uch on solu1io11. The leeond method io 10 uaminc by phySK:<).Chcmicalmctloodsoolutionoof hypo"'hich ha....,dissolvtd ,.,.nou,aDXM1ttUofsilvcrS&lt,anddtduufmm1hcresuluthecom· i-i1ionof 1heoaltscxi11">f!inoolu1ion

TI1c mootimpor1ant of thcsc phyoico-chcmicalmc1h<><4i11halol f'OU•li.-try. When a sil\•cr will: is immersed in a oolution a •mall differ<:ntt indectricpotcmialis..,.upbctwunthc .. ·ire and the ~u1;.,.,, depcnding,amongothcrthings,on1heconcentrat;.,.,offr« "~'·er ionl m doe ooluuon. By mca1uring how thi1 potc.,tial vari .. """h,·aryingcor><rntn.1ionoofchiolulphaic a ndof si h·croah,onc: can deduce ohe compmition of the complexes IOnned under variouo condi1ions..

hi1 nowknowntha1 thc fi rot reaction i>c1weena 1i l....,roah a nd hypofOl"msacompound:'\a(AgS,01),"'hich isra thcri nf<>lubJcand ?"IY modtta1dy ""bk. u.,....,,,.,,., iu cxi<tcnce is tn.ollicn• and it

~~~~:~,)7J~=hi;~ · ~: ,::;=.1bl~~u~:,1;a: T~a:":

i:;:~~f'.£~1~i;Rl;·~:~~~~J~f!~f~r,: Thcrcac1ion1 arcionic,andarc;

(Ar•+ Sp,- - (AgS,O.J­(AgS,OJ- + S,O ,·-! Ag(S,0,) 0)"'

Pos1 fiocation: [Ag(S,01),J• + s,o,-- IAg(S,01)1r•

141 Tllll SCl£JrC£ OJ' l'llOTOClf..il'llr

T111lllt chanlsrnoffiHtl01 Theevideneep::Mnutothef.actthattheooc:ructM>nltakeplau.onthe surfaceol:thccninoJ:,.;Jvcrhalidcra1hcr1hanintheooluuonbe-1wttnthccnins,oothatthernootprobablemcehanism.i1 tha1•":"

[?,~tf i~;~~f~i~§~~~]~'.;~?~~§ 1jl,·crhalidcgrainstruc1urcp,...;ntosolutionwhcnlhcyarclil>cra-

tc~rt ~~~';!~~~cn~,l\~:{'.t;s,~j; corrapo.,ding. to the .ion (AgS,O,J-, thuirulfilsonlya•....-ytraruit(lt)'rolcuan1ntcrm<'<haic productintl..,rormationofthcrnotthighly thiolulpha1cd compl­ua; i1probablyuis11mettlyasane'o·.,,...;;hangingmonon>olccular

~:~~~~,:c~~=.ha~:~gu:'.11'0g~:~j~~;.,~'°:,dy ocwwcrtcd to (Ag(S10 .J,r. \\"c could, thell:IOtt, almoot nq;lcct 1h11 intermediate compound andounoi<kr that theesatnlialand funda.­mcntal ll:&etion in fixation, in spite QI: the many •-aricd oomplcic reactions,,..h;.:hhavcprC\·ionslybecnadvanccd,isa,.;mp1eone:

Ag•+ ZS,O,- - [Ag(S,0,), r

F.cton •«•~tl ng FIHtlonTimt

:£-:E~:?~~{l~?:~t!f.~~T~·;~;2~~~tE ooluhlcoomplcica,and(iii)thc:partialdifl'usionol:tl>ccomplexcsand a lkaliha.lideoutoftl>ctrnulsionintothcoolution. E..perimt!nuha"" indica1cd1hat1hcchemicalrcac•ionisoompaDti'"lyrapMl,andthat mootofthcfi""1iontimcisoccupicdbythcdifflllion~:o;ow

;~~::s~~~~:~o~:~~~~~~=~~~~~~ disturbed and rcrno•·edbyagita1ion,sothat•uchdev1caaorork1ng

:7.r:~;3 .. :~.i~:~:~~;~~;~:,;~;E~:~~~~!~~;:r

~ II-:.: O"'"'" ~

F'f. l. Rota:=,.;;...:~.,,~~h<:~·fi!i::~oed~~''"';,.,,

(•l N•l~rt of £.,..u;.,,.,-Jn general ,i.., ~ t0luble c~lotidc cmul.W.\I will fot moll: rapidly lhan chlorobrom1da, which in tum

[~~~~~Fg}£g~~~}f~z~tIE~:f~~ lha.t any hindratMOe to diff...;on is largcly ofl'~t by the reduced '"""Hin1ofa bardenedlaycr,givingaohontrd1ffusionpa•h.

:~§.~~~~~§::~~~~~~;;;~; fixa1ionitMO<UO<SUptoaconttntrationol20perccn1hypo. hthen ..,main1roughlyconttanttoaconttntrationofabom<f<lpcrccn1, beyond which it J«rnsu rapidly (Fig. I ). This result wao gcnera~!y acceptcdandappearcdinallthe standard tcxd>OOlto,and~vcnoc to1hrrecommcnda1ionnottouschypoofgn:•terconce"u''"on1han

~!'~:~·i~~~~g:~::drJi:~' i; ~i;!'";~~· ::,1,f,~·~~f~~;,,~· ~:

r.,.2. 11.da..,..brtw«n~ .. ,.,.. ....... ..ihypo«on«n<,.,;.,,. .,_.;"C """"nimu"' u.....,. ...i;,...,. liuo;... ..-;,.,..,_

in<rnseinfix..>1ion1imewhenuoingup1070pcrccmo:mccntnuions ofhypo. On fur1herim'<:ltiga1ionil""' °"""r.'edd .. 11hccbssical minimum in fix.a1ion time is obtained wi th dry matttials, but no• wilh ~kn! materials. On uamining f"'>' rte0rdo it""' found thatallearlierin,..,.tiptonhadcithcrusn!Jryfihnsorplatco,orhad not •peclfied 1hcirc:ondition, but fromdocrcouhsobtained 1!.= 1ttms to heliulc doubtthatthcda,.ical"'01'khad a llb<:cndoncwith dry ma1crial,nndthcrcouhs"'rongtyapplicd1onormalfo"'1ionc:on· ditions. h is not surprising 1hat dry ma!erial fl~a slowly in \"cry strong hypo, since ronccntrattd solulions of many salu ha,•e a dcprc:Ming cff.,.;t on 1hc swelling ofgcla1i n. HowC\'Cr, in 11orm.al fiu.tion1hcfilmisalreadyswollcn . .\lorcrecenltC11Smadcwi1h a large ,,.ric1y of materials at nonnal 1empenuurco .w. --1.finlioio ,.,./i1i,...1hownode<:rea,..,inra1c offu..o1ionwhentlocooncen1ration of hypoucuds..Opercent (t"ig.2).

(• ) Uu ef TJr>-J~oflllllnNsn-/l.r.-l'i<•tiM.-Since1hc reactionbt1wttnoodium1hiooulpha1eandoih"erbromideilinreali ty 0t><bn"''"'"thiooulpha1cionsandoihuion1(p. H7), 1hiooul pha1<:1 othcr1han1heoodiumphwouldbtupectedtobtha,..,likeh)"po, PJO'·ided1hat1heydisoocia1cinrolutionandthattloc~1i>•eionhas liulccff.,.;tonthegelatinorsikerhalidc.Thisis1ruel0r1uehu.lu a1 lilhium, pot..,.ium and cakium thiosulpha1<:1, but ammonium 1hiosulpha1cdiaociates togivetheam1nonimn ion {NH,)•, which haoasolvenl actim1onoilvcru.hsand..,.i1u fiution. Rapid fixers therefore, "'"ally oon1ist of ammonium d1io.ulph:u c, or ofa mixture

/llXtlTIQ,V tl ,'(D $Ttl BIL/$tlT I Q,\'

ol h)·po with "" ammonium oalt wch :u ammonium chlori<le. "ll>e ammonium argcn•othiosulpha•c IOrnu:d during fu..otioo is 1 .. 11.t.bt.! 1h.an the c:oncaponding ood ium oah , but a sih·er image fixnl in amrnoniumthiotulphateisnoless11ablc 1han•normallyfixed0n<, p!O"ided1ha1 i1i111-oughly"-...hed. ~l•in orpnic <:om pounds ca Ucd f•lll<r""'J' 1"111 a,.., analogous in

Dl)Jllpoli tion andproper1icowi1hamrnonium satu,andlikc1hela11er theya~lcratefix.otionwhcnaddnlmahypoba1h.

(<) TtmJ>tMllif<.~Mo.\ chemical reactioiu arc accclcra!ed by rising tempera1urc,and1hio i>trueof1hcre•ctionsoceuringduring fixa1ion.Thcrei1,howC\'cr,anadversceffe<t-raising1he1c1npera-1urcwillincreasc1hcdegr<:eof>wcllingandthus!hcdiffu•ionp.ath, so tha1 1herci1 anoptimumlcmpera1ureforlix.a1ionuound60-7o•f,

(/) t>tvu ef &i.-.1;,,,, •f Fin""' B•IA.-T here are three main ehnnicalchangco,.•hichoccurinafixingbathduringU>C: (i) ade­creaRinconccmra1ionofhypo, (i.i) anincr<:a>ein c:onccn1ra1ionof agt:n1othiooulpha1es;and {iii)anincrea1einooncemrationof solublehalidco.Allmwl1oslo-.·d0"<1·n thereac1ionsoccuringduring lix.ationtoagreatcrorlC11tT ex1cn1.TIIC:1hinlf'actorisno1imporu.nt inaba1hused10lelyforfixingehloridcnnuhion,but1heaccumula­tionof 10luble iodide inafixingbathusedforiodobromideemul­sionsh:u a markeddepressingdf.,.;1on•il,..,rion<:onccn1ration,and ocriouslydow• downfotalion.

Thctraniferenccofd""elopedm.a terialdi!"CCttoafixingba1hoon­,;.lingoflOdium1hiooulphatcwoukl rcouhintwoundesin.bleeffe<t1. Developmcntmigh1oontinuctosomet:xlcn1duringthcearly>1aga of fin1ionandcausci/i<i..ic/•$ (P.2'24) ;and1he dev<:lopingagen1 1nnsfcrredw 1hcfixingba1hmightcausc11ainbyOJ<;dilingtothc b~resinousproducu,sinccthe1ransfetted•ulphitcbecorr>CS100 d1lu1edto p""""'""i.dation.

Theoc defects may he fl:"l"lially P""~ted ~an in~crmcdiate ri~ in wa1cr or temporary immcnion in a d1lu1c acid ba th (whoch llOpl further dc,..,lopmen1) he1wecn dc-...,lopmcm and lix.a1ion . HowC\'er, it is now common practice 10 uoc an ac id fi~ing bath.

wl~~ a~:~v~l~~~~ d~~i:~c :~!i,:~Y:~~eio~: .. ~1;r:r:'8;:i~:~~;

Tiii!. SC/EXCI!. OF l'/IOTOCR~l'llY

charJ'!(I). Indeed it;. the pr<:M:nceolhydrogcn ions wh.ich giva acidoolutions1hcir typical acidic propcMia. l'huo:

110 o -Hydrochloric Hydrogen Chloritlc

acid ion ll.SO, - 2H • SO, -

Su'.!:,~~ric H)·drogt'n Sulphate

Nowl0diumlhiosulphatcisuns1ablcinacidoolu1ion,aslhcthio­•ulphaieiondc<:ompooatogi'·eanacid•ulpl1itcionand•ulphur:

tt• + s,o,--ttso,- + s

This ttaction is ..,...,niblc-indeed oodium thiofulpl>ate U IJ\anufac-1urcd by treating oodium sulphite with •ulphur undtt a>r>dition1 which IOra:thcttaction in thcopposi1cdittetion.

AccordingtodocLawolJ. las1Action (p. l26),1hercactionfrom lcftto rightwouWbchindercdb1·ahighconcemrationol1ulphitc ions,oothat oulplturouoacid (whichda.oc:iatcslogi'·ch)·drogenions and 1ulphitcion1) can be used to acidify hypowithoutcaU>ing its dccmnpoti1im>10 t ulphitcand •ulphur.Thc t ulphurout acidrnaybc forrnedbyaddingtothehypo,sod ium sulphilenndasmallamount of acid (usually acetic add ) which "'acts wi1hl0dium 1ulphi1eto gi•·c 1ulphurou• acid. Altemativdy, oodium rnctabioulphitc, an acidformohodium 1ulphite,maybcuocdinco11junctio11,.·i1hcxcea ofoodium1ulphitc,sinceinsolutioniti1 cquivalcnttooodium sul­phiteandsulphurousacid

:>.:a.S,O, + H,0 - "'•.SO. + 11.so, Sodium Water Sodium Sulphurous

mc1abi1ulphi1e sulphite acid

Acid fi~cr1 ha,·cafurther ad,·an1agc-duu h.ardencrHuch as alum or chromcalurnwhicha...,dfcctiveonlyinacidoolu1ionmaybcincor­l'""ucd. A well·Nlanccd acid hardcning-fixingNthcontainssuf· fidcut hardcllcrtomaintain thchardcningaclion until the bath is uhR111tcd

f'/Jt~T/0.!o' AXD STABIL/S~110 ,\

' ractlulFin tion

Although in thtO<y a fixing bath may be ustd until it ccua to ha•·e anyfixingac1ion,iti1 notgoodpracticctoapproachsuc:hdegl'OQof uhaus1ion.1\po.rtfromthcda.ngerolcontami,,..tionoltloc""sbcd material by silver ults, it is u-~ic, s.ince fi~ing lime incrcua ••crygrca1lywithuhaustionofthcb;ith.ltisu1ualtodiscarda fixingba1h,.·hcnitsfucationtimci1about!M'iCcthatofaftt1hNth.

l1ha1 bccnfoundinpracticcthatifamatcrialis rcrn0>·cdfroma fucing baih immediately the cmul•ion i• apparcn1ly clcu, oomc cum·

::;d~i1~% f;,h~:ei';n::;,~3;i~e;,,a~1~~r~;'~';:1•::::;.,:~;,":.!~:~) Tlu• i1 unda>rablcas1hc 1mageishkclymdctcrionueonkecping T hissih·cruhisthc"'fo...,iraolublcinwatcr,anditspr<:M:n°"rnay bc oho>.·n by ttta\ing the 11\aterial with todium oulphideoolu!iOn, andsofonnings.iJ,..,r ,ulph.idc,whichisblac• andooi1...,lublctha1a dark>tainMillbeU.0..·nwithpncticallyanyformol1i l'erotherthan mctallics.ilvcr.

An ern>11COU1cxpla,.a1ion of this phenomenon, fr.:q1ocntly gi>"Cn inpho1ographicliu:r.uutt,Dthatfixationtakcspla""int1•"0•t:r.gcs. Tbt: 1ilverhalideDfintconvertedintoaniraoluble (a ndpraumably inviliblt) C<1mplcxcontainingahighproportionof1il\..,r, a f<erM'hich tlw:filmapptar1elcar; thD•ubocqucntlydisool•·a in c~cea hypo to gi•·c a oolubleargentothiosulpha1e.lla>edon<hisanump1ion,the ad,·iccgi,·cnio1ufixfor1wicethctimcrcquircdforthcmatcrialto clcar. T hcadviceisgood,butthecxplana1ionoomplc1dycrro11cous - ifinsolublesi]\·crsallSremain,1hcy arearcsid1mmof1il>·crhalidc The point at which a film or platejwt clear1 is di Ilic uh to delcrmine, CSpeciallyM•hen1hc1na1cria!isfuccdinawhi1cdioh. [fa film or pla1ei1 ...,fn0\...djuotasitappcar1clcarandis c.<amincdagainsta black Ncltground, a drop ofh1·po locally applied will gcncnolly produce an C\'CR clearer patch. l lw: ooundnCSI olll>e ad~ia: to fix :.:':.a1 long aothcclearingtimc is duc1od>etendency10undcr·

F..~n oounder advice is to tnmd'cr material after clearing from the usedtixingb;ithto a frcsl>fixingbath!Orthcsamctimc.Thisno<only g1,.,.. foll aaurance of complete focnion, but provides a fi~e<l material ror .. 'athing .. ·hiehisoontaminateda[,-,-tsolclywithhypo,thcbulk of docargen1nthicioulphatcsha,·ingdiffuocdintotlocfir1tba1h,and muchol1he1,,.alltt1iducinto theocoond. Themc•hod isnotun• e<::onomic;whcnthefintNthbccomcscxba"1tcd,1l>eocoo11db;i1hD """lao1hefir1•bad1,andfreshfixerwcdfor1heocoondbath. T otal fixingoohuionu .. disnugrcatcrthaninnormalone-bathfixaliun.

Wuhinca11~Hn0Ellmi1191torc1

The purpooc olf'O'<-faa1ion """shing is, ofoounc, 10 elimina<c hypo and1hca~m<>1hiotulpha1et. lf a 1raccol'hypoislcftinamatcrial, itisli.o.bletode<:otnpoocunderlheinAucrw:cofairandhumidityand itac1with thcoilvo:rimagctoformJilvcr•ulphidc•tain. Theoreti­cally it is impoooible lO ttmoYC all Lhcl<>lublc pits by washing--och period ofwuhing ... ;u ttduoc the ph eon1ent of the emulsion <0 a fractionofiulOrmcroonocn1ration,JOthatlikc1McHinthccittlc, "'Cappn)"<hncarcraodnearo:rl00Urgoal,but""""'''"""'chi1. How.,,,..,,., normal washing brinpthcamc:cntration 1osulficicntly lownh>CJlOc"'utttt.&1materialswillttmainunchangcdl>'..,rk>ng ptriods.Papcrprinutttainhypomottllrmlythanfilms<>rplates, as it is a._,.bcd by ci1hcr doc fibra <>roi~ingofthc papcr•upport, but1hcquan1ityttmainingaf1o:rthoroughwashingck>cJnotpn>ducc a.1,..,nccffccuduring1he normal lifeofacomrnctti.o.loramatcur prin1. lfuttt....,pcrmancnccof!"inuisrcquittd,asfurarchival purpoocs,thcnh)·pomaybechcm;c..Uydes1~byllocuocof ~,)fl'Otli/tliMl,,,1.

Alk.a!incOl'oahbathsar<:J<>mctimeoinclud<:dundcr1bishcading, but .'hey mc~I~ accclcraLC doc normal wuhing process. Hypo climrnaton o~id11e the raidual traces of h)·po 10 J<>dium 1u!pha1c (whichi1relati,·dyinnocuous)whilchavingnoactiononthes.ilvcr irnag<:. A Ly~eal hypo eliminator is a mixmrcofhydroscn pcro~idc and am1non1a. '!'hit oc11wcru the hypo tO llOdium and ammonium sulphate, which arc r<:moved hy 1ub!eq...,nt washing. Tra~ of thesc .. 1t• leftin1hcmatcrialha,•cnoadvcrsccffcc1.'n1creac1ion, '"hichi•quall!itativc,i1ufollows:

~.,s,o, + 'lft·,Or + 2~ 11,ou - ~.,.so, + (~ 11,),SO, + $11,0 f!ypo Hyd "'ll<n Anrn'°"U. So1Uum Ammonium \\'.,.,

p<fO>.i<le wlpho1e ..,fpt.a1<

Pho1og,.,,phic. litc,.,,tur<: contains frequ~nt wamingo against the ....,ofhypoehmmatori. Thediff1euhyistha1 thcclfcctofatreatmcnt onthcpcnnanenceofa photographicimaR"can never be guaran­teed. Singlctestsar<:unreliablc;J1aLis1icalapcrimcntsarcrcquircd on nrying '""'1menu of a varic1y of maccrials ou:.red fur many dccade1undo:rawi<Lcrangcofconrli1ior>o. Unforrnnacely,ifsuchan ambitiout cipcrimenl were made, tt>C final couclusioru ,,;ould rd"er lO ma1crials which would be oomplctely outda1cd, and could no1 be applied "'itb certainty lO the new matcri.o.b in oommon "''" The""' of hypo climina'°"' cannol, thcrdi:>n:, be rccmnmcndt:d as hcing rr.,.,rromrislt.

St1lliliution

Anahcrnati,·cprOCC11IOfixation,call<:d•ldilis111;,,,,,hu,...urncda gro"·ingimporta11ecof-cn1ycan.\\'.hcrcasfixationandwuhing ,..,,,_..,,hcur.,tablcrcoi<lualoilverhalidc..-.ainingaf1cr<k;.,..,Lop­rnc111, 1tabilisation co1weru the oih'C1 halide into comparati,·cly ou.ble colourie. compounds. H,.....,.,..,., as •'-compound! do noi

ha,·canci<trcmclyhighdcgT«ofJ1abili1y,aitabiliscdimag<:willno1 la>t as long as a ,...,11.fut<:d and "1Ul>ed image . .\ l°"""·cr, as 1hc oil''C1 compound! r<:main u a .. ·hi1c dcpoait in the emubion layer, ,...bilisationisnormallyappliedonlytophotogn.phicpapcn,, .. hert ouchadcf>Olitwouklnotbcnoticed.Stahl;... tion isespcciallyuocful in the rapid protaoingofpaP"t! uscdfurdocumcnc Mp)·ing, .. ·hcn byappl)·inga,...bilisingoolutM:i11totloc1urfaceofthepapcralwicf, ocmi-dry procao g;ving a damp print 111itahlc fur immediate uoc p.316) maybe1ubo1i11ncdtOrtwolcng1hy"·ctooes (fix.:uionand

"·a1hing) . A Hahiliocd !"i11t will las1 for \WO or th- ycan under normal

off1eccondi1ionJwithou1 apprcciah1cchangc, a11d for many ycan bd0n:thcprintbeccme1illcgiblc.

Stabilising agenu may form inlO!uhle or >0lublc corn pounds. The flrstha,.., 1hediu,d\'an1agcof lcaving all thcraid11al 1il'·erinthc cmul1ion 1ogi,•cpotctl!ialin11abi li1y,whi le 10lublccornpound1may diffutcpaMia!lyilltotheornbili1ingJ<>h1tionandintothepa1•crba>< :\lottO\·er, astaLiliocrforminga10h1bleeomplex "'illgi,·ca print "·hich can al any su!Mc1ucm tirne he fixed and \\'ashed 10 give apri.,1of normal11en""nence.

Ofchc stabilisingagems in use, pcrhal"' 1he mo<t important is hypo, which convcrlJ Che 1il"cr halide into argcn101hiooulphates, though alkali 1hio<:ya11.a1northiourcarn.aybeutcdasstabi liocrs. To photographen,who""UknowthatapcMnancntprimdcmandsthat allrcoidualarg<:ntothiooulpha1es1houldbethoroughlywuhedaway aftCTfixation, it may be surprising tolurn that hypoioutcd.:ua •tabiliocr. Let "'examine this apparent anomaly.

bc~c~:."'d::..~t7;~~hu~~;~ i::.:1~,:~ ~~~;:i~ causcdby tbcmorcn.piddccompoaitiouoftheJOdiumarg<:m<>lhio­•ulphat~. Th is sa!t dissociatn into todium iono and argcnt<>1hio­sulpha1e iono, and 1her<: is alooa ,•t:ry oma!! funho:rdiooo<ia1ion of •hcargnuochiotulpl"'le ionintos.i],..,rior>oand thiotulpha•c ions:

AglS,0,)~'- .. Ag .+ S(S,o,)o-

TllE SCIENCE o~· PllOTOG"APllr

Thtructi,·coil,..,rionsar.:the ca""'o( iiu1.abili1y,owingtothcir tendency IO react with atmospberichyd"'l!en1ulphideor "ithdc­oornpolli1ion produc:,. olthiooulphateo to form .;1,..,,. J11l phide. Small qwontiticso{argcntmhiooulphateinprinu""'uldbemuc:hmorc 11abltifdisooo:-iati.onl<>gi•·eoih·erionsC0<1Ldbe rtprCMcd. l thubttn mcntiontd {p. l27)that,ingcncral,chemicalrcactionsarcfa>'<>Urcd byhighconccntrati<>nsofrcact.anuan<lhindcrtdbyhighoonccnU"a· tionof1hcproducufonntd-aprinciplcknown,..thcLawof:l>I,..,, Ac1im1. T hedi>JOCiatio»ofargentothiooulphalelon imo1ilvcrion and1hi<>1ulph:tteioni>, thercforc,hinderc<lbyhigh«1nccn1ra1ion of1hiosulphftlt ion, and thcargentolhlosulphftlcwmplCJC iswn· ocque11dymuchmorest.able inthepr.,..nceofexceM 1hiosulphatc. Apl>01ographicma1eria!fixed,blotted anddriW withoutwasl>ing .. ·iUG01uain1heargentothiooulphate ... wellasalargeCJCcessof thiolulpha tt(hypo),andwillthercforcbcoonsiderablyrnorc"ablc lhan abadlywuhtdmatcrialfromwhichthcbulkofthehypohas beenrc....:wm.1h<>ughitisl>Qt,...._..bleasa""ll-washedmaterial

Clulpu>l'l

SENSITOMETRY

THl>O. UJl>CTolpr~and rcrotdpl.o.•ographyis tuproduceby chemical and php•cal means"' faithful a reproduction as 1"*'blcoft~original!ttne.Thep1ctori..lphotographcr,on

1he otheT hand, aims to produce a pleasing picture, and be may pt11]10K"ly emphasioc or •uppr'al portions ofa v;ew in order to oon'"Y -... pic1orial impr.,..;on . Such photognphic <Mml is prac~beca-theoriginalsubjectisnotoonsideredtobeidcal. OoulMLessifthepictorialphotognpherl'oundthat1ubjcc1,com­posi1ionandligh1ing""rcperfect,thenhisaimwouldbethcsame "' thatof1he press and rttO<d photognpher.

It "'OUld seem, therefore, that the ideal photogr.o.phic process would give accurate n:production ofa occnc. Normal b!ack·and­whitc ]ll.otogr.o.phy, l.owc-·cr, suffers at the ouuet from two very K rious limi!ations-it doe5 not reproduce colours, and since one lens produces one imllgc only, it doeo not reproduce 1he stereoscopic effec1obtait1tdbythcuscoftwocyes. ,\t best,1hercforc,aviewcan berccordedonlyasa«1ml""itionofneutn.l'1had.,'or'toncs' •uch umiglnbesecnbyaone-c)"'d.comple•dyoolo1><·blindman

WillUntheoclimitations,aperftttphotogtaphmaybedefinedascne rcproducingtoncsoothat thcybcarthesameproportional rclation­•hipthroughout ... in theoriginal. Thutinany•·jew1he tonevalue lyingexacdyhalf-waybetwttn!Mdarkc:uandlirh•tttlORCIShould berendcrcd ... thcexactmiddletoneofthephotograph.andc--.,ry 01herin tervalofioneshouldbeproperlyproponiom:d. Wi1hin limitoanincrco.scdordecrcascddegrccofcontr.utmaybcallo"·ed. but providing 1hat the i11tCt'Vals of tone value arc proponioned cxaet lya1in theoriginalvicw.then1hcphotogn.ph canbcdefoned

'"

T/1£ scH·xc£ OF l'llOTOCllAl'llr

.. .,.rfcc1.Dilfettntwncvalueoola\·~dilfttonlyinthcintcnsity olthelightwhich1heyrdltt10<cmit,and1heycanbtaccuratcly mcuurcd with a pholomcttt (p. 163).

lf-.ljacentarcuofa•tripofnormalnegati\·cmatcrialarcgi'·cna .. ria ofcxpo11ura1tarting from a very low vah•C and incrcuing prograiivclyoothatthccxposurct"CCcivcdbycachllc[>bca,...a conJlant•ali• t<>iunclghbour (c.g.,oothatead111cprettiva doublc1hccxpo1urcoflu prcdttcsoor),and if1hc1triplsthcn p~unifonnlyandnonnally,wcfmd lhal•"Cry"""Jlu. pooura ha,·cnoclfcct on thcmatcriaL- tl>at is, the density is not highcrthanthef•z produttdbydC'o·clopinganune>tpcoedponion. Afttt a cer1.ain critical cxpoourc, J,o,...,,, • ..,., 1uboequcn1 ncps ol incttalingcicpoourcprodoccprograai\"Clyincn:asingdcnsitiauntil avt;ryhigh,.,.lucisob1a.incd,rcpraentingvinuallythcfulldC'o"Clop­mcnt oh.II •he sih-erhalide, that;. tooay, ttptt>Cnting the maximum dcu1ityolwhich the material isaopablc. Hyplouing thcdcnsiti<s agairutcom:opondinglogf: •"lllucs,"·cob1ainacu.,."C.fig.I""°", a lypiulrtoeuh.

Suchacurv<:lsgcnerall)'!Cnncd thcckor<1<1triltU<11r0<0Tll<lnilD '~""of the material-the la nor from the initial• of the two pionecr

workcnin1ensitomc1ry, Huner and Drifficld. Thenia.in thing to

remember about the characteristic cun-.: ol pholographlc materials is that i1 is,..,charactaisticolthematcrial-it ischarac1crinicof1hc combination ol ma1crial, conditiolll ol '""""°"""' and amditiolll ol ~ng.Aphotographicma..,rial may ha•-ean inlini1cnumbttof dilfrttntcharacteri11iccu""COranging,on1hconchand,froma curveofhighdcl\1itywhlchwouldbeobtaincdifthema1crialwcrc lcftindc•·elopcro,·ernight,taauniforrn•crodcnsityob1aincdby ..ccidcntal dc\•clopmcnt in hypo <>n the other. lletwcen these two cxtrcma!ltecurvc.mayvary in>hapcand JIO'.'ition according to facto,...ofi:olour,ofhghtoomu,a,-.:ragelcvcluf1tlumim&tion,intcr­mittcntorcon1lnuouocxposurc,naturcofdevdopilljjagcnt,formula of dc,·clopcr oolution and time and temperature of de-·dopmcnt. A charac1cris1ic curve ol a material, therefore, h:u no uhimate oi-nifiatw:eunlaaexpoourcandJ><O"'S'ingconditiontarc accura1ely dcfil>cd. l norder,thcrdOr<:,togi•·cinlOnruotionon1hcbeha,,;our ola material in .-ma! tllC, exposure and procasing amdi1;.,.,s l11Cd inprcparing1heocnsitomctricHripfordetcrminingacharac1aistic cu.,.~mlllt appr<>ximateto thooicp<C'•a.illnginprac:tice.

For cumplc, a 11rip taken from an amateur roll film lh<M.tld be npoocdt<>lightapproxima1ingtodaylightincharac1eristict--nott<> 1ungstenllght-nd1hctimcofcxpoourc1houldbc:about-/ftce,thc variation in cicpooun: being obtained by ,.,.rying 1hc inten•it)'· l)cvclopmcntconditiorumunap[>roxima.<cto!locav.,,.gedC'o"Clop­mcnt conditiC>n1 UICd byamatcuri or by developing and prin1ing atablioh•ncnll. Obviously a •inglc chancteriotic curve will not rcprc1Cntthcbchaviourofthernatcrialunder1hevaryingcondi tiolll rnctwi1hinpractice,butitcanatleastrcprcscnti110&·tfdgthchaviour.

The charactcristiccun-e sh<>ws that the material begins'" respond toinctta•ingcxposurebygivingincttasedde1uityatpoint.4on1hc diagram,andcc:ucstor.,.pond<>nlywhenthccxpooul"Crcachcstbat

~.:;;'~~;,;i1~{.t:: .. !'J~==~~~~•=u;.:.'~~ logF.valucof,.-.l!"'-cr4.

A kit; ..:ale io a sc;o]c whut! equal intcrvalt rcpl"CIC1lt equal n1iol,andanin1ervalollog E - 4...,,...,..,,tsanitiobct"·ccnthc '"-ocx1ttmesolJO',oruposun:D' isM·crl0,0001lmcsth.otoftl" This ii by no mcalll ntrcme-many ncgati,.., materials ha•-e an C>1potU1"Crll ngcofthisordcr (thoughoinoclhchighco1den•i1lcsarc Mt normally used, published cu'""'° may not indicate 1hc fact). l-tl>\\"CVCr, bc:f<>nl discu,.;ng the information !lt.0t '"-""be: gained from a charac1criS1ic curve, let us >Ce hO\~ we IC< oboul producin)!

1w Tllli sc11:xc11: a~· PllOTQGlb1P11r

TlleSu1ito111eter

:~:-:;:n:~; .:.:::·;=:::i:r::~:.:ly k..;: i:..:i reproduciblcc:.poou,..,., prcferablyoo 1h.o1 c.o.ch bcar1 thci&m< n.tio to iu neiKhbour. A n.1io of 'V2 is commonly chotcn.

is ~~ ~ =.:~1:1 :1o': ~~~~a":'uur:":t:'J'i~;I t::"~:~~~.fy'~;;:: bratcd. lluncran<l O riffw:!d,oftcn known a• 1hcfathcr1ofS<n•i­tomctry, ulCtl aoiandard candle, a"d both benzene and acetylene

:~::rr ... :~·c ~=~;.,'7!1f~' :~~,~~~;;~r ·.;:~:~,ra~-;";1::~·;n;1~::;~; lam pt, runataco11St~ntvoltagc,usuallyfron> accu mulator1.

Sincethcrcla1i,·c1~sofmatcrial1ofdiffcrentcolourw:nsi1i­'"itictwill'"'ry':·ith thccolourofthcCJtpo1inglight,wc1•n1S1 uu:a.

~~=~~:::;:n::~ ~~h!:ify":~·i:~ ':::,=:.: tunptcn filament lamps, should bcapoocd 10 a oourttofcoloor lcmpcr~turc Z,800°K. 1'\cgati\'C ma1crialo used IOr ou1door onapohot apoourctohouldbcc:.po!Cdooaooura:ofcolourtemperaturc

~i!?!:~·~;::;:. .·~~ ::.:~i,:.:. ~";!;~~~:i:.-: :.C~".,~~ ounlight 11obtained by wing a lamp run accurately at a colour

:;;,;ra~u:cni~.~!°a0

~~:~ fi~:~r ~:'..... ~~):~~~~:~,.~~ c1en1lyrcprOOuc1blc,ahquidfihcroon1i1tingoftwo sol u1ions (bag,,donnformulae•'Oh·cdbyDavisa11d Gibson) is ulCtl

hpaaur•Madulotlon

Early .. nsil°""'ttn employed a time ..,.._le modulalor wh>eh took

~:~;:;1;.r:e c;~t:~:.::n~.di:~~~~~~ ~~ ~(::!~ maten~I rcai,·ed a. Sttict ol i~terminent apoourct. This ... as to minim>se the effect of long-period ~ariability in the Jigh1 soun:ct thena.,-ailable.H01<'C'-er,anintermincntapo1ureeani1>elfintro­duce•-ariatioru (- p.rot).\\"ith•heavailabili1yofmo"'oonstam lightsoun:a,it .. -aspoaibletoopcra1e1hc..,.torootha1awhole *"rict of C>tpoourct could be e>btained "~lh a lingle m'Olution. h\llrumcnuof thittypcarcstillinuse.

Time-ocalc .. mitome1er1 do 001, howc,•er, reproduce normal cxpoeurcina<:amera.'wherc,foragiven11cgativc,dcnsitydiffettn­caared1e1"C1uhofd1fferencesinthein(cnsityofligluf3llingonthe

m:U<:rial. l11usinga timc-tc.o.le,iti•p<OOlibletointroducevaria tiono ductothefailureolthereciproci1ylaw(SHp. 199). T hw,inreccnt yrar1 it has become lhe practice to""' an intcnlit)·-..:ak ......;. 1omc1cr IOr nepti,·e materials. Differences in intensity arc e>btai»Cd by placing the material in contact ""ith a step tablet. Th.etc a.re made byautinga di'f>Cl"lionofcarboningelat.inandrac:mbleinappcar­•nceFig.3.

11w:c:.poo.urc time with an intensil)"-tcalc .. 111i1omc1eri1 usually hascdonthea\'cr:ag<:u>edwitbaca.mer:a-usuallybetwttn\1fand 1j 0occ. How.,,·er, for routine .1oensi tomc1ry, ouch u the batch-to­batch1cstingofS<nsiti•-eproducts,atime-scale1enti1omc1crcanbe cql!nllycffecti'"'·

Othcrc:ooditioru,bctidcs tlioo<!ofillumination.ha•-elobctakcn into acco.mo. Tiw: densi1y produad may be affected by Ilic tern· pcmtutt and humidity of the matcrial :u 1he t.imcofnpoourc and.on the inl..,...cal bet"'""" c:.poourc and da"Clopmenl. Proo:asing con· ditiorumus1besimilarlystandardisedinl"ClpccttOll1ccompooition and tcmpcr•turcofthed"'-eloperandmethodofda-elopmcnt,and

~~~~~~:~:.~!~;~E~u~E~~~!2::~;:~~·~;~~!~~:;~2:

T/16 SC / ElfCE Of. J'/IOTQ(ill.APll/"

m.o.rcriab(Britis!>S•andard 1300: l'an l :l962),orlhcAmcrican C<[ui,.oknt puhhshcd ht• rhc Arncrian Srandud• AlllOCiatOo.,.

Th1 M1uu1rn1ntof D1nsitJ

T he blackness of a gi•·cn sil•·cr dcpo$il in a ney~rivc ma)" be rru:asuo:Jin1"·0wayo. lnoncthcrarioofthcincidcn1lightro1ha1 !rans~nn!ed b! thc_dc~<t is mea•utt<l and i•called 1he 01'3Ci1y. _lf l0 !S thcu1.c1dcm11n ensn yand l tha t 1ran•rn it1 cdd1cn theoJ>3Ci<y 1t !J I. T hiS!S t>Otagoodchoice ifit is theafftaT/VltlOf pri ntswhich io '"'P'"'~"' bttau&c C<[ual incrcmcmo in opaci <y do nor appear

~:~~ly •:.t:.,~)"~j.., ~:;,1..;:•c~n~:,::.;~~';,'!c "!:.~;::1.1,0 ;!"~1~:i: uni•·cn.ally 1ncasutt<lhylog<>paci<)",callcddioJi1,J.

=~- =~"- = ~ 1=-;k I= - ;clc, I= ~ -- - - -- - - -- - - -Opadoy O.noity Q.u•n1ltyoil"'' !k"funh•«• - lq

100 Op.cl<r - 1.000 ~ u.,,.;,r - '

Quan1ilr•i1,..,, ~q p<r unil •«• - lq

.-\•il•-c.dcpoo.i11ransmiuing,1,o<11>ciocidcntintcnsitywillha•-c anopacityofl 7>'0,that;,,,anopacityoflO. Tht:dcru.i1)",Whichis losopacitf,""ill1hettforebot l,sincclO - lO•. tr1 .... ..,•uchd<:poo.i,. ~re.•upnimpwcd, the light incidcn1 nn the teOOl>d dcpooit is>'• that

:::i~t,";,,";r ~~~ :;','1;:;!t~7; i:.8:~;~:·~~\:~c;.,::;b;:;:,=.';!:c";,;:; opaci1yofthccombina1ion is thcrcfnrclOO,and thc dc111ityi•'l,1incc 100- 101. Simi larlythc densityofacombinationof th,..,couchdc­l>Olli1tlo3,and oincc thc amoun1 0hil'•erpcr uo1i1 • ,..,,. it 1hc oa me in

each elemn\\oflhcomtl>ina1ion,tl.etotal a moomorsikcrpcrunit area in:ln)·combinatOonis1h.,.,...,tiaUrdirecdypr-opor1ional 101hc dcmi1y. l npracticcthis,..,latiomhip isafl"ec1cdby ou.ncr oflight bc'"''"""'hegr.sint, buthold•O\'crafairlywidcrangt:. "11..,..,,..,]a­tionohipo bct""'ttn opacity, dcnsit )" &"d 'i l•·cr pcr unit""'"""' ilhlllrMeddiagnmmaticallyin t"ig.<l.

Th1 Den1ltorn1t1r

All)" i1111rmncm which i• capable of mca>uring li~lu in1Cns itico could he used formcuuringthcopaci1yofama1crial, andhc11ccits 01>1ical dc111ity. An irutru mcnt which mcasureo light intensity;, callcdap/tol011Ulna ndot><:applicdspttificallytOll><:mcasuremcnt ofphotasraphicdcnsitietis kno"·""' a""'''...,'"· li.lootphoto· mctcrifunc•;,.,>bycomparingthein1cnoi1ytobcmcamred"·i1h1hc imcnsi1yofiUuminatio11froma11andardooo""'ofk"o""nou1put 11ocla11.,. intcnsityiladju.t<d (1HW0llybyadjusting1l><:distancco< !he 11andanl ooora:) until the """ in1cnsi1iet uac1lr match; the dis1a .. cco( thcotandard lamp and itsandlcpo"'C'l llocngiveta mcaourco<1hc in1cnsity nfthc unkno .. ·n100n:c in 1nct.-.:-candlet. Thcpoi11!ofe<[uali1yo(11>ctwointc11si!ieti1ob1aincdby1hcU1Co(a pltoln1<1"/u""-a pie<:cofappannuowhich accura1cly indicata whcnl""Ohuninouointcnsiticsaree<[ual.Thcrearc""'cralfomuof pl1<J!Omc1crhcads,butinordcr toind icatcrhcprinciplc11ponwhich 1hcyallwork,wcohallco11Sider on ly thelllOftCOnvc11ientand most cornmonlyl!led - thc Lummer-Jlrodh unculoc.

TllE SCI E XCl; QF l'llOTQCll.,f.PllT

face,,.·coceligh1 .. ·hiehpasoesnormallyfrom 1hc oppoo;1c<idc of" th.e cubc,accpt in thca:n11<: (s.ikc<e<i) area, " "hich ttlke1tlight coming atright angla. Irttw:twoluminositicsareadjwtcdun1il1hca:nttt palchdi1apptars,1hcnt'.-telightfromthcccntreattaisof"1he1amc in1c,..ityas 1ha1fro m1h:surround.

Ollllllllllll)![llllm

N.,...ifwc"-crc1oea.1bctwttnl""'ligh1lyindin.c<lgl;usplatcsa "'cdgcofli11cly divid<>:llamp-blaclr. 111lpcnd<>:iinl"la1in (Fig.6.o), "'""'""kl OOtainagnduatcdwcdgc\'3.ryingindcns.i1yfromuroa1 011ccnd1oahighdc,.;1yattheothcr (f ig.6t). Dy mcasuringonan accunuc photometcr,dcns.i1icsa1 specific poin1talong the ,."Cdgc, 1ht:1ewuldbcplottcdagains1dis1anu:along 1hewcdgc,and1hc ~"Cdsccalibratcd 00 1h.a1 thedens.ityatanydiHat1ccalongthe ,. . .,dgc 1J know11,

,\ sradu.:ucd "-edge (which may altcrnati•·cly be made by ca1<:fuJ upo1u1<: a1>ddC>'Ciopmentofaphotogn.phkmalcrial)isollcnuocd ind>cW1mructionofdcm.itomc1en..Th1»iflightfromacoiutant ligh' """rcc folio"~ t""<>pathsofequi\'3.lcnt length wa photometer head (Fig. 7),1hee<:ntrala1<:awillbcim•isiblc.lfanunknowndcn­s.i1y is now inserted in one path, and thecalibratcd "'edge in the otherpar1.d1cp<Ailionofthewcdgecanbeadj11ttcdun tilthecentra!

attaof thchcadisagainin•is.iblc.Tioet""irucncddcns.itie1mU>t nowbce<1ual,andcanbcttaddirtctly from aoe:alcohowingthc dis1a1..,.,cl tlw: lightbea m along thc""ulge.

Ana•.h'an1agcof"this 1ypeclimtrurncn1is1ha1,provid<>:ltlw:light oour<:c is rc.uonably constant, its ou1put need not be accura1dy known, s.incc tlw: samc balance point would be obtained i1>dcpen­dcn tlyof thecai>d lcpowcrofthelamp.

WhilcAvi1ua ldcn1i1omc1cr is lllefolror <>e("'ional""Ork, i11rou1inc iru11ruonentothepho1omc\crheadi1 rcplaccdbyaphoto-elec1ricccll "'hich on illumination generates" Cun'Cnt varying with the light in tcn•i1y.A shuucrdcvicei•fit1cdoothatlightfromci1hcr pa1h1nay illumi11a1cthcccllatwill,and thcpooi tion ofthe"·cdg<:i•adju>1ed oo <ha11hcdctlcctionofanammeternccdleistl1coamcforuchp:uh. Today au1oma1ic recording pbolo-clcctric dcn1itomete., are U>cd forlarge-ocalcsc111i 1omc1ry.

P1r1ll1landDlfruH Den1itl1S

The measurement of the dcns.ity ofa developed sil•·Cl" image is com· plica1cdbydococat<criugcl oome of thc1ra1Umi11cdligh1.Thati1to .ay, .. ·hcn 1>arallcllightfalbnormallyon1othcimagcoomeof1hc 1ransmit1cdlighti1dC>·iatedside>.-:a),andp1nepnxtt<l1ina i>ormaldin:ction(•'ig.8).

]f,.-cmcasureon lytlw: 1ransrni1tcd. !-.lltlligh1,-shallbc mcasuringltutn.nsmi tte<llight1hanifwcrncastlre thc"""'1Uof thc transmincdliglu,and h<:nccthcopac:ilyanddcns.ityinthcfonncr willbcgtta tcr.

l~nsi1ybascdonparallcl trammi tte<ll ightmcasurcmen1 alon~is ca lleds/lfflt/••"""i1JMDp.tr<tlkl (D l!) ,andi1gttatcr1handcm1ty ba$cdonthewholcd ifl"llle lighttrammiMion,ca llcd /~<.~igor DJiJ.,, {D J(), One mayuk which is \he s.ign ifica11t density 1n 1he

~~~~:1~~ r~~=r l~~:;:~:~ .. ~,;~a~-i~~. ~~\~;:~;.,~'~,,1~:.~:~

TUE SC IENCE o~· rllOTQGAAl'/11"

,..ith du: negative in a pnnting frame, thcwholcof1hc1raiumiued lightilused,anddiffuscdcruityi$thcopcrati~dcruity.Ahern..-1i,..,ly,ihhcyarcpnn1edinacondc,....rrnlargcr,vinuallyonlythc parallel l""n$mittcd ligh1 will bcuoed in prin1ing, 1hcaid"""'l'l­scaucrcdligh1bcingkio1,00 1hat•pcculardcnsi1yioopc""'ivc.Sincc 1pecul<lrdcnlityisgreatcrthandiffwcdcmi1y,ancgal>-·c1hould, tl1eo.-c1ically,yicklanimageofgrcateroon1rast,..hcnU>ingacon­densercnlargcr1hanwhcncontac1 pn11ting;thi1cfl'cctdot:1infact •riseinpracticcthough itcan1o oomcextcntl>crcducedbyt!attin thcop1ical 1ystem.

Oifl'useJeruitycanbcmeaourcdcq11allywcUintwnways.Onci• thatalrcadytlcscnl>cd,byilluminatingthcdcnti1ybyparallcllight, and mc;uuringthcwholcofthc \r.uJSmitled ligl11; and !11cothcr, morcc<>n,..,nicntinpr-acticc, is toilluminatcbycornplctclydiffuse ligln ,and mca1urctl..,parallclcomponcntof1!"'traiumiuedlight. Illumination bydiffuseligh1cane.uilybeachjc,,'cd by placing the dciu.ityiHcon1aCtwi1hanopticaldiffuscr tuchHopalgla-.Adif. fu..ingcnlargcrthwmakauocofdiffuocdcnti1y•,.Juaofthcncg;o­ti•..,. Thil ,..,Jue;, the one t>0nnally me.uurcd in "'"'itomcu"ic prac-1icc b1· fi11ing an npal gbus at 1hc point a1 which thc dcruity to bc measured ii introduced .

AnOJ1-difl'U>ingdcmi1y""OUldgi,..,noocattcr,and•pecu!arand difl'usedciul1iawouldthcrcfo<cha••cthcsamcva!uc;and 1hcmorc sea11cring the dcmity, 1hc gTUt<r thc .d ifl'crcncc _bch,·ccn these '"'lua. Thcra1iobct,.·ecnspccularandd1ffuseder1si" ct"callcd the C41/i,. totffidml, which incrc;ucs with increasing Kalter of the image. Asscaucrina photogrophicim.age i• afunc1ior1 ofgro;,, si•c, the Callicroodlicient has been proposed ata mcansofmeasuring 1hc grainincu of an image. Ho"'C<'Cr, "'her factors in1crferc and 1hc propooalh:llbccndisc:udedasunrcliablc.

hfl1cU01Dtn1it1

w11 ......... rnakc•positi•·cpnn•froma1"'1Pti•..,,1helattcrilillumin­a1ed unifunnlyand thctr-an$min<'d lightfalbon to 1he printing papcr.l)iffcrcntpor1iomof1hcpapcrrttei,·cdiffcn:n1expooura, bccause1l>cyn:cci•"dlfTetto>1lighti111cmitin,acon<ding10thc tr:amm;,,;,,,, dcruiiics of corresponding i-•ions of the ncgati•'C. On proc ... ingthc paper, therefore, pnmdcnsity•·ana in'""™'Jy "·i1h1ha1of1hcncga1i•'C,<0 thattheµrintilancgati•..,rccordof thcncga1;,..,,or apositi•·eof 1heoriginal1ubjcct.

T he processed print, ho1>·cvcr, is examined 1mderrclkc«:d light and wc 1ec lightandshadcbccauscthe silverdcpoti1it1terfcranot

.. ioh 1raiumi,.ion but ,.;,h 1hc rcHec1;,,,,. The chanc1cristin of

~§:3~~~E~~~I~~:~;2"'~~; natio,.allyagrocdthat1hcpapcrdcntitytobc.'nc"'urcd 1hall bc

~~~~:~·~.~~:::;;i~;.:::i;;~:~~:~!~:~:;:1~J~~r~:: ~:3~~1l;~£~';·!~~:~~~~.:~:~=.~~;:}~2~:~:~~!~;~1?.ci1; ""Claw that:

T..__,,.,;,,lon <l<nM1y - las ~d1y - las ( U~==:eai:.1<.::.:~)

Or••lr "'" «1..alo: las 1 .. -~:":..:::,:;'tj:'t:;,;r;.:;•.:>. ... .w

By.:::: ....... ., -las ("1....!~r:t:.t"irota?::;·:.,z::,..j} "' "'""''ion <l<nM1y - las (U.1<~:~...:t::.ir..: ~j.:;-l

li.i Tllli. SC/li.NC li. O f . 1'110'/"0C11Al'llf

,.,.rhHltJlange

£~~!~£::1;~]~£~.~~~~E cen1.of1ha1n:fl . ..:1edbyaplainp;opcri.a..,,0011om:1t1crhowhe•"'Y

:~~,:;::, .. ;~~~2:~~:;=~·:;•'~£~1·~J.:):~l~;::c~"~~ ""~~~~;:~~~i:,;;:7i~!:~::~:\~~,l~'.~ ~;:~"~;~,.~~~~ .. =~ ~ p;o_in_l incn:aoed umil virtually all the light i>cxcl u<led. The 1ran1-

[~~~~~~~~~~a.Ji~~&?~::~f:~a;~ any blacker.

1~11:1~~7..,::c;_;:~~"fn:~:u:i:,.:;~..: many]"""; Jn general, gkmy_p;open ha,·ca higher muimum density 1hanlCDl•·m>.Upapcn,wh1ehi11mmgi••chighcrdcrQ.itics1han ma1tpapen,wi..:.cmaximumdcnsi1ymo.ybc:ulowul4,corrcs­~l<lingtor.mgcofrcft..:1edin1crui1i.,. (highlight 101ha<low) of l ·

lfa<lja«11Larcuofap.--intingp;opcraregivc11a"'ricsofincrcas-

:~;~~:r.r:~~~~~::~~~~~~:~~~~~~~;J~~~~~~~~a:~

Cilqln !1

THE CHARACTERISTIC CURVE AND ITS INTERPRETATION: TONE REPRODUCTION

Subject Rana:•

T//f; Ul.~/l ... CTli.RIST/C Ct.'Rl' /i

C1meraEllposuNs n d th1 Cllaract1ristlc Cun1 lf1hcapo1u...,ilt<>•mall thatC">..,,,1hchighlighudo1101rctti'..:an npo1u...,a1grcatas thatttpr...,n1MbyA'in •·ig. l,J"'gcl!>8.d"'n thcwholcof1hcitttionoflog£1·5,rcprcscmingthc..,.poou...,rangc nfthcca.menim.o.gc,will lictothelcftofA'. "l1>ern:ga1i•·c""OO!ld t.>c practicallydcarfilrn,and•ho"·notraccofan}'inlagc.

" >

~ -

lld•il· .... _.... - •

Onincreasingtl..,uposurc,thelogEintcr«ptwillrno•·etothc rigln.andwc•hall rcachapoint"'hcnd1Chighlighufalltothcrigln of A", while the shad"""' remain on the left of A' (•"ig. l•). "O.. •hado1<-.of ol1Cnqi:a1i•-.:"illbcrcprtSC11tcda1cltarfilm.and 1hc ba1priU1ol>tainahlcfrom>UChancga1i•..:i•as"'°"·ninthc6gurc.

\\"hcn1hc..,.poiurc;.incrc.ucdootha11hcdarkts1•hado""rctti•..: ancxpoourc'"'lu.altoA",t~willclcatlybc differcn1ia1ion1hrough ·

outthewholcofthcncgati,..,,andtl>ebestprin1froonthcncg•1i•·c ""illbcao•hownin Fig. !b.

\\"i1hprogrcssi•·einc...,a>eincamen.cxposurc,thev:alu<:<<>fcx-

~::"i~l~;~l~c~f~;"1~~~~:i~h f~~~!.;,;~, ' 1:~:~1~h~::-.:;.~g;'~~ ~righlnts•. 111<n•cs l'"""'""i'·ely to the right ~long lhc 10)( f: axis.

lnc,..,...ingcamenexrc-urc:f willgi,..,no:gativcowhichincttascin averagedcnoity, hut while 1hcwholcofthe intercept licolletween A'andD',thercwillhcRcotnpletctJilTorcn\ia1io11indcn•it)" throughout the whole oul~<:<:t range, and the \Jest prin" from the ncga1;, . .,,..;n1how1urpri1inglylinlcvaria1ion in quality (figs. I< and J). Only ll"hen m·er-CXJIOl\!rc i• oo gl"O$I that the highlight exposu,.., extend• heyond /)'will 1herellea11 ab!cr1ccofde1ail in the higblightportionufthe\•icw.

Thclimil0of 1hc111efulrnngcofe>omcraexpo$urcarc1hu•T<:pT<:· ''"'"'d Ly the '"·ocxrc-u""' shown in f ig.. lb and d. ·nu,se ""' displaced by a log Ii <fota11"" uf ~ - I ·~ ... 2-~ (;u "·ill he tcen hy com­paring the position• of corrcspo11di 11g pa.ru, say 1hc highlighu). This con..,.pcmds to a ratio of 300 : l, which means II.at camera eXpo$Ul'Clextcndingfl\"Ct approiirtUttcly1hi1rangccangi'·caccept· able photographic ""'uho with long scale ncgati,·c material. Thi• isusually,..,f....-..J1out11Jmllrt/~1i1-.

Correetllpo1111r1

It .,·ill lie_,, fmm fig. I, O>ap. 12. th.at a typical no:gati•·c c~r~c­tcriltic curvc c:onsisu of thr« m;tin poniono-thc cuf"\"ul port>0n at 1hel0oc,AH,1he11r:aightlincporiion,BC,andthccu""ulohould<T, CD. \\"hcnd<tUit1•is dirtttlyproportional1ologE,thcn:lationship

Tllf; Cl/,tH,iCTKll.1$1"/C CUHl'I:

i1•fio.,·non 1hc gnph as a1traigh1 line, oo !I.at thcttraight line portion, IJ_C, rcpretcnts the rattgcll"i th!n which camera otpo$Ul'CI •houJdlicmordertoobtainJ>n/"''oq."''"'•acconlingtc•ourdcfini­'ion. This~ many. eady tc"'itotnc1ric apcru to ad'-ocatc camera CXJXl"Ul'Clw1thm1ln•r:a11gc.Thcpor1ionABwas1an>e<1 thcl'Nkt­rxpostut pori,ion, RC d>e -•NI u:,,._. porti.ot>, a nd CD the ....,.. ur-•po•tlon.

Thi< a.h-~ is llOUnd iftl>e objut ii 10 produ"" a perkct no:gati,·c, but a nqpu,·c U mcttly a me>ns lo •n md-"1c production o( a pcrftt1poo.iti•·e,us .. a llyintl>el0rmofaprin1onpaper.1 ..... ·illbe mown late•, how~·.,.., ~he charactcrUtia "''!"'printing paper intro­dutt incvitahlc dntort1on in 1onc rqinxluct0011, oo tl1at ll1C produc-1ionofaP?"kctno:ga1i_,..,docs10Hresuhinapcrfn:1poo.iti,·cprint. Thcrcarcmfactdcfonlcdisluh-amagcsincamcr.ocxposuresinthc region HCcomparod "'itl1,!R. In 1l>efirstplaccthccxJXl"Urt:Sarc gn:in<:r than arc '""''"""'Y• ll"hich mca1u tl>at ll1Ce1Tn:ti,-c1pccd o( 1hc ma1cria.l ii dt:'On:axd. Secondly, t!>e l'"""ularity of ncgalives expoocdinthcrqionHCi1gre~tcr1ha11thatof111T-1t!'""""pooedin 1hcrcgionAR.t"mal!y,ano:ga1"·ccxpooedin1hcrcgoon/JC,001hat 1hcd_ensi1icifa_ll on 1hc t1raightlincportionofthcricgati'..,char.11c­tcriS\lccuf"\..,gl\·aoomcwhat1t.~H tOnercproductionon1hcJXl"iti•·c >inot:dist<>ttionofdicncgali,·cintrodu~lbythecun·odportion.-lH

tooomccxtcnt'°"""""llthcdi>1or1ioni111rod"ccdbythcdrnrac1tr· iotiaofdic posl1h·cmatcria1.

In •hon, the •ftlim<J! camera ""I'°"""' i• the minimMm cxrc-urc ""hichwillgi,·cdctailinthose1l"'dOw•whc,..,de1ailisrequircd.1.l1at is to say, camera cxrc-tm: should lie 1uch that s~ath,w cxrc-urc corrC1pcmd110,orio1lightlygttatcrthan.poin1.-l'.

lntt rpr1t1tlonafth1 Charactarl1tlcCune

lfdtc CXJXl"ing a11d pmccssing co11ditio11• uM:<l in determining the charactcrUtic cun.., ofa material arc 1imibr 10 dlOIC "'hich the m:uerialcxpcricncesinprac1icaluM:,thcn1hccharactcritticcu"·c lhouldgi,·cal'""pl'W:alrc1>rcsct1tationof1hcprac1W:alph<Mographic beh.aviourof thcmaterial,andit sl1oOUklhcpoM.iblc 1odedu""from it numerical valuco IOr •uch l'f'Ol"'rtin u rog, •peed, contrast, 1naximumdcmityandexpo1urcr.11n({C. F•l• l'ril: f og is tl>ed(mityobtainod on a matcrialdC<o..,lopcd "~lhout CXJXl"U,..,, and it is common pnctice I<> J>">"''" one ""P of •hcl(tllitometric11ripofrna1crialfl'()rrlUJXl"Urcfotthismca>urc­mmt.Fogvtt.lucis,ofroursc,cqualto1hcc:ons1antdaui1yofthat

"fill: SC/EXCf." Of" PllOTOGR,ol'l/T

the ll.oma11 C). Gammo., thcslopeolthc otraight line portio11, is mcasurcdbycontinuingthc>tr-aight lincportionun1ililmttt>thc 1og£uis,and1hcndi•·i<1ing <hehcijl:ht atanypoi11t•,b1• 1heimer­ttpl t. The ratio•/• is COO\lla..C, no m>.tter " 'here the rnasurerncnb arelakcn ,a,,d this>11h>eiscluorac1cristicoftl>e a"glcO,a11dcallcd the ,.,,,,_1or 1he a ngl~,0< "'~ 0 ( t\g. 4).

fi~1t",:,~;:i~ 1i ~;~~·~ ;:':!~~.:.:":r :1:'.~~':,'.~~c·.~7Y ";~ti~ cxpc>1urehasbcen.,. rull1hatthewholcrangt:ofdc1"'"""lin011 1hc11raisht li11c portion.

In ge nera l, docoptimurn cxpo1urc lies lhuliyon lhc foot of thc cn,...·cwhcred1c1lopcislower,.,.thatthca\"Cragct1cga1i\"ccontras! is lower d1on 1he gamma ,•aluc. The a•·cragc oomraot of a neptivc, altoknownas1li<:td1llr<Ulinbx, 1naybcmc:uurcdbydi••iding thc dc,..i1yrangcbytli<:log£ra11gcofthcoubjcc1;thisis4:<1u.>ltothc slopcofthcdotted li11e ( f ig . 4) joi11ins thclowcsttnthc highest ncg..,ivedco»itics.

lnoometypcsol"mk,thcwa)"inwhichthccotUl'2ltofamo.1crial increasa"·ith 1i""'ofdC>·clopmcntisimportantandcanbecalcu· latcdfromafamilyofcbaractcristic cul"\"CJol thcmo.tcrialrep..,. ocn1ing a oeria ofd.,,·cloprncnt 1in>eo (f ig.!>). It is apresoed graph~Uyby plo1tingth.cgammaofcach_cur>~againot 1!1Coor• ~pondmg d<:\·clopment l!m<. The 10,,,,,,..1,,,.. CUl"\"C, obtamcd in 1lu1 way,isohow11i11 fig.6

SpH d

l'erhap1no aspc<:tol ocrui tomctryhaogi>·c11rioctomorcdiocuSlion andcont<0>·.cny than.'hcdcterminationofphotographic1pttd.11lc charu1erisuccu......,g"..,. 'hccomplc1c 1toryof •hcrclationbct"·""" denoity a11dupoourebragi,-cnphologroiphiemo.1crial,proa:sacd i11atcnainway,and1hrrcfon:itshouldbc ix-o'blc1olOnnanumcri­calntima1coltpecdbythccorrtt1 intttpretationofthceurvc(i{ indeed photOgBphic 1pca:i can be adcquatdy aprukd by a ling le number). Thil ii generally agrced--<lisagrcemcnt has been C(ll'I·

oemcdwith themethodolC>-aluatingaJpt«l ""..Hrfrom thceur>·c. Jn ploo1ographiepncliccspttd isaaaocd in 1cmuofthcmini­

mumcamcracxpoourcwhichwillgi'·c&dcqualedctailinthe1had· ow1, and "'• h••·e ""'" 1hat at this minimum camera cxpooure, the lowcstvalucin1hclog£rangcofthcoubj«:<willlicin1heroo1or1he cul"\·c.Ourforst11epis,1hercfort,todcfone.,.mc 1ignifica111pointin thcfootof!l1echarncteristieeurvc"·hichcanbc~asabasis from whichaccuratc1pccdnu mbcn{andcoruoequcnllycamcraapooure1) can be calculated; thewholediff1"ultyhashi thcrto b«nconccmcd withattcmputoderi\•csuchaspttdcriterionfromlli<:cbarac1eris1ic cur>·c. lrlli<: cluoractcrUtic curvn of all materiab had d >e "'me lhapc,butdiffercdonlyinpoaitiononthelog£axis, 1hcrcwouldbc no diffic.uhy i11 fixing con-csponding poinu suitable for 1pttd mcasurc1ncn1. HO>l"C>"tt, different mo.tcriab gi>~ cbaractcrinic eur>'Cldifl"cringinlhapcas"-cllaspoai1ion,andbccauscof1bapc diffcrcncu, poinuchoocn according to different eri1eria gi>-c dir­fcrent rclati,.., opttdo. A briefrcvicwolmcthod1 ,.•hich have been uscdinthepanisintcre1ting.

&/wWtSJNN.- l 'he&.heintt spttdwasbaocdon thelogCllpoour<: ,.,.JuewhichC<>1Te.JI01ldcd tothcfirs1jug1pcrccptibleimagc (point S, Fig. 7). a11d1hederivcdspttdfigurcswcrcupresoedonalog· arithmic llCR le. Thi s system"'"'" moot unsatiifaetory-apart from the

T11£ SClliNCf: ()I' l'llOroGR.H"/11"

imp<>O'ibility offuting with any accuracy the point a< which the curvc.11""' .. iu_fi?' pettcp<iblerise, 1hi1 point hu norcal1ignifi­cancemdetcrm11ung1pecd,1ince 1h"oon•r'2ttat1hi1point1Stoolow tc>diff.,rcnliaicsatilfact01"ilybc1"""""diffcrcn11on"'-.!"""inaview. Ahhough1hi11ugga1.,.J.,ritcrionof1oWsiti,·i1y .. ·asunsat;.fact"'1'for 1hcscicnU.t andph.otosraphcr,i1"-...agodlt:1ldto 11M:saltsman. lf it is difficult to d"tcrminc a 1peed number with accuracy, it is tqu.ally difficult to disp"""" an uaggcnu.,.J daim, and the sky­rocletingofScheincrsp«d numbcnon the Continent ahou1 thc 1930'1.,-...ducto thcirdct"1"minuion in thcadxcni>ingdep.o.rt­met11s,ra1hcr1haninthclabor•tory.Lll<e:aUinHat<:dcurrcncy,1t..: Scheiner 1pecd numbers l>«amc wonhl<St, and the prcocnt ,... u lledSchcincopccd1tuo'"""°'hi11110dowith1hcoriginalcri1crion - they arc simply oompk:tdy un1pecif>ed ligutt1 (prob;ibly roughly dclcnnincd by pnctical photographic <<Sii) pro>id<d :ll guidts to COl'T"tCI expoturc. T he original Scheiner1ystcm isquitcdd"unct .

11 /

~ l/. aNID.S}wo'.-Thccri1.,,.ioi1mggts1cdby~lur<erandOriff>eld

was a function of the log E ,•alucwl>crc <he oontinuat;..n of the Jtn.ightlincportionofthechan.ctcristiccu r\"cmct thelogEaxis (pointi,Fig.7).Thiscri1crionhasbcenfoundtogi,-.,fabcindica­tion of 1peed1 for modem usage of modem materials and th<:<don: the published H. and 0. figutt1 ha,.., littkor no ttlation to du: original H. and D. method. "11'ey 1uffcttd slight inftation in this oownry,bu1onthcOon1incn11hcir1pecdfigutt1i"""""""'a t ar.1tt oomp.o.n.blcwi1h1hcSchcinerinftation.Thisproduo<dthcphcnom­enon on the Bri1i1h marl<e:t ol impor•cd Continental films ha'ing a n H. and 0. film 1pcul of a different (a11d much higher) order from

"///~: CllAllACrliR/$1"/C CIJlll'li

thatof1imilarBrin1hfilmo,10tha1photographer1badt<>distingui•h bc'"""""t""Oly>1c•no-d>Coo-call.,.J Britisha11dComincmal II.and D.figurCL

DINS,...t.-The fi,..1cri1crion forsp«d determination "'hich could bcfix.,.J with oomp.o.rali\"C..:curacyand whichga,·cttSul11 morcclosclyrclatcdlopn.ctical1peed"""th.aladop1cdinthe Gem1anD l :'<1yo1cminl93l,llK>ughithadbccnuscdfor~)·carS

ptt'"io<,.l1·intl.a1rount'1·· Aeoordingtothis1) .. <em,sp«d"-...de­ri,"Cdfrom 11M:ctpoturcrequittd1Qgi'-cadtn.OtyO.labm..:thelOg l"'-cl (.OO..·n,..pointf>, l'ig.?). 11'iscritcrionisapn.cticalone,u it indica1ts a poin< on the cluorac1cristic cun·c at or near which dcnlitybcgimtoriscatar.1lc1ivingusdulphotographicdilf..,..,.,1-'ialionoftonc'cal""'. 11'iscri1crionha1nowbcenmodificdsomc­""lia1 in •I'""""""' with the A"""rican Stan<brdo Association and Britisl1Starw:l.udo l n1titule,occp.l80.

Mi•;., • ., Uuftd G1tldit•I CriurM.-h """ """'lniscd, 00"'"'""'' 1hatthe1nu:critC1"ioo>OftonedilfC1"entia1ioni1oncof< .. t.U (1hat is, sl.,,of1hecun•c) and no1ol Jt1Ui1J,10tl.atan ahcrnativcmcthod which has~ theoretical merit """baled on the exposure co...,._ ponding to that po.rt ol1hc cun-c which has a minimum useful grad­ient. The value of this minimum 1lopo was 1akc11 as 0·2. and the oorreipondingcxposurc is1hown:i.spointGonthcgraph(•·;g. 7).

Fru tion1IGr1dl1ntllriterion- l .S . .ndl.S.l.Spn d

The penu ltimate 11agc iu d1c t lrugg!etodcvi111;1he rR<»t service­able cri terion of scnsi1ivi1y followed 1he recognition that the minimumuscfulgn.dicnt willvaryacoordingtcid1cromrast ofthc ma1oria!."l1'i1if0.2r-cpresc11uthcrninimum uscfulgradicnt\\"hC11 1heprin1ismadeonoayGradc2hromidepapcr,ancg:rui,-.,ofthc .,.membjoct011 amorc c.<m l,....tymatC1"ial wouldha,·etol>cprintcd on a10ftcrpaper,sayGrad" I. Thegradictu0.2 """uld then fail 10 g;, . ., 1hc minimum diR"=mia1>ou of tone in the print . Tt>C minimum uocful gn.d icnt ii perforoe greater than 0.2 on the more

:"'..:,';'7~"...~:i~":;e~t~~':.:,t,':!"~:~i;';~mfl:.11:t~~!e';.} thcircffecti,-.,cont,...1t.Suchacri1crionisdc1Cribcd&1afractional gndicm criterion. Long Hudia of thc.nriouo lindo of pri_nu oblainablc from each of a seri<S of ncptiva, obu.incd by gi,·,ng diffcttnt cxpoturcl tO each. ol a tct of diffcrcn•. ncpt;,.., mat~h, ledtothefollowingconclusi.on. Thcbel<critenonofopttd;.gt,·cn

Tl/£ SC/£/;C£ Of' l'HOTOGRAP/11'

bvthclog£va!ucCOITOSpO"dingtoapointonthcl'ootofthccun·c where: the gradient is0.3 of'thca,.,,,..,.ir: gradient O\'tt a log a­posun: rangc:of l .5 tothcrightofthis point,Nthown in •·ig. ti. h iol'IOtc:atytodc:tttminctliispointand thcla t ... dcvclopmcnc, culminating in ncw 1tandards, arooc from the: n:aliPtion that, if all filrnt "·ere d"'"lopttl 10 the tame clft:<:civc C0<11ra11, a fixtd densityal>ovefogcouldhcuscdinste<1dofafractiooal gradicn110 determine the speed. The new •tandardo, agreed by the U.S.A., United Kingdom and Gennany, thcn:fore specify tbM speed shall l>ebucdo11 thecxpooureErcquin:d1on:achadensi1yofO.! abo"e fog, t uhje<:ttolheoonditionthat ther.lm hcoodevcloped that 1hc density at l.3 log uni u greatcr than logl-: is 0.9. The arithmetic speedio thc:n0.8/ E.(B.S. l .l!J.60Part!,l962,A.S.A.1'112.5,1060, D.l.N.•512 l'art I, l96l.)

r.,. •· F,..,_. *'""'1ic•>< ni,.,.;.,,,, (t<pn>d-.1 r ..... B.S. 1:!80 19'7.by<OU•O<SY"''bt Bri1iol>Standard1ln•tiM...,).

Whenfirst;ntroducedB.S. >pcednuml;e,.wcrcexpreuedinl>oth log 1<nd arithmc1ical valu .. , whereas the A.S.A. speed• wen: express-

T11i• C llARACT£RISTJC CURVli

<;donlyinarithmctical•-..lms.ThcocaleofHumbc:ndoano<n:ally rnattcrtolonguitcanhcapplitdtocxposun:tabl .. and metcrs. "llle1ubjcctitfortherdiscus><:dinlhechaptcronapooutt,p.'2IO.

Ill THli SClEJ,.C E QF /'/IQTOCllArlfl"

T- 1.,rMuctl-Tlle Relltion IN lwH n Tone Valuu ef t~e Orl1i111IVlewudt110H1f tlie Prl1t

'0.cdcru.iticsofdilfcttnlparisofthcncgativcgo....,.nlbcligluintcn· lity (and1hcrdi>n:thcnposurc) on000Tapondingpo.r1tofthcprin1-ing paper. Now zcrodcru.i l )" transmiis 1hc whole on the incident ~ght,adensityofltransmi1t 11f,dcnsity2, 1h, c1c.,90that negati>-.: dcR$ityi•dircc1lyrclated tolo1 transmiue<lin1e1>1ity,and hence to logc•posu rcof thepositi \•c printingpapcr,c.ccp11ha1as thcdcmi1)· of thc ncgnlivcincrca""',lhelogEofthcpositi'·cpaperdecrc;ucs.

1\dcu1i1ydiffcrc11ccofl bctwccnt,.·oponi o111ofaneg• tivcmca11> acha ngcoftr1m1minedin1cn•i1yin1hcra tioof l01<1l,whichcorre-1ponds toachangci11log/ (and thcrcforc of logE) ofl uniL The unit> of ncga1i,·e dcn1ity arc 1hcreforc idcnlical with the uniu of po1i1i,·c log E, and the ncgat;,·c charactcrioticcurvc n;la1cs tone valunof,·icwwith log £,·alunofthc pooiti,'Cp<inting proccs&, as lhowninFig.9.

ll~ :-<ow• DOnnal \~tw g;, ... a camera image wi1h an intc1..;1y range

ofaboot30: l.1hatis alogErarw:ofl·!>. lf,.'C gi>-.:thcsmallftl camcraupooure in which all 1hcdcnsi1ics licon 1hcs1raiglu line ponion of the 11ega1in charactcri11ic cun·e. the log E valua will rangcfrom210 !·~and 1hcdensi1icsfrom0-!>10 l·6, as1hownin •' ig. I0.1"hcrangcofl~E. (pos. ) willthn1bc1hc"'mc as thcncga­ti,•cdcnsi ly range, which" l·l, and since 1hc best print ,.·i!l we p.-~cti~lly the wh'.'lc of the positi'·.c dcnoiiy range from dear paper tomax1mumdcn>Hy, theapproproatc gradcofpaperwi ll have an

~~~~~~~h~~;~~n~:·!:~~~~~l~;:z~~~ the figure.

lnploltiltll•charac1cristiccu,.,,-.: (orindced•nygraph) thc acnial pooi1ionsofd..,UC1hncnofundamcsualligni6cancc.LogEnw:rtly dcno1ndiJ1anccto 1hcrigh1,andDdistanccuP"""rd1.WccanJU11 ascasilya nd asaccuratctyplolacharacterillU::Cu,.,,·cwi1.hthclogE axiJ • bo-.·e thccu,.,,·c. By plouingbothnegativ<:and p<>1>tivc curva in1hiJ,.•ay,andtuming thcp<>11U,-.:characteristU::cu,.,,-.:1hrougha

~~·i~"A1°(n:;.r7,.~~;';.bi;.11:1~c1';g0l(t::.)'.;~1~c";:,h:i;·~.· ."h';:.~: Fig. I!.

Tl/£ SCIHXCE QF PllQTQCRAPllY

lrtl,.~':'tlo.ut..cn.corrcctlycxpoocd,thepoo.itiooolthepoo.itivc ct...ractcnstlC curve W>ll be ouch that it just cortt:1pond1 with the n.ngcolderui1iaofthc ntga1ivc.

Wccannowtraocthroughtonevaluesold .. originalvi<:>.-101tt howthcyarcreproduttdintheprints.Ir,l"orcxamplc,,....,choooefi•.., equally1pa«<1to1 .. ,·alueswhich~thcsubjectfromh.ighlishtto

lludow,lhown,..4,b,c,J,con Fig. ll,\\'Ck11ow 1hat1heoegivc equally 1pa«d log cxpoourc values, and from the ch;oractcrinic cut\"cand thc ca mcracxpoourcwcca11deducc thcncgati•·cdcn..i­des (• ',h',c',d',t')whichtheoe 1cmevalues produce. h"'illbtdear fromthegmph lhatif""a<ljustthe ntgativ.:cxpoourc to thal"'C uscthe 11raightlineportio11ofthccharactcri1ticcurve, 1henthe ntgati•·c dcn..ities •', b', c', J', t', will be equally 1paced. Theoe negativ.: tltt11'.ties tlc1crminc the log upoour<: values which the poo.iti,..,matcnalrttcives,andfromthepooitiv.:cha,,.etcristiecut\·e

\\'Ctandcd uce tloccorr.,.pondi11gdcnsitiesoftl1c prin1,o ", b",<",d", .-, ~ntl hence the tone values of the final pl>01ograph

lf1herc cxi1tcdaprin1ingpapci-witha slraightli ne charactcr­istic curve from 1Cro 10 mnimum tlem ity "" cou ltl obtain pcrfttt 1onc...,production,1hatis,thctoneva lU<ll • ·,•·,,-,J·and • ' .....,..ldbe«[ually1pactd,butasalli-iti•..,p;optt"clu1,,.c1cristia • ...,.,.,....,.i ,heprin1 1onc\oalucsbccomeunevcnlysp;ocedasshown in 1hefigurc.

Wecouldbcstop<es>thcd;.tQrtionintoncvaluetbymeansola graphoonncc1ingthctone\-aluesoftheoriginalagainstthooeofthe print,l"orCl<a111plc,byplot tingpoinu•,i,c,Juid1api1U1•·,v, o",r/",<". In l'ig ll 1h<sc.-a!ut:1appearon 1hcsamc1tn.ightlinc. Oncsctofvaluco munth.....,fOf"bereplotted atrightanglcobelOrc '"''C can obtain one graph. Thi< ;. oon•·e11icn1ly do>>e by using a line at 45 ' , as ohown in Fig. l\!. TI1is gives the complete tone reproduction diagram. A ricrfcct pho<ograph ,..·ould gi ve a tone

TllE SCIE.\"CE OF FllOTOCl/,OP/11"

1eproduction cul'·e which is a. w.npu liw. A 1tnoigh1 line .i 4$• indicatesthatthcpcifttlprint~thesamevisua.lconu1011as1hc

original view, ,.·hile d""iations from o • indicate increued or dr:crcuedCXNllr.•lt in thcprin1compared";1h the original.

ltisclcarfromaconsiden.ti<>nof Fig. l21hatlince1hcwholcof 1hcpooitivcchancteristiecun·cmU1tbeUICd,andlince1hi1isnC1o"tt m:aigh1,1hc....,ofthe1traiJ!:ht lineportionof1he 11tt~liotchlor:acteri ­Hiceurvccanne•·crrcouhmpcrfcc1 toncrcproduc1>0nonapapei­print. The tone reproduction cu"·• alwayt rcnec111hedi,.ortiou introducedbythepositi,·ccha1actc1ioticcurve.

Fig.13 11>ows1hetone rcproductioncur'"CObta inedwhcn=1>0Sing onthc/..,of the11ega1i,·ccunrc.

lly comparing this with thecun·e of Fig. 12 it will l>e KCll that the former it ncarcrt<>a 1\raighl line. The uie of1hefootof1he nepti,•e characteristic cu,...,.., ha• tl>c adva~"aga of l'C<!Uiring minimum exposure, and gi•fog minimum l""mineJS. We now s<:e 1hatfarfromgi,;ngpoottr tooereproduction1hanthc....,ofthc 11raightlincportion,itactu.ollygivaimprovcdtonereproduciion, linceittrnd1tocounter-balancetheinevitabledistonionintroduccd by1hepooiti•·edu1rac1cristiccurvc.

CAapur l4

COLOUR SENSITIVITY AND FILTERS

~~l~l'.~¥·i~91:1[§:0·gg i~~~~i ~~~g~ic~]~;1~~~:g[f~1~ blue md oflhe•pr:ctrum.

lt ilther<:l'oredcartl1at 11M:dw=ofluminOlityor1hcvarioU1

=~~ :r.::.::.·,~-=..:~,~~r;:~:;;:~t::Oh:.".=~or ~~~=j~~

~:£:~i:~~~~.~~~~:::~~~~:i:~~~~::·:~\::~~~:~:~:~J objec1 on ~ light ba~kground . _ll'he11 the object i1 t<.> pr«h•cc as

~~~~:~~;{e]:!;r:~~i~~~: ~,~.;~~ ];;~f,P~~:i =~;.~~~~7~::"!....1::t.~"'.:~'~'.;:t ;~' :~:~~t~·~ :·~:: ~:W~ii} =~~'."'~;~ •;.::;t~,!~t"::~':C::' ~r:;~:::.cH~ conoidcrablcdq:roeofcontrol in colour r<"ndering in monochrome ~raphy.Thcluminooity•,.Jucwith"·h.icha.pa.rticularoolour as rendered in a hlack·and-wh.ite pl>otogn.r!>clcarly depc11d1 on twofacton: (i) tlM:oensiti•;tyofthematerial101hatcolour, rclati,.., too1hcrcolouro,and (ii)theinteruityofthccolo1>r,r.Jati•·c100the1 colouro. ·n ,coe facton can be independently corurollcd by two ttthniqua:colourtcnsirisingon theonehandancl1hcuieorco1our

"'

Till>' SCll>' Jo'CE OP l'llOTQGlfAf'llr

filtenoo1tl .. 0ther. Athirdf.o.aor,1hcUS4'oflightooun::uofdiffcr­cn~ .. iectn.l mcrgy, may also becmployul in 1pecial CA1CS.

Colo11r$en1ltlrit1of Photograph lcMat1rlal1

lnhismltlyofthecolouaen•iti,;tyofphotogn.phicma1erials,Vogel tteognisctl that 1hc several sih•cr halides differed in this rapcct. \\'hilc1 ilvcrchlorideisscnsitivconlytoultra-violct andviolet, lhe scnsitivitye>foilvcrbromidcextend•tothcbluc."11icgn::atcs\nngc ofscm.itivity,howevcr,i•obtainc<lbya<lmixturcof•smallpr<>por­tiot>e>f1il....,riodidcwithsilvcrbromidc,thc resuhingcm11Uionheing SCtUi<i"" throua:hou< 1hc blue to 1hc hluc-grttn. In 1873, Vogel wasfortunatcinaccidentallydiscm·cringa mean1ofexmidingthe emubionteruitivi<ytolongcrwave-lengdu.lncomparingthecolour scnli1ivi1y of comimrc"-1 materiab, he 1101iood 11 .. 1 tome dry collodion pl<lta obtained from Rngbnd "~ more scnsiti,.., to greenll&htthananyothcnp<C>iously1estod. l ncorpontedi11thcir cmub.ion""''l"'llowd)..,toruluceh.alation,and Vogclcorttetly deduced that the d)"' had scmitUed the emulsion•<> longer,,.,.,..,_ lengthi. Although Im oti.er,,.-.1ion and deduction"""' not im­niediatcly acupted, and caused considcn.blc conum·crsy, furthcr expcriimnt w;u stimulated, and new scmitifo::n were discm..,rul byotherw<>Tkcrs.

T he mQllt imporia11t oft he early scm.itisi»g dyesrnffo was ttylhr1Ui~, disco••er<ldhyE<lcrandoornmcrciallyappli<:<lin l6lH.Thisw.._,$0 •ue«Mfulthati1wasu!edalrno<1exclu•ivelya1nsr<lcnscnsiti"'r for cio..:on fifty \'ears,andstillhasa limi1ed use today.

Until1hcurlypartofthiscentury,thcaddWscntitivitydidne>t extend beyond dtc gT«n, and emul•iomooscmitiood were called orlM<A•-it,irnp!yingthat,r<llativc,ounoensi1iscdcmuls.ie>nJ, I""' <Wrt<I ~~in monochrome. They "'er<l CCTtainly a markod impr<t<o·cment and, "" they wcr<l .,;n inoeruiti,.., to Man~

andred,couklheh.andledinredsafelight. Ahhough comparati,..,ly few oucccuful scru.itioers .. ....., kno .. ..,

beiOn: this cemury, many matcriah were rceognioed as importing 1Cn1iti•·i1y,1houghthcywer<lunusablebecaUS4'ofotheru11desirablc ch.an.ctcrittict.Among1hem"'"'abuicdyestuffknownascy;anin1:. r<lCOl("ioed asa 1ensitiser by Vogel. In 1901. certain related dyes ofthceyanincgroupwcrcfoundtoben>011t1uccessfuluscositiscrs Sincetha1da1ethecyaninedyeshavcestablishodthclll!Cl•· .. as pre-eminently important inthcscn1itisingfidd. \l.cforcthc !91·1-18 war,Gerrna11ywa• practicallythe$Olc$0urceofcyan inc scmi ti•i»g

1-'0LOUR SEJo'S/TIVlrr ,l,\"1) PILTERS

tr~"";~i;".._""''"imul<ltedproductioninthiscountryandinthc

Thcfintd)"'.tQimpartscnsitivitytQthe"·hole•io.iblc1pectrum """* pr«luced 1n Germany in 191)1., and 1och dyes ar<l known as

;;;:~::~~~i~hhe"..'.~~:;:':n~!'s;~~r:":t~of~ti; 1pec1rum,and ofthcinfra-rnl as far"" l,000.n,.. "11>ecarliest""" d1·aga•·ccmuls.i~1rclativc!yioscosit'.''einthegT«npanofthc

E'.~~~·:~~~:Xii~:~;:~l:~:£·:::=c ~;:I·:~~?~~: li<>n1that1>0liglui1rcallysafc,andthey•ho11ldbchandledand 1><0Ccacd inoompletedarkn<:0>.

Elt tn1lonof$1n1itlrit1lntotlle Uttra·Ylolet

forccnainpurp01CSitmayhcn""""5aryt01twlytl>eimageproduced

~ ~!":;~i;:.~·i;;.: e'r;:u~::;:n,:t~= :~=.:":!'.: =~··~11~t.il~;,~!•i:n~h.:::~~ ... ~:~c:~:-~~i~ withoutab,...aktothc•hori .. twav.,.known, l111>0m1:olusc,how­C\'Cr, an cmul<ion records only to 360ml';•hortcrwave-lmgtluarc al...,~hed. by the sla.,ofthc lcosor prism fonning 1he image. The oublututJQn of quart~ for glaM allow• free 1ransmioaion ofultra­vio!ctray1bclowthatwavc-length,andscns i1ivityio thcnc•tcndctl to220m,., l>clow which thcgdatinof1hccmuls.ion absorhllin the ultra-violct.Scnsitivityhclowthatwave-lcngthisachievctlhymaking cmuh.ion1 that ar<l virtually gelatin-frtt, or by incorp<>ra<ing a fflJOl'CICent m.otcrial which con,'Cru the lo .. ·cr ""'-''c-lcngtlu to higher ,...,,..,_lengths that affect the emulsion. l':.ictenlion of the usdulr1C111ofan cmut.ion in<o rcgion1of1hor•cr .,.a,'C-lengtluis 1husnotaquC1tionofscnlitisi11gsil,'Crhal;,Jes 101hotcradiations (IOr enmplc, by ocmitising dyes), but of removing thooe con­uitucnu of apparatus and cmulsi<>n (glan and gelatin) which ~..,nu •he U-1 u1i,,..,·iolct rar• frt1m l'Caching •he sih·cr h.al;,Jc.

Spect rogrem1

T hecolour tcnsiiivityofaphotogniphicmatcrialcanbeassc•scdby (Xpo<ingthcma1 criolinaJfH"••tMPh--anapparatt"whichthro"~a

\

f0,.2. SP""tf"1nm• ol11·pi<ol "onHnuy".o"ho >nd ~· .... ttri.alo. ,... .. ~···~-l< ... !i>o o/; fi1u .... ""f""<'<nl M.Uto o/;

IOm~.

white light tpcclrum 001 to the ma1eri.al. toge•h<:r with calibnttion figuraintcrnuofwa\'Co)cngths.FM•boolu1crau!,.,thclight 1hould i>ctuchthaiall"-a'·e-lc11g1hsateofequalintcn<ity,huti"oo"putt:ly compar:ati'"'P"rpoot:1iti1utualtouoetu11gotc0> ligh1ing. On pro­a:ss;,.g the material, the image and the calibnnion ""''Colength markingowouldindic:llcthcr:angcofoolourtc:ruitivity. By.,,,pooing

CQLQUI/. SIOISITll' ITY ANIJ f"/L1"f;JIS

the material in the tpcctrograph immtdiatcly behind a graduated oplical "'cd.gcin wh.ich thcdcm.ityvariation it at rig.h~ angJ.,. to the •peo;t1um,n11poss>blcto~t therelati'-.:11enoitmtyatvariout

W>o\-.:-leng1lu by the height of the image at each WB\'Colcngth (fig.I ).

1l>e rauhing proecoocd image it caUtd a,....,. spttlr•fT""' and fig.2ti.o.."1opcctrogr:all"l$oftypicalemulsions..Sir>tt lungotenlight it much richer in thclongcrwa\-.:•lc11g1luthandaylight,orequal­e""'l0' ligh1, •M heigh< of•hc image a t th<: red end ofthe•pcctrum itgrcaicr1hani1thouldbc,relati\-.:totheblucetld,001thatitnot a tcriow maner ir t pcctrogn.mt are mod nu:tt:ly r.,.. comparati,.., purpooes. ~·..,... a nonnal spcc1rognm and from the n:lati,.., intcn>i1i.,.of1hc001111i1ucn,.oftungs1cnligh1,wc<:an<:alcula1e and dr.1w the 1pcc1rogram that "-ould be obtaintd by using an eq...,1-cncrgytpcctrum, when it ...;11 be &>und thac, C\'en in th<: moot highlycolour-tcnli1i~ma1crial, .. ·eha,·enot)'<:tsucettdcdin pn>ducinganaddcdtcfUitivityatanywa,-.:-lcngth:uhigh:uth<: originaltc!l!iti\•ityinthe,·;olet. ( t"ig.3. )

i~ 1:L-3J

-- --C..-"11oo°"""'­""Y(LlN<l1H

f"if. , .s.,..,.,;.;1y<un-eool1i><•Y<•ndol,..,. ma,.ti.o.lt00<1<qu.>l<n<'SJ">l'<"l"'m

Uu of llghtFHt1rs

Although much progrca has been made in utending the ten• sitivity Q/l.il~r halide cmulsiotu to longer ""'''Colengilu by means of tc~tisi~ dyes, there ~ no Jtno.,.·". med>Od ?': simul':".ncousl)· chminaung, or"''en ocnowly modffyong, the imual scns>ILvity to 1Multr:a-violc1and•hcbluc.Wecan,i.o....,..,,,.,climinatcMrcduce the effect ofd>OOC ...,.,..,_lcnft"'- indeed ohny .,..,,'Co length-by ab.orbingcomplc•elyorP"'ni.ally1Mwa ..... lcng1hs 1hcmtcke1by moamorappropriatclightfih..,....

G1ner1IPropertl11 or li1htFilt1n

The~glufil!crhas":'beinsertcdinthcpa1hof1hcligh1falling<»>thc

§fil~f,~;J~~i~~~f.~~~f ::r:~ o<rt:WMorpushcdontothcfrontofthele111.

Ughtfi!tc:nusuallytakcthcfom1oftintt:dglauordyt:dgcbtin

~~~~~~~~~~~t~~;~~~~tf.#I~ pl~raph~, and higher quality optically•K"i!rked glass for mou ~~l requirements ouch as tclephotogn.phy or phcuo-mc-chaniul

lleze FHter

,\,iwch•,·c occn in chaptcr 2 (p.35), uhra.-·iolct•Old blue ligh t is

:~·,~,~- :~~ :~::~:~·~:.t~h3c 1Xi:~ ~.1'~ ~li~:n~~;,~:::~ aerial. ,·ie"-... True atmo;&pheric hue .... ncn ,·cry little red and pracucallynoinfra.rcd.

Sintcall photogr,o.phic m.otttiah ""' highlytenliti\'C m uhn.­vi.olc1,unfih~pic1urcsrtt0fdmon:ha~1hanio'~siblc.Thiocan be remedied byusinganuhra·\·iokt aboorbingfihcro-.•ttthelens. ~~I~ fihcn are oft~ made 10 absorb~ small amount of blue ligh1 as

S1i!lgrca1crhuepcnctn.tionmaybeobtainedby1u1>presloiDgall d>t ~lue at>d pa11of 1hcgrcen (adccp)·clloworon.ngc r.11tt) and max11num pcnctraiion "'Uh a panchromatic film by using a red filler. The m~x.imum possiblc _pcnclration is obta ined by using an infra-rt:d ocnS1 U\•ema1cria! wnha filter 1l1at absorbi all but the far red c11d nfvioiblc 1pectru m-o.cc illustra1iom on (>311:" 19~.

__ ....... ~ -

"' '

' ~ '· ~~~

'~. .. ..... - .....

O"r-correctlon

By the uoc: of deep yellow or red fillers, theouµprCMionofthe blue endof•hctpectrum ea n beeontinued,sothattheblu"l'arerendercd darkerinlloe pl'intthantheir visualtonevalun,andthcyellowsand r'((!oeorrcspondingly1ooligh.t.Drama1icpictorialcffcct1m.ay1husbe producalhyattentuatingundulyccrPinlandle;ir.pcfca1ura,and rcndcringcloud1Qstarkwhi1c api,..tamidniglndi.y.Slight0\"Cl"­OCH'"1'1'Clion,m<>r"CO>U,issonu:1imcs ....,fulfor(ivingdiffcreru.iaiion bct""ttn"landscapcfeature (forcxamplc,a)..,llowounlitbuikling) andahackground1kr.f.x•reme 0\-.:r-<:On'e<:<ionisreprescmcdbythe uoeofinfra-rcdoeru.i ti,..:materi.ahwithafilterthatclimin.atesthe uhra-violctand bluemwhich thematcrialisa1sooensitive, Fig. 6

A colourfiherfunctions byal>oorbing sornc of thelight towhich a rn.acerial itoensi tive. The e~posurerequircd to produce a correctly exl)O'Cdn~ativcmu'1COnocquen t ly\Jeincrea•edhya factor ealle<l

CQl.OUll St:xs1r11·1r r ,.,...D HLTIU.~

ihejJtnfal#. h ~kl \Jecmphasiscd howt'ICr 1ha1,contraryto

~:u~=e';:..i:o~~::no":~=..i~~~f!::;:-~~~ a )-.:llow filler will require much lea increa.o: in e><pooure if usai in ooujunc.1;,,,i .. ·itha pand~rorn.a1icfilm that ishigh1yoeru.itivct<> thc transmi tted yellow than 1fU>Cd with an "ordinary' (!Jluc-scnsiti,•e) ma1cri.o.l.Ag:ain,1heinereascinexposu rewithagivenma1erial"·ill \Je less if the •·iew is illuminated by yellowish (for cumpl c, tungsten) lightingthanbyblui •h(forexample,tlaylight)!ighting.

Thusthtfi hcrfac torissignificant onlyifthc typcof liglu ing(tlay• lightM tungsten) and the ocn•ili,·ity of1he maleri~I ('ordina ry', <>rthoorpan) are1pecified. Where they a,.., not 1pccified, it is wual toa .. umc that the filter lii t<> \JeulC<I with pan material in daylighl.

Contru t f lltt n

S.,far,weha•..,eonsidcredcortt:etionfihcrs (oomctimcscalled•IM­<lu-U. filtcn), whooc main function;,, 10 produce: a photogr.aph in whichcolours1how1hcaamelwnin<ISi1y\"alucu 1hcy,,.._..,,.,,hc q<.

Fihm arealoo largely u-1 ddibcratdy to falsify 1ueh ,one values, ,.•henoccasiondcrn.andl.Whiletheeyeeanrccogniu:<olour001>1rast

"' ,.,

"' ;;~~~~~--·----·----·-------;;~~~~~:;

mimin><~ol«>l<>o., ... docum<nl {•)Tri><><•ipO,.ioh .. dandbl ... .,.,....,,.,..,. (•J lll ... <lihM,..trdbypholosraphy-'oodiaary"~I ...

(<) ~~~~~i ... ....!byphot"f"'pl>•-P*"""'""-h

a.s " "e ll as brighrnao con1ra>t, doc black·and·whilc prim can diffcremiatconlyl1ylhcla11cr. Thcdc<ailona1taincd microKOpc •i>«imen, or the lcucring on a poottr may be diu:ttniblc only bccauseofcolourdilfcrenoe.Rcdlcncringonag...,.,nbackfj:round ofthc .. mc luminooitywould r«:01'l on a fully corrected lilmaaa uniform g<C)\ and to meet that •ituation it might he conJidcrcd pr<:f~rah!c to sacrifice truth in lone v~lucs in order to bring out dctad ( v,..thclcnuing).Apanlilm"·"ha~lihcrwouldgi,·ca prin1inwhichthercdwouldappc:a<ligh1cr,andthegrttndar~

t~n1hcirtruev;.ua.lluminoiityvalucs,.togivcqui,cl<::giblcliglu

prontlogonadarkbackJrt(lund.Ahcmam·dyanorthofilmwithom filter would give a print with dark lcm:ring on a light back~round. Fihcrsoou>Odarcgcncra!lycalledro•1rtmji/11T1.

Octallinanyonccolour;,.uuallyenhancedbyr-cndcringthc oolourlightonapoint,lhatis,byutingafihetofthes.amcg<'ncral coloura1thatbci1>gphotographcd.Foraamplc,inpbotognphing furniture, it is uoually d~ir,.hlc 10 give cmphui• to detail• of marquetry, parquc1ry or wood grain pattern. Pho1ograph1 on 'ordinary'oronhom.ottrial...,ndcrthcfunii1urctoodarkand lackingindctail.Aponfilmwithc:onutingfiltcrsh<wo-.1hcgnin orpattemasthceycsccsit,butoftcnamor(:ple...ingdftttcanbc produccdbyphatognphingll"'rcddi<horbrownwoodwi1hadeep yellow or red filter tnae<:entuatc the pattern of doe woad 1mface

COLOUR SENSITll'/Tf AND FJJ,Tf>"llS

pa~;~'.~;~is, ~'::a~ •:;~~!-:in~e1:•J~~ndor~~eh~~ ,\white paper, wuh printi11gin twodoff"ercnt colours (say, rcd ar>d

:~".:'~~~:ir.i1:.c~t;:_~\ a;:p~r~'~':::~a~-~;~.~~'.~:~~~y fi~~- r~~ :-.;p~,ti;:'~!;';:",,:~.:~i.k!~.:~t~:.:,il~~~!:~: lca,•1ngonlythcbluelcttenng1nboldrclid:Convcnely,<:opy•ng onan°ordinary' (bluc...,nsi1ivc)m.otcrialwouldeliminatethcblu<: printing(Fig. 7).Thcoucccssofouchtliffercn\iation willofcoune tlepcndonthcscpara1ionnfthccolours.Whi leiti1easy tosepara1e rcdfrnm blue, it is more difficult 1oscpa.r.ueo:nnpletclyrcd from )"Cllow.

ln<:OJ>Yingpaintinp,the...-1faithfulrcproduction1arcobto.incd bywi~gfullcolour~rr«1ion,butonenoldpain1ingshavclostmuch aftheiroriginaldeta1l,andinthc.:oumoftimc colour•·a!ue1ha"e been falsified hyabrowndiscolorationnftbevamish. Thcuscofa deep )'Cllow filter .'end1 tO dimin.atc thi< discoloration, oh.aw up dcta}l"?1casily,,;.,,blctothceye,ar>dgi'·carc1ultinwhichcolour lumrnoooticsapproachmorcncarly101hooeof1bcoriginal picture.

Ofien old documcmo and p,;n ,. arc disfigured by brown siains. I f the origin•I ink i• black or b!uiJh in colour, the otain ma)' be di_min.ated and the printing preserved unimpaired in a oopy by u•mgafihcrthattrarwnitsonlythcligh1.-dlc<::1cdbythcbrown stain(brexamplc,ad«:pyclloworrcdfiher).,\ltemati•"Cl)·,ifint hasfad«!toabrowncolour,itmaybcrccortlcdasblackbythe u1Cnfacomplcmemary {blue) filter, oriuequivalcnt, 'ordi11nry' (bluc...,nmi,•c) malerialo.

Thcapplicatioruofcontrutfihe,.arclcgion,andonlyoncor tM> •ypical examples ha,.., been mentioned. lbc appropria ic fihen maybcsclcctedbyrc1n<mbcring1heprinciplcthatafilterofd><• ~me gcn~ral colour as a ponion of a vi.cw renders that ponion lighter rn the f>'"mt, and l hQ\\"S up iu detail. In the cx1...,rne it may be ttndercd.tolightastobcindistinguishablcfromawhitebackground, andooelimina1M.AfihCTthat;,oornp1cmcntaryincolourrcndcri 1hc~iondarkerar>d1upprcs:!CSdetai!.ln1heextrcmccaoc,it maygiveauniforrnblack.

ll1utr1ID1n11tJ Filt1rs

Thcocfihcrsucdcsigncdtoab.oorbvil.iblclightnon...,1tttivclyar>d arc useful when it i•dcsir«l torc<lucethc hriglunao nfan image

TJI#,' SCIEJ<rCE 01' '110T0Glf,4'1/J'

widtQIU rcsor•i11g to a decrea«: in C>IJ><>IU"' eilher by using a •h~>T1':r tirne or a 1rnall leru aperture: \Ve could ,_ for er.ample, be US1ngafilmof,•cryhigh•pttdfo•1ub)tctoinlowtllumination,but ha,·e o<:<:Uion on the oamc film to phot0graph a subject in full 1unlight. The ,,...,.....I)" rcdu.r:ti.on in Cllf'OIUrc may be °"uidc <he nmgcofshuucr1pttd1andlensapcrtu«oia•'llilable. l nthiscasca ncutraldtnli1ylihC1"of2would rcduce11M:dfec1ivc•pttdof thc film tOOtima. lnanochcrca.. .. ·ern.o.ywishtoU>elf,.fuULcns •1"'r1urctott<lucethcdcpthofficldandtheU>eofancu1ral den•ityfiltcroflwouldcnal>let"toopcnthclcn•byjus10'·crthrcc

""!"· PolariJ:i1g FHttrs

Thoughnotfihcriind,.U>ualocnR,thcycanbcuscd&Jamcansof darkcningtbcolty,andtoalimitcdcr.1cn1,tt<lu.r:inghazc.Thonatu..., ofpolarizingliherth:ubccndiseusocdonp.36.Polarizingfillcna..., c•peciallyvalual.>lcincolourpho1ographywhc11iti1 dcoircdto obtain maximum colour saturation l>y 1heelimination ofiurfacc reflections from "on-metallic ourfaca. A basic incrc,..., in Uf'OIU"' offrom 2to3timaisnttdcd.

Saft lightSCf'H lll

/\l entionhasa]...,adyb<:cnmade(p.24] ofpslightpaper,oonamo:d becaU$<:iteanbchandlcding:;ulightwithoutfogging. Howcvcr,i" practiccitispouible1ocmployfihcnofdilTe•cntaboorp1ionsforthe handlingandinspcc!ionduringproccssingofmaterial•ofvariouo 11<:nsjtivitit$.Thote,.·hicha...,ocnsjtiveonlytoultra-\•iolcta11dblue ~ght ean bcaakly handled;., light from which th;, region oltho opectrum hos been tt:lt10'IC<!. This applies generally to chloride emuWom and thus allows a '""'1' bright yellow light to be used. Somewh.at greater ><=re<:ning is required for bromide and chloro­bromide priming material•, while orthochromatic materials require a veryhighah10rp1ionoflight,uptoahout64-0ml'. l 11gcneral, panchrom.a1ic rttatcriab and colour films and papcl'I must be handled in complete darkncso, but ''"'1' ~mitcd ...., ol a aafelight paa.ingonlyO-Ol pcrantollight arounda wave-leng1holS20ml' !Or panchromatic and 0-l ptt <:en! at 600ml' for colour film ;, poosible.

RECIPROCITY FAILURE AND OTHER PHOTOGRAPHIC EFFECTS

T;.:~~:~~:;i: .. :~::;! ~~=~~~ i~2E~~2 doooo ,.·Mntheemubionis•"bjectcdtoll .. a.ctio"olliglnor0<hn radiations. The<e arc many """h photochemical rcactiom occur•i"g "'natu...,,r00<1tof1h.:mofahighlycomple•doaracter.

ln1l,.couneofastudyofoomc>irnplecr.a1npla,lluro>cnandkOOCO< found,aswouklbcupectcd,1hattheamountolchemicalproduccd duri,.g thercactioninc...,a>cdwithincttasingin1emi1yofillumina· •ioo>, and aLM> with incttaoing time ofilluminaiion. T hey fouold, rnorco••cr, that thef)U~ntily of material produced wudepo:ndenl Onthe,·a! ueofi11tc11iitymuhiplicdby1hc1irncofapos\!tt.Thusif agi,•cnarnountofchemicalisproduccdl>yRcer1aininten>i1yo'·er a gi,·en lime, \be .. me Amount ,..ill be produ-i b)" half1he in1cn>i1y acli ngfordoublcthc1imc, orindttdforany•·a!ucsofintemi1)·and timc,l"'\>'.;dcd1hattheir1>rodude<juab1hoprodu.r:tol1heoriginal onttnlily and oimc. llun>en and k oococ thUI found Iha\ inltnlil) and timcwCTCpttfectlye<jui,·alent.l'h;,ohK1"\";11ion"'"' gc"eral~ ;u the /h.,1H••ROKH /.11w •f f'/wi1«/onni.:4/ E'l"iraUJtU.

The photographer t'ound 1ha1 a >imilar ...,latioo> hoMs ()1."CI" a limiocd tangcl0r1hodc11>ityproduccd on photog'."phic m.at~rials._Thuo in ~encral, densj\y inc...,....,. wi1h inc...,umg light m\enouy and

:~~;~:;11:a;t~~f;; ~i::;~~~dti~; :e~~~:,;:~"~.~ ~~ .. : ~~r;~~: '"

Tl/£ SCllll'C£ 01' PllOTOCRl<l'lll"

produa:d by halftht imc111ity aetingfordoublc \),.time, <K f<K double tltc: i111en1ity ac1ing for hair the time. H°""(:VCT' if marked changes ar<: made in inttrtlity a"d time, e.g. OOIC•lenth or one­hundr<:<hh of the inlensi1y acting for ten or a hundr<:d ti,,,.,. as long,1hedtniityproducf<i,,,..ybemarkcdlydiffcr<:nt,C\"Cnthough therNw1ofin1enii1yand 1imer<:,,,..irt1QOfl5!.ant. FM the density producf<ionphotogr:aphicmatcriab.,thercfo,..,,ther<:isfailur<:in the oruc rcciproci1yoftimeand intensity, a phenomenon kno..-n as Rttir«11;1/,...,1'-.U.tormcn:ly~17 1'.U••·

T his has bun taken by oome as t:\idtnce •h.a• •he llu"""'·Roococ Lawofl'hotochcmic:ol Equi•"llltnccdoco not hold IOrthcphoto­cMmical ttac1ionoccurringduringthcupowrcofphotographic cmuls;.>n,butsuchargument1ar<:qui1cinvalid. Wch.avCKCllthat theprimaryproduc1ofligh1actionoo1apholographiccmulsionis electrons, and we ha•·• no ttaso11 to doubt th.at 1hac arc formed <JUantitativcly in acconlancc with the Rurucn-Roococ Law. Thcu arcmany•u~ucnt andnon-<1u.antitativcuactionsberorcthc

liber:uN cl«1rons give a photogr:aphic image-a proportion of the cl«1n>ruoombinc with 1ilvcr ions to fonn 1uJ..microocopicspcd1 of metallic sih•cr (die la1cnt image). Some of thCIC may act as d..,..,.Jopmentccm.-.:1andca1alyu:thcr<:duc•ionofsomeofthcsih·cr halidegrnin1tomctallic 1ih·cr,"·hichhua11op1icaldtn•itydepcnd­ingnotonlyonthcamountbu1alsoon\hcsi•eanddistrihutionof the silver grains. It i1 ccrt~inly not1urprising that the production of Optical density is notalnolutcly in acoord with the Bunscn-Roswc Law-the surprioing part i• dt11t reciprocity bct1>·ccn in1cnoity and timc,inrcspcctofpho1ographicdcnsity,rcmain1 tothccxtcntthat itd~.

hperlmenl• ID• lt rmlnt lonor AnlproeitJ FallM r• Thercciprocityfailurcofamatcrial,,,..ybcdetennincdbyascri"" oftime...:aleupoourco. Foruamplc,variot11portionsofastripof matcrial,,,..ybeexpol«l1o thepmcintcniity,,.·ithexpoou...,timc increased by1hePmeralio,PydoubJc,forcach11cp.OndC\"Clop­mcnt, a u:rico o{ dcnsiti"" inc...,aoing wi1h upoou..., time will be obtained.

if,..., "'peat tltc: upcriment, ui.ing only h.alfthc intensity bu< gi•ingeacht1cpt"·iced1eupoou...,o{tl1eaNTC1pondingoncintltc: f'<"'ioul experiment, we •hall find liulc change in the densities of CONCSpon<ling11cpo. lloweo."Cr,bygiving1hct1rip\'.•rl"f,CIC.,of•he

RI!C I PRQCIT/" f.t/LUR.I!

intcnsityandincrc ... ingthctimcs1ocorrcopo11d,"·•ol>allprobably find a considerable change in 1hc dcnsi l)' of a panicular otcp, although 1hc expoou...,o{any 1•cp, mcasu....tL as / xi, remains con· ttam. The u:ri"" may of course be utcnded in the 01her dirtttion, successi,~lyincreuing thcintcniity..-hilccorrcopondinglydtcl"Casing

thcupoou...,ofcach11cp.FiJ.l1ho>.'1a1ypical.-.:1u!t. Wccouldplotthc""'Yin "·hichdcnsity•.,.rics"·ithdiffcrent

intcnii1ies a1 conotan1 upoou..., (/1), tha1 is to Py, we could plo1 1hcdcnsiticsalonganyhor:i~ntalsctof1•epoin•'ig. l,bu1a'...-y differently 1haptd cun·c would be ob1ainN IOr each otep. The """"'""°"ldh.a•"CliuLeoignificancesinceitwoulddcpcndnotonly onthcrcciproci1yfailurc,bu•abolal'$"IYonothcrcharacteristicl of 1hc ..... 1crial. A more oignific:ont """Y of plouing reciprocity failurcfrom1hcexpcr:imcntalrr•ult1ofFig.lis1oplo1tMexpooun: rt<juircd IO produce a tMJ/4111 density al nrioUI intensities, ""hich gi,-..rcciprocityeunuushowni11Fig.2

Sir>ec .. ..,areplottinglog/1againstlog/,hor:i•ontallin.,.onthc graphrcprC1Cntcorutantcxpoture(/l)atdiffcrcntvaluaof/and1, andvcrticalline1ttprcu:mco1\llantin1t1uitybu1diffcrcnttimcs,and hence diffcttm //. Poimo representing con11ant 1ime but different intenoiticswill notonlymovcalong 1hclog /axi1,sinceimen1iti"" ,·ary,butaboalong1ltc:l0tc/luis,si!lttcon1tanttimcbutvarying inten1itymu11mcanvaryingcxpoture(/1).Theyarc reprcoentcdby lines a< 4~ ', aJ shown in the graph.

F<>rtuna1<lythcminimumofthcgraph,whichrcprC1Cnt><hcleaot c.po1ur<:rcquircdtoproduccagi•·cndemity,andhenccconditiom givingtheh igh.,.tefftt tive1pttd,occursaro11ndthosc intcn•i ti"" !lormallyincidentond1etna!crialinphotographicpracticc.

Cose of Rtclprocltr Fall ura

Contidcrablt experiment and •peculation ha•~ been dirtttcd t°"•ardsancxplanationofrcciproci1yfailurc • ndthecvidenccto datci11dicot.,.anentirelydifferentcxplanationof1hcincfficiency of1hc~athighin1ensi1iclfroonthaca1Lowintenii1i.,..

Withabricfhighimen1itycxpotu...,,1heeltttronsarerelcascdto<> rapidly to be handlN by tl1e rclati\~ly oJo,,· OCCOt><I stage. The upoture isal10\"Ctbcfon:moreth.an a proport;,,,,of1hcclectrort1 can be neutralised by ohc •ln>-·er movfr1g lih·er ions, •he wa.1cd elc.:trorurecomhining,.•ithbromincatorrutorcfonnoil•"C<bromide. Mot"CO\"Cr,tl..,...el«lrot111hatdo!Ormlatenti""'gcproduccahigh P«>i-tion of internal image and allO much tuh•laten• image, both

TJ/f; SC/t;NCN o~· l'llOTOGR .. l'llr

of ""hi.ch arc ""''cd oo far u normal processing is corM:ernW. Small wonder then that brief high intensity c•poourc is 1 .. cffocimt 1han upo1urcainonnalimemi1ico:n1d1ima.

Low intensity indflciencycanalsobcc•plainal by lhccvidcncc alrudy rc,·icwcd . El..,1 ro111 arc rele-.1 at a \'cry low ra1c. They arc u·appcdand ncutralioeda"d mullrcmainasisolatcdsiJ,•craton\SfOT muchlong<:rthaninnonnallatcntimagefon ruuion. l thiualrcad)" bccn<>llo<,l"'•cdthat•uchu1rcmetub-la1entimageisun>tal>lc,andi1 ;. pootulatcd th.at ineffi(icney;.cauocd by many i$ola tcd atoms of oil>-crt..ing1hciracquir'\"<lci.,,,1roruduringthcp<:riodofinSlabili11·.

f11.l.S..,.,<o( ,;.,.....,.i. .. p:>-

''"''' """';"' «<~!1~~t•r

1\n analogy, tf..,.,.gb not a \"Cry good one, can be ofTcrW b)· comp,ari11gclcctronsl<>bricks andthcla1cntim.agetoanedificc •llldcrconS!ruction. lf1hcoptimum numbcrofbricklaycro ford1c j<>histc1.,andthcj<>bcanbcfinishcdintcndays,a1lwmsand hricklaycrocouldnotbcc•pcctcdt<>d<>thej<>binone-1cn1hofa da)'-thcrc"'<>Uldca1ainlybchighintcnsityfailu...,jn1uchbrid­la)'ing. Lo"•intcnsityfaih1...,maybclikcncd t<>lhccfforu o(a,-cry <>klandxnilebrickla)-.:rwhocanworkonlyatonc-1cn1hof 1hc 1><>nnal121c.A<hclluK>do 1hcjobalonc,hcmigh1bcupcctcd I<> complete the building in a hundred tima the time 1ak<:n by t!.c 1eam of ten , th.at io, in a it.ow.and days, Unfortuna1cly he cannot maintainthispcrfonnanccbccauM:ofins1<1bili~inthecar!y 11agaofcon""'ction-y,due co1l1cdalructi\•cac1ivitiaofsmall lJoys who kick the bricks off the wall under oon11ruction. This p,allimcisna1urallylimilcd tothcfirotfc"•courses, bulwhilcthe)" candestroy1hewallsnco.rlyai;fututheoldmanbui!dsthcm,and 1husaccoun1forlowin1en1ityfailurc,thcylia>·c]ittlccfTccton1hc .. pMi .. "Ofkofthcte1Mr11ntcam,whichsoonra ..... thewall1t<>lhc 1tahlc limi1.

Thcanalogy maybcpusheda11agcfurthcr. lrtheji"1fcwcouroeo wercbuiltbytheten-mantcam,theoldmanco\!ldoontinueathis full cff1<iency, unhampered by the small boys'dOltructivc clfOTIS. So with 1he latent image: ; a uniform, \"CT)" small •~poourc to medium orhighintcnsicylig!,.,1uch1U"<Mlklproduceonlyalowdcnsityon d~lopmcnt,,.·it1provide1uflicicntlatcntand•ub-latcntimage 10 act IU otablc moclei for fun her low in1cnsity upowre. Low intensity

THli SC/1',\'Cli OF 'llOTOCflAl'lfr

failurcis1h<:rt:byclimin.a1edand1hcrcciproci1ycurveisuthownin Fi1J.l.Thclow-interuityla1cn1im>.geit1husconlidcr:oblyin1cn•ifled byahigh-intcruicypre-cxpoourt:.

Thcclf<:<:tcanbcpu11ou,.,in1wowa).,.Whcn1he 1ubjctttobc pho1ographcdi•orsuchlowinten•ity1ha1e><posurt:1imcc1ucndsovcr .on1chours, u inastronomical pho1ography,a unirormomall pre­exposurt:tononnalimcmityligluwiUrcm°''Clowimen9'1yfailure and 1hcn:fottcnhance1he1pttdcou1idcrablytolowintcmitylight

j,Jtm 1111111111 ttttn lo lo o •O •o

Jn modem astronomical photography, how"'"' this treauntm is "'"'ly uscd,as•peci.al platco a ...,nowa,·ailahlcin "'hich lowhuen­tityfailutthasbttncliminatcdbyod>ermeam.

Thcclf<:<:tcanaltobo:uscd1oadv.aniag<:whenam>.1tti.alhas\Jttn gi•..,nextrcmcundcr-exp<11u"'toanormo.l-intcouityview,10thaton development 1h<: highlighu produce only a low densi•y. A •u h­sequcn< Mflijliflll expoou"' 10 low intcruity ~ght will 1how ."° l'CCipm­city failu...,wherethctt is already latent or sub-latent •mag<:, and will therefore build up the Intel>\ imag<: ofhighligh11 and mid-1ones; failure will occur where there i• no pttviou1 latent image a nd have liulc cff<:<:t on thcohdov. .. ofthe 01iginal view. The original faint latemim>.g<:will thus bo:intMl.if>cd- proccso co-~bytheponmanteou1erm/11k11Jiffeati6re.Torcturnto1l1C

brickla)•inganalogy,theorigin.alunder...,xpooutttononn.alinteouity ,.·oukl be n:~ted by the team often men buikling up •o a few cou.-ona:rtainJCCtion1ofthe,....,.J1only,lea,•inggap1.Sublcqumt activity by the old man woukl l.M: largelydenroycd where chett .,-...., no wu.- laid, but would build up unhampered on 1hc ""''ion1 when: the team had laid a few wu.-.

L~ten•iflcation ''"" used during the war when importa nt aerial photographowcTCSuopcctcdofl>Cingundcr-""~·

lnt1rmltt1ncrEfect Anotherpbenomcnon,.owlr.nown10bccloodyconnee1cdwithreci· procity failure is 1he i.un.iiu., ifm. It was d ilco\'Cttd by eo.rly

•liCIPflO Clrr FAILURE

1tudentsofscn9'lO<netry,whouscdJCCtorwhttlslOgivc 1imo--.k expo1urco (Fig.26,Chap.l2). Thcycouldgivccxpooun:1byrotating 1hc wheel slowly to that only one r....,lution was requil'Cd, or they couldgive theoamecxpo1uresbyro<atingthcwhedx 1imef,a1 fast, a ndcxf>O'ingforxrota1ion1.lnthclauerthctotalcxpo1urcttccivcd byeachotcpiothcaggttgatcorxintcnniuen1expo1uref,,eachat l/xofthcoriginal1ime,thoti1,cqualinall tothccorreoponding time of the oingle-ttvolution exposu...,. The interui1y during the illum.inationl11hcoamoincach,and)'etin1cnruofdcnoi1ico pn>ducedthen:sul,.an:diffcttnl.

The ttao0n is that 1hc photographic material bo:ha,·.,. likc 1he cyc;in1crmittentexpooutt1of1uffi<icntlyhighfrequencyarenot rccogni.oedao•uch,bu1appeariden1icalwi1haunifom1expo1urcof lo,. . ., in<cmity. T he nttasary frequency for the photognphic material torecordthtC>tpolfUrccxactlya.acominuousoncwhosc intcnsityit the11<1trog<o'"er1hetotaltimc,i• muchhighcrdrnnthat forthceyc,.othatm0$tin1crmi11cntCJ1po1urcogiveaphotogrophic effttt IOJilcwh= bctwttn two extttm.,. One cx1n:me ;, that givcnbyanon-intcmiiuentcxpoou...,,tha1is,by the ~1u_altimc and

intcn1ityollightactiononthem>.terial;theotherug"'Cnbythe ""'.,. intensity opcra1i,·c .,.,.er the total time tilin to gi''" the intcnniucntexpooure.

TI>eexplan.otionol1hcintenniuc""Yeff<:<:tin1cnruofphoto­g,.. phic 1htt>ry is •imple. r.1<pooun: ofa gn.in_10 lisht releases clcctroiufrombromidciorts, andthcel<:<:trol\oex111inafrtt"atc fora •hort period bc!On:bcingtrappcd. Thenumbo:rof~recclectrono per unit volume (thcconcc111rationorelcctron•) <lurongc><pos11rc ""illi!lettasc,.·ithincttaoingintcruity.Ouringintcrmiucntcxpo1urt:, thelightls shutoffaf1ercachex poouttincn:mentl>Cforc1hccon­«ntnotionanreachthe'-alucappropria1el0thatintcouity,a11d1he totalcff<:<:titequi•-alcnttooneoflov.'C1"intcraityandlongertimc.

Th1C1a:tden Elfect l t h:urrequentlybeenob9e,..,'Cdtha1alightningflaohoccurring

~~~~;r-.: d1~;~~ght~:x~~,:.:i~n;ie~;a.fi~;"!~i~~ lt::.~~n:;:"~n :~~ curiotllphcnomcnonknownat bl<1<kl11At•••:,alM>called thcCla;µhn

e~~;.·~;::~"~r.:.i~';~\~i~~!\· 1'CCCllt .. u<1ico of lucnt imo.gc

:!:~:'n •,.";!, :"~!';~;h ~~;:;.:.~~: 'f:~:~·h~ ::

1111: $CH·xcli ()f l'lf()TOGRAl'Jlr

t:1tpooure at nonnal inten•hy. Black ligluning chus appears on" photograpl>""hcn 1helightningOQ:unduring1hecarly .. ageo/"1hc day~ght upoou..,. T he ini1ia1 high intcn>ity ••I"'"""' forms, .u ..-c ha""tttn,aJ1ttatdcalofin1crna lla1cmimalJCwhichioanefficic11t

fl1.4.Clay<ieo<lf«1.Tt.oblockli1h1nin• <>«u•l'<dd1ori"8 'h<"dY•tav• of<Xl'O'U"'•'howhi0<lil:h"';"fl•1u.{M.l<'J' K°"'k•U"'"'"'•lf•,,_

eleccroi; tmi> (or the 1ul1SC<JUCt0t c•po1ure at mmn•I intcn>ity, compcHng with surface trapt $0 that mud• of1hc 1ul>M:quem latent imagcio wasicd in lheimerlornflhegrain. l ndced thcs~rfoula!eQt image produc«l l>y lx:>th e•poourcsi1 less than thatofthc..,.,.,nd (norrnalintens.i1yexJ>Olu"')a lone.Dcns.ityi1 1huslo"·er"'he"'thCTc h.u be<:ninitialhighin1ens.i1ye:cpolutt

The Ke,.,chelEft'ect

lnl 840,Sir j ohnllcnehclnotedthatrodlighthadoomcthing akin toablcachingdfecton the print-out imagcofs.ih·erchloride papcr. h has1ul>M:q""mlybe<:nditco>-.:n:dthatthcl• lt>llimagconan uposcd emuls.i.on ,..·hich io not colour scmi•ikd 10 the red or infra­red is aPP"rently des•f'O)·e<l on """""""' 10 chnc radiaiiont.. The effectiok"°"·n.uthelltrMIKIEJt<I.

An=pts to uplain 1hc phenomenon ha•·c caused much con· 1ro.'Crsy, bu1 in•"CJtig:ruionsin ohelightofourf'""C'l< knowledge of

l</iCll'l<()C/Tff"A/LUl<f;

lamu irnagcfurtnation iodicated that latem intag<: is notd<Sl•oy<d by red light- it iomcttly r,JU1ri•M~ii1hroughout the grain. AftCT red.light illuminaiion thee<: it «:nainly lea nuf«• latent image, but intern.allaten1irnag..isincrea1cd, ataltoio1ub-la1emimag<:.)\lorc­cwer,if1he1urfaa:latcmi1nageisdes1ro)'C<lbya"oxidi.ing agent, and the material with iu rcs.idual i•W-' la1e111 image then t:1tposcd toredligh1,redis1ribuoionagain1a kcsplaceandourfaoclatent imagcisformedatthcapcntcol"io,.ernalimagc.

Why don ttd ligh1 make doc s.il>'cr of doc laicnl image mobile, Kl

1hati1can"" nderthroughoutthcgrain?Thc: ans"'Crisiodica1edby ~.uuranentsofthc,..,,, . .,.Jcng1hofm:uimumaboorp1ionbylatent image 1il•'Cr and the ...,.,..,,lcugoh of maximum Henchel effect. It io 1igni6cantthatthescareidcruicaL. "l1oc"'a>0nablehypothco.ioha> bttn advana:d that 11oc aboorption of energy by the latent imag<: opeckcauscsthc •il"er1oernitanclccuonandr.-.'Crttoa oi l•"C1"ion, 11M:electronswandertoothcr s.i leswlocttllocy maybc1rappcdand ncutraliscdbya s.i l•·erion.Thc 1octrctuhiothatthclarg<:rdustcnof oilvcr atomoatthc1urfacc,whichconstitute1>011tU\llatcnlimagc,arc dimibul"'1lhroughoutlhc grainasi11elfecti•..,;n1crnala"d 1ub­latentimagc.

The"' io yet one 1non: importa nt effect which a more compleoe knowl"'1gcofthemechani1m ofpho1oly>iohase nablcd ustoundcr­ltand. Photographcr1""'U know that a•expo•utt i11crcaocs ootoo don the resulting demity unl il it reachct a ma•imum ••aluc. This rtlationohipiscx prcaed<Juantitnti•·clybythc charac1cristiccurvcof ama1crialundcrccrtaincondi1ionoofproccs1ing. l f1heexposurtio inctta.cd well beyond lhat which gives maximum dcn1ity, it is frcq~ently found. 1hal /.,,,,, dcrosities n;sult ~ rilj'., 5). This dtmast in dcro11tyatveryhoghexpoouresiscall"'11old"""""'·andthedcgn:cof *<>larisa<iMdcpcnd1on thccmulsio11and !locprocessing••>nd;1ions.

Tl/£ SCl£NC£ QI' PllQTOCRAl'/11

l.ct usnowoonsidcrthcaplana1ionofthe'effcctandoornt:funher C""id"1>C<:uponwhi<ohitisbased.

lnour p«:'l'iowcon>idcra1ionsofthepho•olr<i<oproeei;t,_we_ha,·e

~~.,,:~::~·.:~;~t~ ·~~~e ::t~:=:~; ~:::!~h~!~"~ a!oofonna:l"·hen si!,·crbromidedccompooa.\\'e ha,•csccn thata bromine a tom can, in effect, wander in the Cl)'ttal merely by trans­fc...,ncc ofan elcctron from a ncighhoming bromi<lc ion to a bromine atQ11>,wheu1heirrolesa rcrC"·cncd,the ionlxcoming lhcatomand 11oeatomtheion. When a broinine atom rcachcs thecdgcof thc cryital,ll"cha\•elJCCllthatitreac1>withre<luci11g1ullstances,moot prohablygclaiinorasil,.,,r-gdatincomplCJ<. Gda1i11is ... id to be a

~~:,7,:.'r;;:~:t:;,";;:~'.:;::::'1~c:,;:1 :~~0~~';';~;~,i;:~:~~ ~~"!~!~p~f !":!,";. ";~t:~~4: \~ ~;_~":;" :n,t~~t~ at~~ ~:·:·:.~h~:~~:::.~i~""ia~ n;;:::it~~·j~:,!:: ~our;;-,;:; uusoi\·ccxpoouresofthcoolarUationrcgion.

[h1ring 1hco.capoou,..,.largclaw>1irnagc •pccksa...,fomu:don the 1urfa ... ,a11df...,.,,b<omincfintoatunucothcwn'OUndinggelatin, thc11build1u1> &<'.0n""nU'ationoffrttbromincinthe1p,aceobc1wecn 1he gDins. On comi>lction of1heupoourc, the cxcm bromine rcact• ,.;th1hclatentim•gcspccks(ao•ilverandbrom1ncal"·ay1do inthcdark)toe<>at1hclatentimagcwilhala)"erof1.il,·crbromidc,

:::,~e,'.:f~;~~~ - i.;.:~";.eatt~; 1~~';!7::·,e~1~e ~~~~'(,~~ ~~m~~·i: fonne<l, the more 1he latent image specks are coate<l w11h oilver bromide, and 1hc lower the dc,·cloped dc11sity.

If1hisoxplan.ationiooon:ect, certain pre<liction1 a rc l?""iblc. l_Ve should tx1>CCtth;,,tif,..,,remforo~1hehalogcn-aboorb1ngcap,ae11y ofthegclatinbyaddingafurtherhalog<:n ac:ccptor lo!MemulS>on, oolariPtionwoulddecn:ai.c. Wefind,infact,thatahalogenacccptor ,...,has l>:>diu m niiri..,, or certain organic d)'CllLiffs, oomplc1ely climina1a sol:irUa tion.Again"·clhouldupo;ct•nia-1d"'"lopcr oomainingoil\•erhalidetoh'enttoun00>·crthcla1cn1 irnagcopccb and ra:luceoolarioa1ion. Thioagain is found tobc true. SoLarUa1ion

~=:r:;.~~;h .... ~h~~~:i:":~,;~:::i'~7i::~~.::~~~~ ool•·c111- llOw1oome\\·hatlcssoolarUation,but ade\•elopcroon•

~:i~~.~~:;,~ oo~?~l~:~• ~·~i:~:.l~t~y;o ~1,~v~c:'~~;r::~';Z gen•tionnl"thegDins ioa validexplanat>Onofoolarioauon.

C~l6

PHOTOGRAPHIC EXPOSURE

I T. HAS B~'. J:::N.' Slated in a.n carlicr.chap!er (pl73.) that correct cxp01urc may be considered ao the ininimum ncceua ry to yocld a negauve from which aoausfactory print can be made.

llcl<>re""reviewthemc1hodoforarrivingat<hilvalueintcmuofa gi,.,,nlcnsapcr1Urc and1huller spccd,itistocecsaal)'tOeU.mincthc problcmof 1oncreproductiona~nlc~clOlely.N0<inrnpcct

ofthclin.earrcprnduc1ionoftoncs,sin"""'"havcalrcadyoecnth.o.1 thccharxtcris!iccurvcofthcprintingmatcrial makcothisimpoo­sihlc, bu1 in1hc~uctionofagttatcrthannormalDngcof bri1h1nessaanda001nthcrcndcringof""rtain'l<cy'brigh1""""" ouch ao brigh tnmet guatcr than white a nd in the rendering of !ightand11l:ldowin 1uchsubjcctsasportraiu.

Fin1ly,1hcbrightnesorang<:maybcoonsidcn.blyhighcr1hanthe 30:lalrcadyquota:lao1hcaveragc.Subjec1SphotogD1>hcdagain$1 the ligh t,intcriors litbyasinglcwindowand ..::cneo whichinclude bolh interiorandex1eriorbrightn<:n lcvdo may p...,.,.l\\ brigh!neso range9as greao"' [()()(): 1. Thi•may even exceed the cxf>01urc rangeofthcncgath·cmacerialandiseertainlyfarincxcmofthc mnere1>roduc1ioneapacityofarcflectionprint.ln 1uchacau: thcrcisobviouslyno1uch 1hingas 'corrcct'cxpoturebu1Dlhcra oompromilcuf>O'U"'inwhicheithcr thcligh«rordarkerbright­ncu ,.,.,, perifioed. True there arc ways and meant of tack~ng '""ha1i1uationin1hcprintingp<OCC11:thcuscoflong-scalc printingpal'C">loealCJ<poourcoontrol,ctc.l'hetca,.,,discusscd la1crin1hischap1cr.But1hereisa~basieapproachwhichcan often be adopt~ ~ tk CJ<poourc is made, namely tha• of reducing the bright""" range. This can take thelOrm of• fill-in lightforthcdarlterarcaoor,"'hcrcthioio impoaiblc,theocloction ofavicwpoin twhercthcbrightnC11Dngeismon: manageable.

Thc task of 1hcpho1ographcrinuhicving • n acccptab1e lighting b.olancci1furthcraggiavatedby the factthat 1hadowoa1>pear da rker in a ph<>tOflDph than when viewing an actual ..::cne. The cxplanatioui1 thatthccyetends tolookintothc1hadowoofan

~

T/I H SCIHl(CH 01' l'HOTOClltl./'/11"

actua.l••><">e(ma.ki"llautoma.tiCMlj>U<mcn .. <oi .. diaplira.gm) "·hilc the brain tends to imerpret 1hc area. in shadow on the ha.sis or; .. a.ccu.alt<>ll<:rathertluni11ai>P"'retntonc,arcac1ionkno"'n u MitllMSI .-1""'7• llo"""'"'' • it doa >Kit rea.c• to an1·1hing like the Pmed<!grtt ,.·hen ,.;e..inga pho•ograph.

llttauscofthis,ancxper;.,ncftlpholograpller,.·illcndca•·<)UrlO illurnin.atcthc 111adM"tloabrightRC>1b..,lwherethcy"illappea.r right in a photogn.ph . .\lethodt commonly cm~ wru.ist of the uscofrellec1orhoard1pbccdtodircc1tomeof•hemainligluonto the ohadow areas or the uscof1upplcmcntary lighting in the IOnn offtaoh,ctc.

CHSta•tll'lghtnlfS l.Hll

One oft he appr~d>et t0 ilu: p~lcm ofupwurc ~ tl"'t of,.~ki"g 1oaamstantbrightncsalc-.·~l.Tht1canbcach~vai.mthestudmwi1h

::.::ir:a!~.~:~ '°:e":: a~~ ~it :::";~'.~~~;J,':~: '::iio';,.~~u~~ btight~knowntogi~eacceptahlcr<:<ul ... intheprojectedprim

~!~l~:~E~:::~:~~~: ~.~:~rr~:~:l~~%~:~.:~ .. :t~~ pct~ic..:o ~~~,·:1ru;.o~=~~ rc;; .. ~:r ~'.~"'~x~:;:in;~;,;,,g, the l>righmC>O level varitll a grc;it dc11I. For example, the cxpooure rcq uircdforphotoi:rapli ingaocc11e litby~n!ight.(takingmto account rccipmcity fai lure) is more than~ 1n1ll1on Innes greater ihan thatt"Cquire<l rorthetameocenelit W• lh noonday sunhgln.

Th1S!l*1clotS.n11t1 .. 111s terial •

Thevarious 1y11em1foruprct1ing1her1umerical t pccdofancgati'"' materia[ha,..,bccnditcufledclocwhcre (p.!77).Theyareb.o.Kdon ttandarddC\..,lopn>cntandanupoouretimeofabout 1/$0..,.,,,...J. Thcuocolafine-graindC\..,lope<ma.ya1<11C.o.rcduction in•pccd ao><I th.it m..., lie u.kcn iruo account when making 1hc cxpooure. fu<1cs><lcd CSIPCJOU"' timt:1 to eon>11Cnsatc for low in1cnsitics m>)" also cauoca 1ublt.1ntial rftluction in effco::ti,·espccd and wh<:rc manu· facturer"s rcoommendationl are no< a•"ailable. a ocrin of trial and error mun be made. l'or Qtthocbromatic ma.tcriah ("·hich ar<: in>em.i<i\'C 1ored) theopccd numbcrforoungncn illumination ( .. hich

l'llOTOCRAl'///C EXl'OSUR£

~:~:e:~~b1~!..;i!1 .. ~;!,:t;o~,::,~[:Ch.>~1 ~~-'.~ :r.1.::~~h~ tung>tcn iUuminationasi1i1richcrin infr:a-n:dn.diMion. The use offihcn to adjust tonal ...,J.iionshil"' may aho be ooruidercd as a lfcc1ing_the~of~hcma1eriaJ,thoughitismorcusualtotrea.t an1·rcqu•rcd•"""""""mupooure asa fihcrfa.ctor.

Odocr fac1on may affco::1 the 1pccd number. Scnsitixd materiah areoubjcct 1odeteriontion (dcpcndingonttoragcconditiont). 1llc manut"uturcr usually givu an cxpiry date on the bwc Qt canon afocrwhj,,h he n.olo,.gerguarantca lhetpccificd cha~tcristia of the material. Among <Miier lhinp, t"""' could be tome loss of Jpttd.1llcspecdsofthccamcn.aliu11crmaybcinaccun.tcand if an upooure meter iJ uxd, it may be gi~ing mi>lcading rtadings. A Loa ofspttd may a.bu result from reciprocity failu..., (p. 199), ,.....t commonly when upooure limcs of JO tcc0nds Qt lli<)rc ... ..., necdcd.~n_Cllpcri~photographer,hQ,...,,.·cr,ma.kcsapraclie<: of detennuung 1he spttd or a new film ,,., 1he basio of his own equipment a nd procaiingconditions.

MethodofhHHlnghpasun

T he first serious ancmpu 10 climiuaic gu<:11work from upwure wcrcundoub1cdlytl101Cof lfortcrand [)Tiffiekl,thoughduring thc latterh.>lfofthcla11ccri1uryocvcra l kind1ofupotureaidt including tablcs, calculatortand 'me ters' wtrc put forward.

'lliough C>tpotu"" tal>lcs and calculators still exist and form a wcfulguidc 1ocxpooure undcrvariout ligh tingoonditions,thc manufacture on a la rge sca le of1ma ll , photocell upwurc metert, eit her as a component of !he camcr:a or as a ocpara10 unit, has now takcngucaworkoutofcxpoturefor 1hema.jori1yoroerious photo­graphcn,ama.teura1><1 profcss.ional.

Thcba..uofcxpo1uretablesitthatofclassifyingdaylightundcr such headings as 'Bright Sun', 'Huy Sun', •Ooudy Hright'.and 'Cloudy Dull ', alkw.ingan increuc in upooure of <imeo 2 (or l •top in aperture) for 1ucoeosivcly lea bright conditions. Funhcr •ulk:livi>ionsareusuallyallonftl to1hckind of oubjcc1- light, a\'cragc,or d arkintonco--andforfrom,bdeandback-lighting. Cak:ula1on111ually work on the basioofaddingQf1ub1n1c1inglog nwnbcn, thefin.alanswcrbcingapplicd totomekindofocalefor t::on•"Cl'ting 1he number intoaperturco and upwure times. Obvious.Ir thccfficiencyof1uch i.ablaor.:alcula1°"'dcpcndsonthcin1cr­prcu.tionofthc 'c1atoct• and the a,.., with which 1hc amdi<ioosare .......

THE SCIENCE 01' P/IOTOCflAPHf

The rore-runncr ol 1he phouxdl mtt..,. toolt the l"orm or an •ctinomct..,. which m<asurcd the 'actinicity' of the light in toms of itta.c.tionon•)liea:of""1Uiti•-..papor. ' lllctimetllc:nforthepaper tobJackenga•-..anindicuionoflheintclUityofillumination.Other deviect1ook1tM:formofavisualpholometerinwhieh•grcyoealc wasusedto'extinguish'thcdctai!inaM:lcctcd1hadowarca.t"orthis <e..On they we<e known as 'extinction' mcLcr1. Altloough qui<oker to use 1han the actinometer, they we<e open to crroro due to \he ad•ptationoftheeyctolowintensiti ...

Photo·ll1ctrloEJcposu~Mat1rs

These made their appearance a• pho1ographic acCdlQrics in the early30'oand ha•-..rapidlybccomcthc11andard method of deter­mining expoourc. 111e m.ojority <>I such mc1cn employ a 9<leniur_n b.lo..-icr-la}·ercellwhichcon•-..rtslightcnergyint0•flowofelcctnc curttnt, whi<oh i1 uxd to dcflccl a highly ocnsitive galvaroomctcr. The cell iseffccti•-...,....,. awidcrangeofin1cnsity, butf.albrapidly •tlowintcnsiti ... Thcddlcctionofthegalvanomcternttdlccanbe -.1 togi••ca numerical reading whieh is 1hen applied toa cal­culatorwhiehtakeointoac<:Oun1th<:1pttdof1hcncgativem.o1crial (or colour film) andgiveoarangeof1hu1ter1pttdjlcn1apcru.'rc combination! which can be applied t0 the Cam(:ra. When embodied ina canttra, thcdcAccti<>nofthcexpoou<emcterm.oybeih•>wn in 1hc viewfinder {allowing adjuotnttnt to be made tO the lens diaphragm) or it may be directly coupled to the lens diaphr.ogm or1hutter1pecdselcctorso1ha1thccamerai1automMicallyadju>tcd tolhcreadingorthcmctcr.

f'llOTOGRAPll/C EXPOSURE

A more sensitive type <>i: meter embodying a cadmium tulphidc photo-rctiswr cna.bla readings to be made ofint"'°'tia u low u 1batofmoonligh1. 111thiscasc1heceU acQaoa variablcrcsistort0a llowofcul'TC111fromasma!!batteryconne<:lcdt0agalva1>0mctcr, •hc -.:tionoflightrcducingthcrcsistance,•·;g.z.

A 1hird type ol cxpoourc meter is hued on • pho1omcter ""hi.oh inc.orporatcs •'standard' ~ght oourtt. Such expoourc photometer> arc intended lo mcuurc brightnesses of small attu .,,d arc not 1uitablcforin1egratedeJq><:..urcttadings.

M1tbod1 oru11ng bposur1M1hr1

Th= a"' a number of ways in "·hich a phot.,..lcctriccxposutt mcterm.oybcusedforaQe!!lingupooutt. I~ /jzhl INl/tod: The meter is used to measure the intensity of theligh1 comingfromarcgionbehindthceamcr11,themctcrbeing directed awayfromthc1ubjoct.Thusthettadingisrelated to the ligh1faUi11gonorincidcnttothesubjcct. To integrate the light, themc:tcrcellisCO\-er<:dwithadiffusingdevicewhiehoftcntakcs a conical or hemi1phcrical !Orm "' as 10 collect light from a wide •nglc.

Theino:idcntligh1mc1hodisusua!lyttgardcdasthemootrcliable asi• isno1inftuenccdby 11nequaldistributioruofrcfl«tedbright­n.,... from the 1ubject or iu 1um>undingo (when the aoccptanoo anglcofthcmcterucccd•thatofthele111). Onthcothcrhand,it takanoaccountofovcr.oll tonal diffcrenccsofthe1ubjcctwhich in practice would req uirccitheragrcatcror lcsscrcxpooure.

1"/U: SCIHXCli Of" l'llOTQCl<Al'll!

llitlrli1h1,.,,lltod: Them<'1cri1uocdh>rud1heligh1,..,llcc1edfroma mau white surface, >uch """ pi<'CC cl card or while handkerchief, pJacnlsoas 1orcc.:i,•c1he 1n<1in light coming from behind d<C camera, It is ,.;milar 10 the incident light mcd>Od a nd can be uKd with cxpoour<: mcteri "°' dnigncd !Or incident light readings Therc>dingooobtained mustbcdi,i<Jedby~ifitis 1obcapplicd

toana>-cng.osulijtt1. ILowa•cr,itl><>klogoodforcop)·ingdocumcnt1 1o·hM:heo1..Ut of an image on a "·hitcJ>"pcrbatc. Gr~..J-1/toJ: This isoimi lart0 tloewhit....ard or highlight method cxccpttlutther<:flccla1occclthccardisroughlycqualtothca"en.g<: 1ubjtt1. Thcreadingsoob1aincdmay thusbc a pplieddi1tttly1<>an a ' -cng.o 1ubjtt1. lloth th io and the white-card m<1hod can be useful 1o·hcn dealiHg 1o·i1h1mall1ubjtt11 which do not provide ... mcinn urafora <e:flccted ~gin reading. Rr.Jk<Wlitlll.,IMJ:"l11cmcterisdircctcd1owartlothe1ul~ttobc

photogr:aphcdsoasoomcasurcthelightreflcctcd..,.,..,.rdJthecanw:ra. It is by far doe mou common mc1hod in use and g;,..,. satisfactory rnuhs,chiefly because tloe majority of1ubjcc111how '"ry little variation in the lotal rcAccted ~ght under a gi>..,n intensity of lighting. llowc,•cr, i1 io likdy to gi'"c misleading readings (or incorrect CX]XMl!rCI if the meter is embod ied in a camcr.o u an a utomaticexpo1urccomrol) when tl>C 1ubjcct contains large !ighl or dark arcu. Thi• .; iua1ion can be greatly aggr.o\"ated if_ the acceµ tance angle of the meter is subatantially grca!er than that of the lens (in"'1riably the case ,.·hen u1ing narrow angle len.., unl'31 special precautions MC taken). The most common CJCample of •ncxccosivelylightarca (tendingtogiveundcr-e•poocdraul11)i•R gcncral\"icwcontainingalnrgcareaohky. lnthiseaocitioruh-i.able to tih the mc1cr<lown.., ao to exclude most of doe sky frnm the ongle of the meter. The reversc.;iuaiiou ari.., "·hen a rda1ivcly omall hutlight1Ubjccti1 placedagain"adarkhaekground. l n thio -:=

1;rcflcctcdlightreadingwould tcndtogi"e""o'"er-exroocd

l"olllm up, thcexperic1>Ccd photogr-.. phcr will use an cxpoour<: mc1cr u a guide, cmploring a1 1imco either one method or the other a11d modifyi"3 the reading indicated according to his experience of oi •nilarcondi1io"sa11d1he kin<lofcffccthe dcoirntoobtain.

This 1y11em originated in Germany and ""'"' fin< lmown u 1he light-value 1y'1cm after •he word IK/u,.,.,1. llOWC>"tr,since it also

l'llO'fOCHAl'lllC li"Xl'QSUH/i

1akes imo account doc 5emi tivi1y cl l!>C flhn, it wu la1cr, more o;M"rcc1l y ""med CJCpoou,.., value 1ystcm. l< utilioeo a log ocalc of numbero-1, 2, 3,4,and ..:>On-in which adiffe,..,necof l rcp­rcM:lllO a factor cl 2. The "" me ratio, in terms of light pusjng ""'l"'city, applic:i tO the/ {mHnbcr ocalc, lhough in thio cue the 11umbcngoi"ll1Ctcrft:t/f'J.,ff'J.·8,/{4,/{S•6and ooon. h mnain<d toironouttheirregularitft:t ind1C 1hut1cr •peedocaleto!lutthcoc a1so,...,rcbasedonar.o1ioof2.

The purp<*: cl the cxpoourc ,.,.Jue l)"llC•H "'"'"' tOoimplify the opcra1iollofscuin1 1loercquittdcxposure onthe u mcn.. Thetw"O rno>'1;mcn\I IOr changing leno apcrturc and nposurc 1imc "-er'C made

ODl1lplcmeniary oo thaiutloe si•ecl thclcn1apcr1u"'waod""rca...d 1hc cxpoou,.., time""'"' incrcucd by a rompcnsa ti"g amo11111. The '""""'lcs,.·crcroupled and a ohirdi.ealeclexpoou"' ,.,.J...,.wu in1mduccd which ""' "'lated to tl>C aclual CJtposure (based on film •peed and light in<cnsity). The act cl octting the e~pooutt \"aluc 1Calc 1emporarily uncoupled 1loe apcriu,..,..shuuer 1Calcs to .,,able1hcmtoheoc1toaba.iercla1io,..hiFwhichga,.ctl1Crcquircd cxpooure. llowe....,r , once $Cl , any cornhln:ni<m cl apcriurc and st.uner1peed could then bcsclcc1cd at wi111ogi"can cquivalen< oxpos11re. ~·or ox.o.rnple, if a fut lpeed was required for an ac1ion 1ubi""t, 1hclen1apcr1urc"·•1opened uµa1 doesame1imc

llulde Numbe r S11temforFl11ll

The b,,.;, of1hi•1ystern hns nlready bcc11 exµlained on p.39. In practiecitis ncccosary1ntakein toa""°untthe lightrcAcetedfrorn •urfaccs adjaccn1 tod1c•ubjcc1.Guidcnumbcrtahlcsarcnormally ba""l on an "'·cr.oge room ""ido wall• ofa'"erage briglnnns. L.ao oxposurcwould be required ifthcA u h were used in aomall room with "·hi tc wall• (1uch u a bo.1hroom or kitchen) and mo"' cx­posurc in a lari,<C hall or whct> thc Auh is uscd out or doo,..

The ,,.Jidity of dot guide nuinbcr aloo depends on 1hc type of reflector fined 10 the Aaoh holder. Small ,..,Accton, dcoigncd for portability, may need I stop mo"' expooure ODnlpattd with one dn.igncdtogi,..,rnuimumcff>Cicncy.

Su bj1etMore m111t

Thecffcc1of~~tclpanorwholeofthe1Ubjtt1 at !he time of e"po1u,..,willbe~n!in1hephotographao~hl11rrin~ofdetail

Tllf: SC/6/VCE Of. f'HOTQGlfAf'llr

in 1hedir«tion of1he ,,_.,,mcn1. Thecxtc11t 10 which 1his becomes apJ>l'rml dcpend1 on a numbcr offaclon which include thedu,,.tion of •hce><pooun:,thc•pttdofmo..-.:ment,it1dir«1ionn:la1iw:10 U.. lcru axis and the """Jc of 1hc prinl image. The amount of blurring isuiuallycommllcdby thc1imc <>f""pol""'and1abJcaof•hutt<r sptt<b for pt.o.og.,. phing ,.._rious kind• of ac1ion hav.: been in uis1cncclinecutca:nl888.

Wi1h a fait-movingobjcct •uch a< a "'cing car, it is common practicc to •wingorpa.nthcca mcrainthcdireclionofrnovcment a td1c timcofmakingthccxpoourc. By thio mca11J, rcla1ively'l0t•g' cxpon1rc1imcacan l>egi,•enwhilcotil!rctaining •ha rpdc1a ilinthc 1ubjcct. S1Mionary objttu in the background bea.mc blurKd bccauseofd•c movcmcnt<>fthecamera, but 1hisu1uo.llycnhanccs thcpictorialdfttt.

Tooal elimination of blurring in an ac1ion photograph usually (i\'Cta"fro&en'cffcctandactionisof1cnconveyodbeucrbyn:tain­ing a certain amounl of blurring. Indeed a kind ofaaggcnucd n>O\"ement blur obtained by uling Jong""!*""' timca has come 10 hcknownas'Auidmoti.on'.

<:Mpurll.

AFTER-TREATMENT OF THE NEGATIVE

T~!~G~£;~[~~::a;;~~;:17;:;~·!:3 by Jtain1, opoll or 01hcr blcmishca. Undc.....,,!*un: may °"""r undcrpoorlightingconditions,raultinginancxccai•-ely'thin' l"'l"ti•·c,whilcuccasi,·ce><posun:mayrauhfromtoom!lcb caution in making an catima lc. While a dclllC nqati\-e may 11iU yield .. tisfac1oryprinl:f, itmayalsoin,...iw:ancxccai•-ely long printing upooure lime. Variatioru in dC\-elopmcm rauh in diffcn:nccs ir. contr.ut•nd••·cr:agcdcnsity,hutpoliti•-em.:ucriabareno"""da~ 1uppl~ in1uch ~widcr;1ngeofco.1tr.u t< thatnegativ""CO\"cring •-erydilfcrcntdcnoityrangacan l>eaccom!MK!atcd.

NC\'crthcl ... , ifd .. iKd, it i•J'OS'i blc •o reduce thedcns.i ty ofa ncga1i,·o byrutidisin!{oomeoflhcmcrnllic 1i lvcrof 1hcimage toa 1ih•cr 1., lt anddi...,Mng thclattcrina ouitable oolvcnt. T hio pr<X ... , in whi ch dcmity ii reduced, " ·ao quite krgically called by photo-1rraphcrsm/.,1iM. l ti1 unfortun.atcthat chcmisuuse thitPmc tcrm fOT, among other proccs•es, the producoion of a metal from iu <a h. Photognphicrcductionisachic,·cdbyoKKl ation,whichi11hecxact """'""""'of chemical reduction. Whcrc a mbiguity ;, likely to arise, thcrd'ott,1he1ennrt0t!ia.oshouldbcpn:fixcdby1l>e"......tµ.w­l'•}/oi&ortWi<M, u thcca>Cmay be.

ltis abop<miblctoincrcti<':orin1c..,ifyncg•li•-edcnsiticaby i!""gc·"·isc dcpooition of sil•= or other material on the mcl.Ollic 11 lvcrofthcimagt,or bycon•-.:nionof1hc imagcsih·cr intooomc od..,r 1uh$1anccofhighcrpho1omctriccons1ant,1ha1 is,ofg...,,,t<r lighMtopping J>O"''Cr. Thit proccso is known u i•lluifi<•t~.

Th• O•ldatlonorlllttlllloSllver

Tllli SC/liX(:li 01' P/IOT(lt::RAPllr

sikcr of1he image ha• been "'"'°'"ed. The end of the procas io nonnallyaaet4e<lbyiruptttion.

Not all oi<idising agents arc 1ui1able ford;.olving 1he metallic lilver of1hc: photogn.phic image. Nitric acid, for eumple, it a typical cht:micalo><id.isingagentwhichreod'.lydia)l,u li l•..,r.to form1he>Olublesalt, .. lvcrnitn.te(p.89),bu11tal.,attacbgclat1n, oon••:<tingit in1o>Olublecompou11<1.,., that illU>CWOuld invoh'<: thcdangerofkioingthewholcol" theimagelayer.Suitableoxidising aiicnts for the li lver of pho<ographic images """• in W:t, compan.­livcly few in number. \\"ewill 1oowoonsidertbe"-ayinwhichromc of1hc""""'impo<1.ant°""'fonc1i.<>n.

PotaulumP1rmanput1 Some clemmts arc capable ol" fanning compounds in ,,·hich they cxhibi1verydiffermt•"ak11ciQ.{p. 124). llle•.,.kncyofm.ongar..se may be as low H 2 in manganne •ulpl>ate, and as high :u 7 in J>Ot""ium pcnna"lanale; in Olhcr compound• it cal\ abo oho>.· intermediate valcncief. The i lruO!urnl formulae for the two oom­poundo nam«l arc:

Mn(:) st: .\!•nga.....,oulph"<

K- 0 - :>.ln = O II 0

l'o<.,,;umpe <m•ns•n•tt

l n thcprcscncc of1ulphuric acid,po1"11iumpcronangaua1cac""' apowcrfu l oxidi1ingagcn1; it r<:adilygi•·cs up its execs oxygen lo cornpoundscapablcofbeingoxi<l iscd,andi1iuclfco11\"i:rlcdin to

;::a:r ... ~C:~~~~~~~~?~.'. ';,~-i~i:~'.~,:~i;·:r ~~~·~.~~~~;i;:a:;~~ if::!~ sih·cr•ulph..tc (oolublcinwa1cr), thut:

2 101n0. l'ou.Mium

KlSo. + 2 ~•nSO. +~ As>So. +8 11;0

l'o1a•um ~·- s;i_. """"'' ..,lpho<o m!phoi< >ulpluoi<

Acidpcrmangana1e t.a.liulecffc.:1ongcla1in,ooth.atthcproa:ss

~[~;::, ::t~~ :.;17:;:-a.:"~~. :~-~i:.~ ~~.:r.="::~~ h.alida (IOrexamplc,chloridafromtapwater),""hichwouldre.oc1

AF Tlill-11/A"ATMli/>"1" OF TIU: XliGAT/l't: 219

;;~i;:~: ~u~~:..t:!i"';,,"'~:.~~~~i~n ~cu~:~ ~~;e~; uxd in ""''ersal proca$i11g1eehnique (p. 133).

'otu1iumDlcftr11m1t1 In the prcscnceofoulphuricacid, potusiumdichrom.otc ...,actiin much the <an>c way a> po1a..ium pcrma11ganalc, the chromiUm ,.,.1cncybmngfrnm6103, d1us:

1UCnO! + 1 11,:>o. + fAI ::= .. :!r''"ic !.~:1;.

=um + ~! + !~ + !..'.'"!.? oolpho1< >Ulpho1e wJphooo

Chromium salts•uch as chromium tulpha1e and chrome a lum (Ht p. JOO) hav<: the property of combining wi1h gelatin 10 fonn a com­plex which is i"""lublcin hot "'"'er-in oth<:rwonl., oflumlnrillt gclatin,andaince1hcamounlofchromiums11lpha1cf0nncdatany poin1 io _p~portio~a l 10 t~c amoum of lilvcr, du:: har<iening or the gelatin "' 1magew1oc. 01Chrom.o<e it thc""fo"" of<cn -.i when imagcwioch.ardcningio required, as in the Carbroprocas (p. 270). The sa~ncprecau < ion againtl pracnccofoolublo halides it necC11a.ry with <hchromate a• wl1 h pcrmang•n•<c

F1rrlcCblorld1 1ndPot111ium F1rrl~11nld e Iron can. cxio< in <hc/trri• Fonn, w~t h a valmcy ofS, or in theftrrolLI f01"m, .w11h a valency of2. Ferric salu are capable ofo•idioing metalbc 11lver, 1hu1

,.~ "" ,."" ~ Ferne Sllve• t'•rrou•

chloride

~:::!°") ' + 1$11;., 2 t"...0. + ~?' t· ........ wlpho1< ... 1p1t.o1< ~ .... Thcrc is,i.....· .. ..,,.,an a hcrnatlve andmo<coonvenicnt""'Yof

a pplying thcferric-fcm>U1r<:acti.<>n1otheoxida1ionoflilvcr.Both fcrricandf<.:rTOU1aah1wlll l"i:actwith potqiumcyanidc<oform ferric and ferrouscyanida, rapttlively, but ifmon: potassium cyanide is :oddcd, •complex aalt is formed in which the iron atom U in the «ill radical. Potassium cyanide fOrmt soluble compklt

THfi SC/fil>'CE 01' l' llOTOC ll.trHr

mci.allocyanideo of this type with a la'll" number of metal salts, audindccd itsfixlnivcpmpcn iesatcductothcformalionofooluble a<g(:ntocya nidc. The reaction of potauium cyanide with oil'"'' chloride, fcrrou• chloride a nd ferric chloride is thus a two-stage n::ict;onincachcasc;hutthctotaln:actioruare

?!&. 2KC.'1 - K O ~~~· ~-cyanidt <hlorid< """"IO<)"&Bi<I<

(......,ooM•l

··~ 6 KCS - ? KO + ~:~)t , .• ....,... ""'...;.'" ,.,,.,..." .. ·- ~nidt <hloridr """"'J•r>i<I< , .• a, 6 KC.-.: - S KCI + ~~:!;~~· Fen;,, l'<>o.auiurn "°'""'"'" ch!oolde cyon\de ohlorid< f.rricy&nid<

l'otaaiumfcrricyanidc,.•ill oxidi .. mcto.lli.o s.i lvertcadily,forming polassiumfcrrocyanidcand1ilverfcrrocya nidc, thcla ner bcingin­oolublc. lnthcp<eoenocofaoolublcllalidc,po<ai;&iumfcrriC)•anidc ,..;11 con,·cns.ilvcr1os.ilverllalidc:

=~)• + ~"',,+ ~ ... - ~~)' + ~! 1e,.,.;.71midt btomi<lt lcrnxyani<lt bf-omid<

Potass.iumferricya nldc is 1nuch lcso toxicthancyanida,and h:u *C>'cralvaluablcpropc«iet. l n i pitcofits abilitytooxid iic mctallic s.il\"cr, it is onlyamildoxidisingagcnta~d, unlilr.CJ"'nnangana~cand dichrom.o.tc,willnotoxidiocoodiumtlnosulphatc (hypo).Am1x1un: of fcrricyanidc and hypo-known u Fannn'J .,dw...-is ~ for oxidising mctal~c s.il>'<=< and s.imultancowly d issolving away the inoolublc silver fcrrocyanidc which is fonned . AI>Mhcr ad van tag<: of fonicyanidcistllatitdoano1n:adil7a1taclr.organiccompoundo. ltis ~,chcn:IOre,forJucltpurpoocs:urcmovingoilvcr(romcolour malcrials, while leaving dy<: images unalfc<:ttd (p. 2M).

locline lla logeni will, of courK, react with metallic _silver 10 form s.ilv~r halideo(p.89).Thcmo11convcnicnttoapply.,iodincdiuolvcdm potassium iodide; 1hiJ iJsomccirnauocd:ual«dl r"-n(photo­gr:aphic)for prints,1hc silveriodidebcin:g•ubsequcntlyfiudoutby hypo. A mUnun:ofhypo 1lnd iodine iJ lmpnu:ticable u the 1wo react

AFTEft · TllEAT M !."NT 01' T llE NfiCATIVE 221

1oforma•ubstanccwhich""!'ilhoero.Udiocsnorfixcs,but aniodinc­cyanidcmixturecanbeuscduapowcrfulphotographicrtduccr. The two substances re:u:t 10 form ,,,.,..ft" w;,u, which is a powerful oxidiocrofmctallic •ilvcr.Thc rcactious whichoccur an::

11 + i.:c~ lodin• P.,.....;um

~ni<lt

+ C." I + S KC.'1 -eya_.. roe-;... .. iorude <y>nlde

Poll111lu111 Penulphmt1 l'oio»i um or ammonium pcnulphatc is a powerful oxidising agcm '''hose••J>trJm>/H>'h4""/action(dc.cribed la1rr) malr.cJit ofva luein photographictt:duction.

0 II

K--0---S--0-K . 0

l'ot.ai•m1"lph>.1<

0 0 II ll

K--0---S--0--0-S-O-K n II 0 0

l'o1-'umpmulpOa10

f rom the structural formulae of potassium sulphate and JI01as-1ium pcnulph~tc, it will be .. en that the latter has two !inktd oxygen atoms. Jn the piucncc ofmc1allic sil\-.:r, thi1 linkage is cas.ily brol<cn toform potutiumand sih-.:r"'lpha•a:

Plteto1r1111hlc RH uctlo1 Oifrcttnt rtducing solutiom act in diffcn:nt wayt. ~me ]0\\-.:r a ll de nil tics by an ~ual amount, ~nd are called mblro<I,,,. mJu«r1. The contrastofthcncgativcisthu11ubstantiallyunaltcrcdancrrcduc-

~~:; ':!a~~;~::i;~~ "':',~~t~~~~ r:::=. ~~a.::~;~;~ d~tyduccogronovcr-cxpo1un:. fam>er'1 rcduccr,madcby

~n~ .. ·~~~ ;;~, ~;i;.t~:::~ :"!:.'"plain

Tiii! t;CJ£JfC£ OF P/IOTOC!t.lPHY

OthcrOJ<idisingago>llO>'.idioeo.il,.,:rrougbly in proportion to the aonoum olo.ih-.cr locally prtxnl in the imagc, oo that ncgativ.: denoitia a1c reduced by the same proponion, ir1apetti,.., ot ori1inal demity. Such rcduo;;cn arc called ,,..,,..,;..w tnlwrT<. A typical uamplc: is a mix<urc of po11us.ium pc:rnunp.nate and poi•um pcnulphatc solution, acidified with 1ulphuric...,id. l'ro11ortional rcduccr1 thwt reduce 1he «11\t~t ofa negative as "·ell as it1 g<:ncn.l density, and arc most ouitable for oorn:otly CXl'O'<"I, but<WCr-dC'·dopW negatives. ~aroughg<:neralisation itmaybc11ntedthatproJ><>rlionalreduotionur1dOC1dcvclopment

l'inally,tomcoxidisingagenaexhibitathirdtypcofbcha\"Wur,in "'hichoxidi1ingactiom>Cetmtobc11mnglycatal)'f«l(oraccole­rated) by themetallico.ilv.:roftheimag<:.·11icn:ductionratiois therdOrcgreatcrforthchighcrthanforthclowerderuitiea. These a•e the: •~J>n ,,..,.,,,,i«ul r""-<.,s, of which ammonium or pot.a$o.ium penulphatc"•ith1ulphuricacidisancxamplc.Supcrproportional reducers arc,,_, •uitably appliM to 1hooe ncptiva in "'hich •Ucmpll have been made to corn:ct IOr under-opoaun: by 0>-.:r­d.,,,•clopmcnt, o.ince hcte thc aim is to rcduce contnut ""hilc affectin1 thclllMlowdemiti"'aslittlcaspoaiblc.

Recently (L952) ane>o·and,,.luablc rcduccrhubccn ..xled to our armoury. It i• ,.·ell known that a neptiv.: left too long in a fixing bath is liable to undergo reduction ofdc11$i1y. The o.ih..:r image undergoes aerial oxidation and the hypo dissolves the resul!ing w mpounds. Dcmity redU(;tio11 is much more rapid in ammonium thiosulpbate than in hypo (IOd ium 1hiosulphatc) and more rapid 1ti11 in add solution. A solution of ammonium thio-1ulpha1e and hardener, acidified ,,.;,h citric acid . isa mostoon· trollableproi><>rtionalreduecr.

h 1hould be emphasised once again that l"'""' reduction ofa ncgati•-.:ohould be necessary only in Jpc<:ial cireunuu.nca; on the other hand, Uui reduction ofhighlight1ofa ncgui,.., is "'"'1' valuable 1cchnique. Interior vie>o'S, for ....ample, frequently cm­bracc an ex'rcmcty ..-ide brighu..,.. ra.ngc, .. ·hich if comprcsocd to

the confina ofthcdeno.i<y range of printing p.open would result in an unfortunate reduction in ovcra.U conlnw. A much more pleuing effect may be produced by locally reducing the hiJ!hlijl:ht densitiesandapplyingthcrcduccrlocallywith,..,y,a1"'"'bof ODllOllwoo.[.

11tlH1lll1111tl01t ln1ensificationcanbcu>Cdonlytoraisecontrast10 thclcvclncccs· ..,ryfor1uc.:e.sful printing. Since intensification merely inoreaseo

AFTlill•TllE.lTMEJfT OF T llli NKC.lTIV!" 2?3

density,i1cannotprodutt.i-io..-detailwherei1doatM>talrcady cxis<-it.c.anmcrclycnhana:itandraiocitfromanon-printable 1oapnnu.blclc.-.:J.Corrcc1Wnl7orunder-cr.posureistherefore •·crylim.ited.

lnteno.iftcalion may be achi.,,,·ed in oevcral """)'S. The image may ~enhanced by depooi1;.,,, of.•ih.-.:r or mereury metal or1 1he o.i lv.:r image by immcro.inir the negauv.: ma solution w ntaining a chemic.al reducing ag<:nt (for example, developer) and a 1i1'•er or mercury ..,It, The: oilvcr imageg,.,.iru ""las nuclei for the reduction. The pro<:eui1calledpllyJki>linlauiji<al.ionandi1 analogow1ophysioal development.

ln <ktrtkl>linltnrifaa1il»ltheo.iJ,·erimagei1oxidi>Cd lo •i h..,r halidc: by an oi<i<lioing ag<:nt which io iuclfrcduccd to an iiuolublcoom­pound. The: augmented image is then chemically darkc,.W. ~Jc,.. cul)"intcnsif>ermaybcquotcdasa1ypicalcxamplc,1houghsaJ110f mercury, chromium, coppe<, lea<! and u"'nium alt Kl as in1ensifie"-

Wbc:n a negative;, immersed. in a ooluiion ofn>cr(;uric chloride (corrOli,..:oublime)1bc:imagcbccomcs..-hitc. Tlie1il•~hasbccn

converted1oo.ilwrchlorideand1hen>cr(;uricchloridcround.,.,ch gnin 1..., beenwn,..:rted tolnsolublcwhltcm=uroufchloride.

HgCI, + Ag - AgCI + HgO ;\lcrcuric Sil'"' Silver M=urous chloride chloride chloride

AOcr thorough wa•hing: the white image m.ay bc blackened by ammonia, lftheimagei•blackcnedbyde••clopinginadcvclop•:r containingno1ulphite,>ilverandmcreurymctabarcf(U~lllilali\•cly

produccd,at1dlhcproccssmaybcrepc:atclasoOena• desired,ev.:ry oilvcr atom of the image acquiring 1, 3, 7 or (2'- l) a torn• of mer­cury aOcr I, 2, 3or~11agesof in1cruifica1ion retpecti•·dy.

A valuable 1inglc.oolution intensifier w~• described in l!H:.. h io kn.o .. ·nas thcp~Ut~1wlf>M1tiwlauifar, andproduce1agreoter

degrccofintcno.iftca1ion1hananyotherkn.owno.ingle-oolution intensificr,lhemaximumbcingoftheorderoftentimcs.

In gcnenl, in1eno.ification gi,..,. rise to increucd gnini...- and, unlcaitiscardullyarriedout,lsliabletoproducclocalirrcg1•· laritics.

SpetlaMStaiH E•-cry manual of photognphy emphasisco the neccstity for dun and e>.rcful working. Nega1;,..,. may bcso eui!y 1poiled by•po1$and 11ai1U;itismucho.implcrtopw.-cntthcir occurrcncc 1han10dimin­•te them ona: they arc formed.

It i1 pWiblc tha t 1pot-fonning impurities may be prestnt in the

THE SCIENCE Of l'llQTQGllA/'llr

sens.itivc rrta.l(rial as j>Utcha.W but, bccau .. of lloc elabora te prc­cau1ion1 a11d utcni.ivc 1<:$•ing by rna.nufactur0T1, the chancc of encountcring1uchdefec11i1r'f!du«dtoncgligiblcproportiont. Spou a rc 110tmall)" catned by p.>roiclc. o( air-borne or liquid ·bome impmitiathatcomcinoontact ""ith 1hc..,rui1iverna.1ttia.I. Even inen duS< may cause blemishes if it is iw-nt on the material during cxpooure.Toavoidwhi1c 1potton1h.c11tpti•·c, .. ·hicha"'1hcshad­ows ofsuch 1peeb, rhcinteriorofthccamcr.olhould hcdeanM pcriodically.01hcr 1ypesofdw1 a ,.,1.-cdangc...,.,.,ai1hcymay ha,·eanadVCTKchemicalcffec1on1hcncgativematcriaJ.

Rtu1y pipes or corroded copper may give rise <oopcch which may hc cithttair-bornc or liquid-borncand which may causc .. rious damage1ofilM1orplata.tilack1poumayarisefroml"'rliclaofoolid dC"'·el~r,ifthede\'clopcrhaano1hccttprnpttlydiAoolv<:d,orifa doodofpo .. ·derMd.,....,lopttrau!ufromcarelas"""ighing-outin thcdarkroo".'.Findy-dividMoilslobul.esrna.ybeptt>Cntinairor liquid and, 1f1hcy com.: in to<Uact "'""h "'ns.iti\·e material, they locallyprcvcntdC"\·elopme11ta11dfiu1ion.A...,.herfaultthatmay a rise1aka the fonnof1harp-cdgcd whitc•poucauoed by airbcllo trappcdon1heourfaccofll1c plate or film. This fault isovercomo: by careful introdu<:tion of the material into thede•clopcr, but the fault is much rattr 1han it u>ed tobc,owinf{IO thcpuvalcnt""" nowadaytof .... ·eningagcnls in proea.ingoolutions.

lngcncmlprcvc1uio1>isbcl\erlhancutt,butwhi1c1poucanbe removed from a pro<:atcd negative hy loral appl ic~iion ofa pig­men t by mc~ns of a brush, a nd black opols by remov ing loca l u cassilver (and gclntin) wi!h a knife blade.

finger marks arc a 001nmo" defect on homc-pro<:atcd ma1criat.. While one ca11 with rela1ivc impunity touch the surface ofa dry ncgativ<:ma1<:rialwid 1clcandry hands,iti1apracticethat1hould beavoidMumw:hat pouib!c.lf1hefinger"1aredam pcdbypro­~ng oolu1ion1, orc,•en by pc11piration, thc finger- print ""ill almoin ccnainly appear on the prooeaed negative. Dilute hypo imrod~ locally beforcd"'""'°I""""' by aoontamina1cd dip or by di<ty hands usually produca dcvaolating stainL

Undtr the cattg<>ry of 11airu may be conoidercd 'idr<N f .,, a ~cposj1offinely-dividcd oih"ct"oocal]M bccautcitappeandifT,,,..,n1 LR colour ""1~11 Vicv.'cd by rcllCCICd and trarumittcd light. Sil\..,r appcaninth11)"cllowish,find)"-di,i<Jedou.1e"·hcnitisl0rmcdb)" thcaaionofd.,.,-.,lopcroni.ilvcroa.l111h.atha,..,goncinwoolution. lt isthuslia.blc1obcfonncdwhcn1hcd"''Clopcr oon1ainsaooh'en t ouch ai hypo, ammonia., or ezcc. of sulphitt, but It is more com­monly formed if the film, Atura1cd with dC"'..,lopct, is pa..cd wiih-

Af1TR-Tlli"AT.t/E.\T Qf" rut. ,H."CATll' t. n~

out i~<emiedia1e rin"". or 11op b.:ith into a neutral hypo bath, Hptt1ally1f1hela.ucr11cxhau11ed.

Another form of fog blemish i1knownai..,rial/•t· II io formed ifthcmattrialisdC"'..,lopcdncart!ocsurfaceofthcde'·elopcr, "'ith in•ufficicm agi1a1ion. "l1oc 1urface lo.1·erofdC"'..,loptr, rich indis­•oh·cd o~ncn (OJtidiscd dC"'·clopcr) , can ta""' •pont.aneous fogging ofthcfilm.Ahhough..,.ia.lfog io gcnerallybla.ckincolour,itD,b)· tlocnaturcofitsl'or1nation,usuallypa1cliy.Suc:hoil•..,rfogoarc,of cou....,, hcs1 a,.,,fdM bu• if formed they ca11 be rtmo,·cd by 1he judicious application of a pl>01ographic r'f!duc;cr. Tloc image is, ofcou~!anackcd1ooomcu1em.butthel'og---opttiallythc findy-dovidcddichroici<>g- i•prcfcrcntiall)"fCTRO''ed.

Other01f1 ct1lnll111tl'n \\"hen a procascd aold .. 11llied pl•ttcorfilm Dallowed to dl)", the surface waler may collect in drople11 which re<Juirc much longer local dl)·ing 1imt. Tioc resuh may be a permanent,,,;., ,,.,,.1; on the ncga1i,..,. Thcoc can be J)tt\"Cntcd by tt1r>Q\·i11g ourfaccwater, ei~her t.)•wipingwithchamoislealhcrbeforcdryingor\Jyafinaldipofthc material in "'aLcr con1aining a wetting agent, The latter reduces surface tension and allo\\~1urfacewatcr1odrainuniforml)· without colltciing in tlroplc!l. Drying marks. once formed, cau somctim" l;crerno,·ctlortlttreaocdbyrcwcningonddryingconectly.

i\nothcrncg,.1i,..,defect isanopalcsccntappeara1'Ct. There arc .. "el'"~I 1-.ible cau..,, for this. /\ hardcrocr bath Iha\ has been allowed lo become 100 alk~line may cause n tlepoiition of,.lumin­iumhytlroxidearo<lanacid ri11 .. ,.·iUrcmovethisscu!!\. Hartl watcr, loo, m.>)"Cause a tlepooi1io11 ofc~lcium carbonate during washing and that also may bcrctno\•cd by acid rin ... Again, gelatinoflhe tmulsiou or l>ackiug ma y become ttticulated, \hat is, show a pancru ofminulchillsantl valleyo. Thcgtlatintluring pr<>ecssing i••ul>jec1cd!othe actionofoolmionsthalca" .. aherna<eswclling and comrac1ion, and if the coniractio11 l><'COrna too ,·iolcnt the surface may become pcrrnantmly dcfonned. S"·clling and con · tractiondepcndupo111henaturcofAhsin solution>,theiroon<:<:n-1ration, doc dcgrtt ofacidi<)" or alkalinity, and 1hc1cmr>trature; reticulation is normally a'"Oidcd by ctlluring that proc-.ing solutiomand wash ""aterareatapproximatclydocArn<: ltmpcra­rn,..,.Ano1h.cr1ypcofopalcscct1<emaybcductoforccddryingby immcr>ioi> in alcohol after washing. To a\"Oid 1his, it is recom­mended that tlocalcohol oolution ohoukl not Le stronger than 80 pcrccm. Op.>ICKC11Cecauoodbyakoholtlchydrationcanbccur'f!d hyrc-immcrt.ion in h111cr and normaldt'}ing.

THE GRANULAR STRUCTURE OF THE IMAGE

l~c~~:i1~=;\:0~~~i~~:;,~h~~~·~?:i~~~~~~~:'ff:~~ ln any one nogativc there is a range of grain 1i•e1; a,·crag<: size <'arieo from ucgati,·c to negati,·e, acmrtling to the cmul•ion and 1)'pcOfdC'-.:lopmcnl used.

Graininess J. lanyofd>egrainsappear!o occurindumps,whil<:bo:t"""""'hc1n there ar<: rcla!i\-.:ly larg<: anpty lpKCS. Clumping may only be apparent; •he grains forming the clump> may in facl be quite dil•inc1a11d1tparate,butoccupydiffcrcntlay<:nindocthid:ncuof 1he emulllon. This u.......,nn= in diltribution is qui•c charu1triotic of a completely haphuard arrangemcntofparticlti. !f,.·e coun! 1hc !o!al number ofgnins in Vig. 4a, p. 9-1, and akulatc zhe area ofa oquarc which u ""GUTag<con1ains !en grains, "'C might fit thi11quarc0\·cr arc!a1ivclycmp•yspaccoo!ha! itcon1ain1only one, or even nogcains,orahcrna1i\'C ly, l>yincluding,.clumpOT lwo, m• ke it cover many more tha» lcn grains. Thus area• '"""Y much lnrgor than !he individual grain 1i'.c will show a \'ariation inlighMtOypingpov.-.:r,ordensi1y,whichwillgivcrisc1oagrainy appcacanc.:. This would not happen if we could give the grains a geomccricdiHributionoo that they were ,.,...,.,1y.•paccd. HowC11cr, as cmulsion gni111 arc""' '!'-.:nly spac.:d, there 11ad"8"""ofcn­larrcmem,muchlCllthanthatrcquircd101howindividualgrains, a t .. ·h;chantnlC\-.:nncsoindcmity•...,..,iuclh1agranularotructurc of •hcimage,calledl'•i.U-S. TI>ercisaroughcorrtla•ionl>ct"·'""" 1hc anragesizcof the •il'-.:rgrainand1heamountofcnlarg<:rncn1 ncccs.arybcforcgrainincss;.apparent,oo!hatingeneralfine grain«! emu]"°"' •how 1"'11 graininess than coorscgcainedemul-

F~. LS.•ic•ol<nluf<m<nu(•) ><I,(~) x•,(•) ><16,((1 ><6i. (•)><UO,(/)>< l,OOO lin<0rm"llnifk•oion.(l•- .n..G.C.F"'odl).

'"

·n1c principle on "'hich grainin.,.. is measuced is simple and Jtrnightforwanl. J1 i11ornakea1Cri«olcnlargcm<!l1Uorcnlargcd op1i.calirnagcsandcsti1natcbyC)'Cthatat"'hichthcgrainjust '"°"-..,In pnoctitc the estimation is far from eaoy a1>d ol-.cn·cr .. cri1cri.aof\ilibili1yolrrain,-aryfromday today, t.csideibt:ing dilfcn:n\ fordilfen:111 ohoe.,,..,n. The earlie11 method projcctnl a much-cnlargnl irnagcof1hcsa1np!con a whitescrc.:n \>'hich """' obscn•cd by n:l\ection in a mirror. Hy """·ing the mirror. the apparcntpooitionofthescreen w1Urno••cdawayfrom1hcohoe"·er umilhecon1idcrnlthcgrninin.,..tohavcrcachcdaccr1ainlow •·aluc. TIICdistlu>Ce•t whichthisooxu<rnl waocalled thc .i,,.Ji•z JUw.u. fkcauoc o( ll"''"ria1ion fromol-.cn'ertoohocn<cr, each obocn..,,. abo made atim.ata of the diMarw:c at ""hich !llC pancm in the imag<: ola half-tone ICK1:1l di.appean:d . This cnabltd all cstimatcs 10 IH:hrought ioacommonHan.dard.

Thcmootclabor.ltccxarnplcof1hi1 mcthodofmca•uringgrnitii­nesirw"'dacril.>Wi111951.TheohocT\•crut inaroominwhichthe cond ition> of illumination "'ere kept fixed, and cs1im~led RI " "l•kh magnifocation the cnlarg<:<I imag<: of a omall .ample appear«! rrainl.,..onatra1,.pan:ntdilfusin~><;rttn.Apina1tandardsamplc

""'nttasaryaoarcfuenceMandanJ.

Gr•lnln.u Den1lt1hl•tlen

lfthcgrainin..,ofdilfcn:nt l'"""ofB ncga1i,·ci1mcasur<:d it io found that ;,.,·ah1ei1markcdlydcpcndcm upon 1hcdcnoi1y.,\t >.ero density where 1hcn: i• nodcpooit, it is non-c,;i11en1; it 1hcn increa.cswithdcnsi1yup1oarnaximum.be)..,..,.i"·hichi1decr<:aoa. 1llC dcm.ity at which •he maximum Ottun is \"ariable, according IOthcintensityo(illumination.

CRA .VUL AR STRVCTVRll OF TllE l.tJA CE 129

,\1 low lcvcb of illumination 1hc ma~imum grainine11 io at a dcns i1 y ofO·S. As the ~llu minati':'n is incrcaocd, th~ masimum move• 1owardshighcr andh1ghcrdcrunico,a1indica1ed1nFig.2.

Gr.,.Ml•ritJ \\"eh<n'C":vicwcd oomcof d1cditfocul1ie1invoh"Cdinmcawring1hc

~~~~a~"i;;'~~;.,~,.:~cc,,i:;;•~y';[c,::1~;·~.:~:io':~~a ~-~~:~~j

Tl/£ SCIEXCE OF f'l/QTOG/t.l,f'/11'

of 1hc ocanning•i-of1hcmicrodeu>itomcter, the product i1oon­>lant,andis k11o"·n ;u 1hcp,,,..i.,;17oflheo;unpk.for d1Cproduc1 10 bcronttant, 1hc ocanning si- nnlll be a1 "''"*' l0 or20grain dia..,,.1tt1 acrcu, and, iri1 is much larger""'" d•iJ, Ilic gmplc musl be C:01'cfolly pt'Cparcd and cho«:n 10 di>pla)· no co;it ing, ><:n•i1i"ily<M"proccuing,.,.riationsacrossit.

,\ny pil,1•i<•/,,.,..,•,,_,,lofthcdcmity \·a ria1ionforagi,,..n ap•:nun: oi•c (ihat is 10 "")',the special \'aria1ior1 In the dcn•ity ol'thc ima11e, mcaoured <>bjcctivd)' in 1hi1 "·ar) ;, 1crmed zranularilJ lo di•dn11ui•h it from graini.ws, which io the cis•al imfmssio• <>f inhC>nl"lft:rlCity in 1hedc,·ckiped imall". Since OOthgrainin<:>Sand grn11u larityllcmfrorn1hcsamccauoe-1hcti•candarr;rn11cmcn!of the imag<: grain•- il ohor1ld be posi.iblc t<> rorrda1c them. The rcla1ionk1wttn1hc1wo,ho"·"'-cr,isnolaoimplc<>11<:. T hediffcr­cncc kt .. ·ttn ll1Cm i• well illu'1r.ued byroruidcring 1!1<:cffects of density on each. Tl1<:ula1ion ktwttn drnsj1rand graini...,., h'"*alrcadyl1tenco...;dcred (Fig.a); thatbc1wttn<kmi"·and granu larityis dilft:rn>t.

llrl!lularil1·Den1it1Rel•tion Whcn "'C""'"' ""''hcgranularitrofdiffcunt<knoi1inofa1>eg:>ti'-"• we find, nuurally,nogranularityat •CTO dcrui1 y, hl!! thcuaf1cri1 r~progrcai\'clywi1hdcnsi1y( fig.4). Thcn:iJ, in gcnC1;tl.no

maximut11gra1111brl1yandcc1'tainlynonc:it11!0w dcn•i1)'.

'l11c1wo \'ery diffcuntcun·n, l)c...;1r-Grainine.. (fig. 2) and l:>cm.il)'•G1;1.nulari1y( •"ig.4),canbccorulatcd.\\'hcnwemcuutt<l the grni ni!\css-dcn•il)' ulation, the oource of ill•11nina1ion wao con""'"'.., 1hat the 1ra1,,mi1tcd light "'""'i"!( 1hccye. l>r "'hich

Cll .l ,\'{,' LAR STR!:CTL' RE o~· Tl/Ii IM.i C!.' !Jl

graininat"-ai""""ured,d«'.-cascdp~.-....iwlywithir>er<:aoing dc...;1y, uwl the Lacltolv..un-.athighcrdcniitits wuducto poorcrpen:eption .,ndcrLo .. ·cr.iltumi~1i<>11condi1io~lf,ho""''"'• ;uindica1cdin•"ig.'l,"'Cadjustthc1lluminationa1cachdc...;ty tnted oo thal all gmplcs appcarcq..al!y bright, the grainin.,.. mu1ured mwlcr theoe circumnanceo incu •&el wilh dcnoi•y in much lhe p mc way •• granularity .

So far we havc opokcnorgrainine.. a"d granularity in term• of 1nc;uure1ncn1ofnegali\'CS. Thishasbecndonc CMt:ntially forcon­\'enicncc.Butrcally,iti1 quitcob\'iou11hattheprinteri•a phy1ical dc\'icc and what ii deals ,.·ith is the granularity of the "cgative. 1\ow in making a print from nonnal ncgati\'CS it i1 alway1 obsund 1hatshado..,and highlightsarcnotoo"·cll produced•• middle toncJ.Thus1hc1ranularityof1hcncgati,·cisnotoo"'Clluproduccd U.1hcprin1inlightanddarkarcasaoi~middlct<>11ct.This":""ld 1cnd 10 produce maximum v<Wdttrily m the print in the middle toneo{b«auoc •hcgranulari1yofthcncgati1"Cincrc">Cfwi thdcns­ity). \\'hcn""Ccomc1<>lool<atthcprint"'Cfind that the granul­a rityi1 le1;1,...,U..,.ninthcdarkcrauao.Thisl0rccsthe rn.ui~um in1hepc'1"Cci1·edgn.nularity,tha1i1«>oay,in1heprin1 gn.inmcu, ll ill more 10..·ards the higl1Cr l<>nn. In faci the muimum grainine..

;, ~~1~:~!Y :: :f::::i,,,~ '.:':::.:.~~=~.~~ .. ~;~;, 10 the print grainin"'* \Jy 1he print material, small l>cca<UC print materials are fin~. grained, and smaller 1till iftht prin~ io "" cnla'll~d one.

HowtoObtalnMinlmPm PrJntGrl.lnin111 \\'eha\'c1CCntha<printgrainirtC10Uduetoncgati\'cgranularity, modified by lllC printing proc,.:ss. We will finl, 1her&orc, review the

fac(~.17!::!,~~l:i~;:.i~;'.:.~n";i.:u~~~t~ia.,c, matcriah of

~:l~t:.· l~~~ .. :!;,~...,1:~~~~:~ ecroc:: .. :=i accompanicdi>yLo>.·cropeed,sothat thcmatcrialchoocnl~uldbc ofthcLo>.·nt1peedadcqua1eforthcpa11icularphocographtct'"*k. This is nol alwa)'tl prac:•icable. For uamplc, a roll film mult lla\'C

~g:;:~fb~t~~~~:g~;;~~~~~~1f.~J~iE

TIU: SC/£,\C£ QJ" f'//QTOGlllAl'llf

bu1 1his in,"()l•·cs loss ofcffcc1i,"C1pttd ; 11 .. rc ii nom·crall gain m·cr a finer grain material with nonnal d"'-clopmcnt.

HavingchoKn1hcfincstgra.in ma1nialappropria1cl0rthciask, ""Ctbouklthc11trytoobtain1hcLo"·csta•"<:ngcdcnsit)·ofncgati•·c by gi••ing doe maximum C~P<'"utt, so that the dcru.iti<s lie wdl on thcfootofthccu"-c (p.l7Z).

{b) Mi~;.,...., /'rilll Grai~iNss.~FO< a gi'""'' dcgrtc of enlargement, gui11i11eacanbcrcndercd!C11noliccablebytheuoeofadiffusion di..::in1hccnlargcr,orb)·throwing1heimagco!igh1lyoutoffocu1. T hio µr acticc, however, u•ually resuhs in au appreciable loss in definition, •11d is not nonnally recommended. Thel'C it little else which caro be done in 1hc priming stage-print dcno ities, or 1hc overall conlrut, c;o11n<>t be gl'Catl) modified since the grade off"' per choKn 1hould just attommodate the ncgacive dc111i<)" range. lfow"'"'' • if the subject can be rcpres<:nted ttasonably "'ell by a lowningcofpooiti,..,dcnsit>es,theH1hcoof1cs1 pouiblcgradeof papcr,.·illredllftgraininC11..Similarlyapl101<>sraph1akcnundcr con1ruty ligh1i11g 10 gi•-c a ncgali'"C ,.•hich can be prinled °''a ooftgradcof1M"pc•willgi•·clCS'lgn.ininess1ha11onetakcn";th10ft ligh1ing,andprin1cdonha<tlerpapcr.

Thctypcofoubject,llO""C\"Cr,ma)"havc ama1kingcffcc1011print gr::o.ininat. Prinll " ·i1h Lo.rgc an:u of uniform medium dcnsitia (oca orsky)ll1owupgraini"CS'lmarkcdly.whctta1dctailina1ubject obocutt:1ittoalargce"'ct1t.

Flnellr1lnOev1lopment ltha1bcen mcntionedtha1ifdc•·clopmenlofancmul!iongraini• ob$c"'·"'lundcra111icroscopc,iti•11ttn to1tarta10ncormorcpoim• 0<11hc•urfacc,andmsprcadthercfrom 1hroughout1hcg"'in (p.Jl2). Undcrdicscconditions,onccde•·clopment has,.artcd, i1 p<Ottcd• " ·ithl'1>pidity,and1hewholcgrain;.a]IJ'IOl,1in1m<diaielyconvcr1cd in10 mc1allic <ih·er. h ""S. 11 .. rclOrc, concluded 1hat a pa.rtiall)· dt\·clopcd emulsion must cono.ist largely of completely d"'-.olopcd andcompletcl)·und"'·clopcdgrains,with•~fcwpa.niallydt\'CI·

oped graiJ'IL This conclusion.,...., errontt>u1 a1ld based°'' the assumptionthatcondition•onamicroscopellidcatt1imilar1othoK inthebodyofanemuhionbycr.Apa.nlyd"'"Clopcd filmorpla1c doainfactcomainahighproponionofpanlyde\•clopcdgrains­ifi1didn01doro,thcttwouldbenosvch1hi"ga1finegraindcvcl­op11u:n1

Thec•p<:l$<)llgrain1arccomp/,u/yconvcrtcdto1nc1allicsilvcro11ly when thecmul•ion i•1ifrcnfullM.-clopmc11t (•·ig.!>.o}. ll'henat1

emuloion i1gi,•ct1 partialdt\·elopmct1t '°" Jo,,·crCO:tll,...t, moot of the exposed gr~ins ar~ only partiall)· com·cned, tO gi~·c •mallrr panicla of metallic 1tl•·er (Fig. Sh). T hi• w•ll l'CSl!I! Ill a fine• grainC11 jn1nge of lower grainin=. lower granulari ty, ~11d (<incc ICll •i l\'cr h~s been formed}, lower dcn>ity. 111 order lo l>l"Oofoce 1hc sa,.t dcn1it)· a1 thio partialde,·clop111c11t. mottgrain1 "'""be 1xutially dt\•clopcd, and he11ce expoourc mu>< he increased ( t.ig S.).hisfor thisrea'°nthatfi"' l'•ioikrtl~o/1°11"11<"••id!Jo«r•.,· ,...uJ •.1 lw •fs,,U. The loos of•pced for a gi'"" inctta~ in fin""?" ofgninmay•·aryshghtlrwithd1ffcrcntd"'·do\>c,.,bm 1hc,.,.na· ,;,,,,.an:of•-dord..-magni1udc.,\numbcrofproprie1ary fiNl'•;,,d.,.,..,lopcncla im1og;•-c'"Cl')"highopeo:dl,and 1hcdainu an:gcncrallyjul!ificd. Kot only is 1hc spttd conlldcrablr hi!ll..,. 1han thatgi•·cn hya nonnal '.\l.Q.d.,.,·elopcr, b<u tloegra1n It

cor<ClpO"di11gly larger. They att actu.o.11)· highopced. coancgrain dc,·clopc,.I

ll.ollfilrnsan:notnormall)·dC'l·dopcdfully.astoohighaf<>11and contrait would rcoull, and hence nonnal dc•·clop1net1t is to some c• lent fine grain dt\..,)opment . Howe•·cr. •!>Cein\ de•·clopcrs hn,·c bce11 devi$Ctl for 1>roducing finen= of~rai.,.

THE $CIE.\'CE OF l'HOTOGRAl'llY

LlwAeti•ity On1lop1n Such dc>o'Clopcrt are normal :\l.Q.dC"elopcn.. with II"' >Odium carbona te repl&ocd bya le.activ.,alkal.i ouch at bor.ir.a, and wually withtheoulphiteconcentralionincreascd;a typicaluamplcis th.al devi$W by Carh<m and Crabtree in 19'29, and marketed by Kodak as D.76andby ll rordat l D.ll.Irsuehadevclop<:risuocdtogi>·cthe .. mcoontrastaoanormal M.Q.,it gi=nok..inopeed,andno markcdi1nprovcmentingranularity.llowever,itisnormallyuoedto producealower oontrastandhenccalo"·ergranularily.ltmightbe argued 1ha t thi• io no advantage, since doe low oontra>t nega ti\'C requires a harder printing pap<:r which cnhanceo in 1he print 1hc rtduceduegativegranulo.rity.

Ho"·ever,it iogencrallyeonsidered 1hat thcuseof suchadC'o·tl­oper givcsa finer ovcraUgrain, and ifthisis true, lhere are 1"·0 f>C*ibleuplana1iotu.Thcre mightbea1cndencytc•producca oof1rr print from a >Oft nep1iv<: developed in a bor.ir.Jt dc>o'<:lop..-1han from a plucky ncgati•-e produced by oonnal :\ l.Q.dc>o'Clop­men<; -dly, C'o"" if~nto .ir1he same quality .. ..,... produced from each, theooft ncga<ivewould oonnally ha•-e a io.,..,, ,.,·crag<: lkarig, .... hich maygh'Can a<h;ontagein print gnininca.

Perhapi 1he mainjw<ification for including bonx typ<:dC'o'Clopcrt amongfiMgraindc>o·clop<:rsis thatthei r sl"""'' actiongi•·csgreater oon trol, and hence a desired degree offinen ... ofgrain is much easiertoachic>o'<:. Ho"'Cver, the.-ci• nodoub1 ,fromthe popularity offloraxdcve lop<:rs,1 hat1heyareoomidere<lad>·antagcow,and the be lief may be well founded.

hlv1nt D1t1lopor1 Aool"<:n\ suchu hypo or potassiu m thiocyanalc ina1low-aeti!\g de•·elop<:r will produce, a t 1hcsamc oomratt, OC»>lidcrably Lower gra nularity than a nonna.l M.Q. dc>o'<:lopcr. Presumably 1hc lih·er halideg..,in1pa<1lydissoh-eduringdc>o'<:k>pment,gi,ings.mallcr de>'Clopcd lil•'Cr grains.. At the same time 1uch oolvenl dC'o-elopen gi•'C much 1 ... effccti,·e•pecd than a oonnal M.Q.developer. Theyha,-canothcrdisadvantage:1bc lil\'Crhalidcdistol>·cdby the oolvent of II"' dc>o·elop<:r is bablc to be reduced to metallic liker (that is, dC'o-eloped) and '"hen silver is formed from oolution in1hisway,itisliable toappo:arasdichroicfog {p.724).

Otllerfln18r1IRDI V1 lopers Ahhoughingcneralde,·elopingagentowillfunc,iononlyinalkaline 1<>httion, 1hedeg reee>f alhlinityretiuiredvarie1 from ager1tto agent.

GR.t.l'ULAR STRUCTURE OJ' Tl/Ii IMAGE ?Jl

'.\ktol,IOre:umpJe,requircsonlyamildlyalkaline1<>lution.lndttd

::;;::~~t:::n :!~:...:~::..~ ~.~~"~! :~~; ·:r~~~ typeofdc>o·ek>p..-.Pcrhap1becawt;ol tbc.ol•'CHtactionof sulphite on >i ker halide it gi= a finer gram than normal :\1,Q. dC>'dop<:r, 1houghof?l"rx, attbc expemcofspecd.lfo"·ever,m.:iol->ulphite de•·eloper11ootsoliablc1ogi"<=dichl'Olcfogatfincgr;i.indevelop<:n oomoiningoolvc!\to1uehas thioqana1eorhypo.

l'~rapheroylencdiaminc (p. 124) has a solvent ac1io11 on oi l\'Cr halide, IO lhal "'heu normally compoundctJ, it fonc1ions like a

~~~;:a~:;.~:i:r .~.a~d.:irr:'~:·~~:,-::~r.s;;;,~ r;;;:~ d'!:~~.~~: :;:;,::;:~~:ng .olutJ01u 1hat arc h1ghlj' rnxic a11d loablc tu gh'c

Tho llH ofFln1GralnDuelope l'$

1'01ummarisc:,JM.·crivanulari1ycanbeobt.ai11cdiuancgativeby dc>o'<=lopi"l tO a low cont121t, but although this is common pncticc, the advanlagc obtaintd is largely offic1 because the ooft ncpti,.., rr<j11ire1aooo1tnotyprintingpap<:r,whiehincrcasaprintgninin.,...

•·or comparison ofthefincncosofgrain produttd by different de,·clop<:n,r1cgati\«smustbedc>o-cloptdtodoe1,,,,..oon1r;i.otandthc , • ..,a,·en.geden•i ty. Under1hesccondi1io"',1heool''<:" l a11dother de,·clopersdcscribeddodtfinitclyprodu<:e l"" ncga1ivcgranularity, butlongcrexpoourcoarercqui re<l toproducccomJ"'ta~lenegati,·co Because or 1hi1 dTccti•·e speed 1,,.., fast materials and fine grain dc>o·elopmcnl have noad,·amageo\'er1lower, finergraiu m:gati\'e ma1e.-ial a11d normal dc>o'<:lopmcnt; in fact, the Ja11er i1 t() be rewm· rncndtd.

Turbldtt1 andlmq1Sharpnus

\\'eha,·csecnthat1hcocaueringoflight"·i1hind"'cmubionla)·crby refl«tion and refnction from the lil•·cr halide cryotals causes irrllJiGftM, or .W....-ays >pread of ~gh1 beyond 1hc con~ne1 of the optical image (p. 107), This phenomenon can be demonstntcd b)· ttpotinga..-ruitivema1eriat";,h a knifetdgeincon1netwi1hthc ourfaco::on proccssingthecmubion,itwillbefound1hatthesih·e.­imagee"'end1 intothe1hieldtdportion. 1'hc1Cam:rofligluwithin 1hcemubion i1duc 1oopli<a/h,,bidity, and militalcsagain•I image sharpness.

l f by mc~n• of a microdenoitomelcr we me~• u ro d oe way in which

rut: o· c u : .. 1ct; OF 1·11QTQGR.•l'lll

dcnsityfall•acrouthcboundaryl>et"'"ttnc•pooctlanduna;poscd areas, .. ·e obtain cu.,,·es of which •"ig. Go- and• arc •n>ical. Thedenoitybtgimtofallbcforell~boondaryisrcached,b<:causc

atan)'poim"·ithindoea;poonlportioniti•duc tot!~in.cidcntligh t

andlightsc:a11crnlinfromeithertidc,andncarthcboundarythcrc

t·;,.6.CooooounDld<ml<r•<.tu.•p-od•..,.. (10.••-0lm,...)

isli11!eliglu ..,a11cttdinfro1nd1cuna;poonlsidc. T hc 1h.apcofthc cur.•n•howingthcf•llolfindcnsitywilldcpcndon<wofactors: firstly, thc"·ayin "'hichlightin1cn1itydiminiohesa11hcboundary isCl'ClP<><l,which<lcpcncbonthc1aOtlcringpropcniesandtheop>· ci1y<>f 1hccmuloion;scrondly,thcwayinwhichthclight in1cmitin arcron•·crtcd i11wdcnsi1ics, whichdcpcnd1on 1hccharactcri,.ic cun·c of the cmuloion. ,\ oontruty emul1ion ,..;u thus gi,.., much >harpcrd ilfcrcnti•lionbctwccnlightanddarkarca1 1hanawft emulsion "'ith ohc "'""'scattering propcrtin. 'll>c abili<y of an cmulsiontogi,.., a1harp rcplicaof1hcop1icalimagcfallingoni1ioa func1ionofthcoiccpncaofthcdcnsi1omctcru·aceofthcboundary 1Jc1.,"ttnanupooctlandun""pooctlportion,•s•howninfig.6, :and isbc<:oming knownai thc ""'ldMtof lhccmulsion.

Thisimpor1antfnnc1Mioiisdepcndentonouchfac1.,...ascmu~on

thickness, :and sih·..,. h:alidc oonccntruion. Aihincmulsioog"·cs lcssopponuni,yl0rsidc,..ar•..,.u..,.1hana1hickooc,and thcdis-1ancco•••,.,..hichligh1isocattcttdlide";o)'l;.dccl'U$Cdb)'ttducing tl~path bc1wcc" thcgraiotJ, i.e. by reducing 1hcgela1in to1il•n­h.alidera tio. 8ytheseand0therd~ces,filmsof•"Cr)"highacu<ana: h.a•·e rcocotly been made in this muncry. They hau:, naturally, ratherl<:S<la1iwdetl1a" ll~old..,.double-coo.1f:d1ypc. ).IOf'CO\'Cr,

1hc1h.arpncnofboundariesbct""ttnhighandlowcxpooutt<:ionbc

GH..tSU L..tR STHl.:C1THt: Qt· 111/l 1.IUG/l 2)1

incttasnl ~dC\·cloponcnt effect (p. 131 ), and dC\·dopef1 gi"ing high defin•tion ha•·c been marketed.

II : : II

II : : II

II: : II

II: : II

11: :u ...

'"

::;:'"

k.....t·····-~·<h .. t.

II=

II: : II

Bclowacer1ainwidthili1impouiblcl(ldistingui1h •hc scpar.ue lines when 1he ncga1i,.., i1 C>GOminf:d under• magnif1er; 1hc maxi­mum number of lines dis1ioguish.ablc per millimetre is called the rtulrotl '*'"' o(tl,. emulsion. Rcsol\'ing power ••arie:o wiih the 1ypc oftes1objocl-whi1c linctonabl:actbadgroundattwuall)· photogn.pbed1ogi•·cadC\1'1optdimagciore.·ersclOre>GOmina•ion. If instead ofan<>paquc bbck bxkgroulld. IOr the while lines the contr.utof1hc1es1obj«1 isl<>wettd b)'usingagn:y badground, \'tt)'differcntreoolvingJIO"n-figun:oart:ob!aincd .. ).f""'°'.,.,.•

1U Ill 111 m m 111 111 m 1 m ~ w m I I I· I I I I: I I I l I · : : ' :; : :: i

Ill Ill I . Ill Ill Ill Ill Ill

m m 111 111 111

I I I I I I I: I l I I I · I: I: I I; I J: I

Clulpwl9

PRINTING THE NEGATIVE

T;,~~,~:=-~~:~~;;,,~::~i;;;~~~~·x~~~;:::!l~7~~ pooi1ivc1a1l<: inwhichthcdcmitioofthcp<intimag.:an:roughL1· proportionalto thcbright""'*"of1hcoriginal 1ubjcct. lt i1, infact, poaible1oavoidtl1Cproduetionofascpantcncga1i...,byadop1ing a method ofprocaoingwhich carne.thccalll(>'ll.c;cpooed imag.: to afinishedphologno ph (p.135).Sensiti,-.:matcriahda.ignn!IOrouch proc:...On1 an:k11010·nas,..,,...,..,.l filmsandan:ntensi,•elywcd in colour pholognophy for 1he produc1ion of tn1"p;ircncia and arna teur ci11CfitR"d.Bu t1hcl'C'-.:rsalproceuisnot thcidca lmctl>od of obtaining a number of exact copies of 1hc sa mo photognph a nd it it chiefly for this reason d 1<11 lllC nqp1i,·e-potiti,·e prott0< oominucstotlourish.E,-.:nintcrR"dofa 1inglepho1ogra phitoffers dlC additional ad,·antages of funher con1rol OV<:r the image in temuofexpoture,tonerangc,gcomctry (e.g.OOTrec1io11ohenica l1), selectionof 1ubjectmatter (cropping), maokingincontrastcnn1rol, colourbala ... :c incolourprinting,anda.-arietynfnptica ltrickl 1uch a1diffuoion,di1tortion and combined images from mon: tl"'n one n~ative.l"d~tomanypho1ognophers,1hcp<inting1tagei• 1he moot P tisfying p;inofthephotognophicproc:e>I.

A photographic image may tab two ba.OC fortn1. h may be an imageonanop;oquettfltt1ingbaoc (usuallypi>per) forvie .. ·i"ll br n:flecledliglu:in1hiscaseitiskn<no·nasardt<:etionprintorrnore !limply u a print . Or ii mai· be an imaii:e on a trarup;ircn1 bale (filmorgl .. ) forviewingbytransmiucdligh1orbyopticalpro­jcctionon1oaocn:en:itisthenknownby1uehnatne1"'diapooiti'""· trarupan:ncy,lantem J.lideandcolourJ.lide.·11,. ,.,...,romuareto oomecx1entinterchangeable,asardtec•ionprintonathinbaoc whichhub<:enmadctnruluuntbysoakinginoilc.anbevi"""t:dby tran1mi11cd!igh1 , whilea'thin'trarup;irencyplaccdincon1ac twith awhite i urface willgi,·caretl<:etio11 print.

Char1tt1ri1llu afPrinti •gMat1rill1

1\ "·idc "'"&" of printing materials are made and we Wll deal chidly with d>00e used in general pl>Mography. for •he moot port they consUt ofoil•'Cr halide emub.ion• w..ted on a paper ba.c, thougbnon-oilvcrproc.,...,.arcu;cdror ouehpurpoaa ascop)·ing line drawings, when photographic quality i• not a $<:riou• con· oidcration. Exarnplco ofnon-sikCT printing papen are blue-print papcrs(util i1ingthcimtabili1yto lightofironoah1)anddye-line or diuo papers which make tUe <>fccrta in organic compounds. They ..::<1rcovcrnonnaloilvcrpopcrsincheapnt:lland 1implicityof handling, butarcinfcriorinimagcquah1yandpcnnanencc.Other printma.tcriabarcdeloigncdtogi,·crclid"ima.p(forcolourprim• ing,p.27 1),whi leothersofarcsinousna1urcbec:omeitllOlubleundCT lightac•ionandarcu>edforetching(c.g.ph<>to-rcoi>U.,p.302).

Silwr halide papcrsa,..,ilahlc at the pra<:nt time arc confined to da'Clopment gelatin cmubions. l'rinting-.1 papcrs (P.O.P.), which depend on theproduetion<>favisible•ilvcr image by pro­longed expo:.ure to a strong illuminan1 such as sunlight, ... .,.e ddcontinucdintheearly<KI». CA/.,jtU ,..,.u;.,,,, These arc chieAy med for contact printing rapers where their relatively low..,mi1ivity istuiied boih to their upo:.urc and haru::lling. Though they may be handled in weak artificial lighl (hence the name 'ga.<light' P"I"'"• p. 24), theu>e ofthcappropriatcprdight allowsamuchhighcrle-·clofillumina­tion. Development is normally completed inabout4-0>econd•and yicldsarichbluc-blackima.gc Rro.,ik """"~' The emullion con,;sts lar"Jl"IY or •i l,'Cr bromide though, ai with negative emulsions, \here is usually a small ad­miir.turcol1i1,·eriodide. Theyarc,·crymu<h faster than chloride emuWons and a«: chieffy intended for projection printing. They may be coatocl on a paper base as enlarging papers, on film for printingcinencgati\'CtOrminiaturcnegati\·et,andonglaufor making lantern •lidco. 0.:..'Ck>pmen• a•'Ctagts 2 minutco and yickb aneutnolblackimagc. CJJ .. Hto.iiidt ..,,..u;,,,, , The emulsion i• a mi>Uurc o( •il•'Cr chloride andoil•'Crbromideandrnayha,·caspttdcompanobletothato{a bromide emul,;on. Nonna! d"''Clopment i1 about Ii minutco and producco a warm Wack lma.ge<>faquality itlcally•uitcd IO por· IRitsand landscapcs. l.lyrcst.-ainedd"'•elopmcma much warmer tonemaybcobtaincd,insomecascscxtcnd;,,gioabrickrcd. T hisis due IO a phenomenon known as light Kaller (p.35). lhc silver

PlllXTl.\"G Tl/£ N£C..tT/l'I;:

grai1u bttooning tOO•mall wiih tt:Mrained d"'·clop111c111 10"""11cr the longer ""'"<:-lengths of~ght l .....,d/, ""''"'I ,..,..s: These consi" oltw<> dilfcrcm cmuW.,..,, o:oaced~>eon iopofthe<>thcr. One emulsion ;.o(hlgh rot\1r"l•'1

and >C11Sl1i•·c onl)· tohlueligch1 while the other i• ollow•:<>•llr.u\ and >CllSili•'C only t<> i·ellow light. Thu• by 111ca1u of fillers, 1he a1110'.1111Qfup<.>1urealftttingcachlayercanbccomrollcd1ogi''C varimgtlcgrttlofcontraot. Used ,,·i1hout a fiher, 1hc papcrgivco ancxp<.>111rcKalcc<1ual1oanormalgradeof1>aJ1cr. l'afUhr•ma1irpaJ!< .. : The>c arc fully colour-$<:nsi1io«I cmul•iom or ,;1,·cr bromitl~and oih·er i<>dide oonipo•itiun de>igncd for making black-and-,,f111c prinl< from colom· ncgati\'Ct. A 1yp;cal example i1tlmtofKOOnk"Panalurc'papcr.

P•/'<tMM:l'apcr1obcco.a<cd"·i1hphoiographiccmubiommuotbe ora high dcgrtt ol"J>Uriiy. FOi" =ample, minute s1x:du of iron t()Jlpcrortulphur. unimporiant in anrotl>er UKofpopcr, might ca•"'".'JIOU whi.ch """Id make it unuoablc flll" 1n.oking pboto­graplncptil\U.

l'apcrofdilfercn\ weight and teuurc is used aru::l ma)" also be dyocl!ogivetuchtirnou"i,·ory'and"crcau"lomet:tthctUlctof IJ0"1rait a11<l pictorial ,,.<>rkefO. Prior 10 cmul1ion cooling n\Olt parc11 rue coatc<! with a bycr<>ffmcly divided lxirium oulphatc •usi>entlrd ;,, gda1in,_ known in the trade a• l>aryia. Thio gi""' a onK.1<>1 h,lughlyrcllttt111gsurfaccto1hcJl"pcr,1hedcgrttofgl00>i­nCM l>eing controlled hr the •i•c of the bari·1a cri·11ab. Thus •urfaecoouehasmau, smoothandgl°")"Cllnbeobtainnl. l\"hcnthc highotpooslbleglossiJrcquircd,1hcbari·talaycrisr•/"""1rd,tba1is. ro111p...,...,.il>etwcensteam-hca1edrollers.Itcanalsobccm~

byrollcntogi,'C•u<h•urfacesas .... :ltandli11n1. ·11,., b.:ori·•a coaling may also con1ain a blue-white llUOl"eOCing

d)'C 1oim!"O''Cthe rcflcc•i<>nofthebasc, ra11>eraftcr1hoostrleof toap J>Ol'"tlttS which gi•'C 'whittr' white<. The elfttl will be more adnn1agco1H"'hcnthooprintiJviCM'Cdinligh1riehinuhra-\iolct radiation ouch as day~ght and Jtriplighting and lt:ll so wt.on •·icwcdin1ungm:nlighting.Suchadyebec:omtsadisadvantagcina print which is to be used forphotomtthanical l'Cprotluction,•incc any rccouchiug with ordinary pigment> which do not ffuorcocc ""Ould gi\'e different rC$uho when phowgraphcd with rncrcuri vopourla111~onaprcdominantl)"l<ltra-\'iolct>ensitkcmatcrial

. ~ 1of-~-t-~--1~1-1-++++-+~-1

1 .. ;f-~-+-~,_..'--'-_,_.,__,~-+-~-'

Fil"': Clouacetmo OOie i1 u !<'d ror print ma10<iat.i n1cndod for film rnakiug or for filnMt rips. The baoe i1 u1uallygiYcn a>ub­coo.1ing<>fg<:la1inandcellulotcestcr {p.lo.l). Sheet film of thicker gaugcisuocdwhenla<¥•lra111parenciesan:tobemadc. G/<U1 :

0l11i1io 11SCtl for print material intended for making lantern

sbdt:1 or when 1he minimum image di1tor1ion i1 roquitt:d in tech­nical uoes. Tho glau i11t:l<:-c:ted for i!Jfrecdom from llawsand ls sub-ooatedwitha layorofhartlenedg<:lalin (p.l (M).

lf wc""Cf'Cconfror11Wwi1haprintingmaterialha,~ngafixcdcon­tl"Utoropo1un:ocale, a print utilising 1hcwholct0ne"""lefrom white1oblad::couldonlybeob1aincdfmmanegativch.avinga matchingC1po1un:oeale. Thilil, in fact, 1hcsirnationin printing motion picture ncptiw:1 and aloo applin toa number of colour print matcriab. In ouch a o.ituation it is 1hcrdOn: necas.:>ry tu

PlllNTIXG Tl/Ii ,••liG A T/l•li

<ontrol1heligh1ingbalanu ooas 1oob1ain ancga1iv.:ofthe"'luittd charac:mistia..Suchcontrolionotfeasiblcingcncralpho.,,graphy and ..epti,·tt of widel y differing •~poourc ocalc an: 10 be cn­countcred. h 1h111 bccon>tt n-ry to ha'..: a rangc of printing p.openofdiffcrcn1 contr.ut (11'Mln inord,,.1oob1ain acocp1ablc prinu.. In the moot common ly lllCd printing papen ouch as tt-<: for contact prin<ingandenlarging,asmanyas.U. g.-adcsmay be a\'ailablc. These may be allotted namtt ouch as ""'••fl, •fl, --"'and1-il,batcdon 1hcqualityo(1hcncg;iti\'<:,ornumcrical gradtt ouch as 0, I, Z, 3, ~ and $. Chanoctcri>1ic cunu for a rangeof"b<om;.;lcpapenar<:gi,•cnin •·ig. I.

Varia1ion ofcomnost byde>·clopmcnl (••in 1hc caseof..epti•·• 1na1criab) offc'1 ''Cl")" Jinlc control with prin• matcriab as the maximum bl""k;, oblaincd only ""ilh foll d"'·dopmcnl, (t'ig. Z. )

Variablccomr.u•p;oP""pl'O>"i<Lea.i.imilarrangcofcon1r.utby 1hcuoco(fihcnry,•cr1lu:cnlarg.:rlc11s.Thcyar<:chicllyofintc=• 10 amateur photographcn wt-<: n«do an: rnod<:ll and for wh<>m a .U>e:koftc\..:raldiffcr<:ntgradttofprintspaperwouldbcuncconomic.

f'6. 2 .. '< .. ...ai ~<:r-* pop«~ r"'d;fferm,,; .... rn­,;; .... •~-i .

TllE SCIENCE of· l'llOTOGR,tl'Hr

MHlmum ll•~k M4I T1M11 Rm111

The ,..,.nge of lonc:o from white to black depends 001 !he ,..,llcctancc of the baoe and 1he maximum black yielded by cxpoaurc and de-..,,lop­mcnt, The maximum black depends on 1hc colour of 1hc irnag<: and1hc1urfaccoftheprin1ingpapcr.ThusahighlyglOS1ysurfae<:, bcinglesssubj«t1olightscatter,willgi,·cthcdecpcstblackand amauorrough1urfacc1hclca.<1. lntermsof,..,ftec1iondcrui1y,thc formcrmaybc at high at 'labovcthcbaoe,givi11$a 1onc"' 'lleof 100: I and ulowu ! ·3for amatt ourfacc,gi,•ill$&"'ngcofon1y 20:1.

k cturni11gto1hc1ubj<:ctofto11ercproduc1io11,itcanbc..,.,n that a n avenge printing paper with atone r~ngcofabout50: I is justaboutad«juatcto,..,produccthehrightncsorangeof ana\'C"'!" subject. I n fact it falls ohort of this rcqui,..,mcnt ifo11e "'"'' 10 co<Wderd..,actualbrighmcssrang<:n.1hcrthan1hatofthe;....p ofthesubj«1gi,.,,n b)· 1he camera ]cm. In this case 1hcavcng<: canbctakcnuapproxima1dyl50 :l. ~1-·.,.,·cr, •he purposcof 1hc photosn.phcris tha1 ofproducio>g

an acceptable in1crprctation ofthcsubj«t rather II.an attempting 1orcproducci1Sactual bright...,..r:angc. This,ofcounc,would be feasiblel'or1ubj«isof iuhc'l"cndyl""·contrast,bu1the""u1tto moot people would be unsati>factory and w<iuki be dncri bo;l b)· ~nauy at being toojlaJ. O ne definition ofa r?"" print (pictorially) 11that it mak.,,uscofthewholcrangcofprintu>jl 1onct, even if1t contains on ly •ouchc. of maximum black, at, for example, in high kcy111bjecuwhicharc ofprcdominantl y lighLtonct.

Oncof thcmajor problcmsofto11e ,..,production arite1 froma t ubject having a n u11balanccdrangcofbrightncso. Alan uamp!e wcrrl<lylakcthccaseofa 1ubjectwhichincludc.partofanintcrior andaview1hroughawiudowordoorofabrigh1lylitcxtcrior.Thc tou.lbrightnetoMlngerrl<ly bcathigh aslOOO:l, hutitisno1mcrcly • eaoc of oomp,..,..;ng a long neg;u;,.,, cxpoourc tea.le into 11.., limiis of a '-et'f 'Jolt' printing mat~: the bright,..,...,. conWt of two main zones, one of which "~ll fall on thcfootofthccur." and the o•hcr toward• •he ohouklcr of the uegati'" ma1crial with a kind of wipty n.,.mau't land in between. If the prin1ing expoourc is adjustcd1ogivethebcst10nc,..,ndcringofthevic\\•tttuthrough1he window, then the interior will be rendered at linle more than a blackfr,.mc:inthercvenccase,thcviewthrough1hewindow"·ill co,,.;11ofwhi1ebaoe .

One tolu1io11, already diocu.IM<l in the chapter on cxpoourc (p.'ll0), i1 that ofadjusti11$theligh1ingbalanccbcforc1 hccxpooure

l'RIXTJNG THE .\'EGA Tll'll

is made. In the example gi•·en, this could be done by increasing thciUumi11ationofthcintcrior.But thisit1>0<alwaY*pouibleand it1hcnbccomancccaal'}·toretort to localupoourccon1rolinthe printing of the ncga.tivc.lltiocan lake thcfonnofthadingor holdingbad1he'1hm'an:asof1hcnega1i,.,,,ci•hcrbyholdinga pieccofcardorthehandin1heligh•bcamumilthc•dcnoe'an:u ha•·cbccnadcquatelycxposcd. T hcelfectis rather Lhatof'tcle­w:oping' the tw<i brightness zona. If the dense ar~at arc fairly 1ma!l,acard with a hole in it can be utcd to'pamt' thea,..,as wi Lh cx tn light: in the re-.·c~ea.e,asmall piece of card can be otuck toalcngthofttiff,,·i,..,and used to'proiect' the weak areas fromtoomuchlightaetion.

Yet another problem is that of rendering a brightnca lighter 1han'"•hi1e'andi11casethissecms!ikenonsensc,iti10n!yncccaary tocoruideratttncwhichcontainoboththc"·hitetaibofapcht fully illumlnaLcd wi1hsunlightaJthesparklcof1unlighti...,lfou the wa1cr, :'\f)rn'l..:l.Uy the whita in a subject arc rcprncn!cd by 1hc clear hue of the paper. How then arc ""' 10 gain the dfeet ofoomcthingbrigluer? lfthc,..,!\cctioruoffthewatcrgi•·eninthc abm" example arc rciw-nted at white, then clearly the white uibmustbercprcscntcdbya•badcofgrcy.Thisislikclytobc unaccep1able in a print 1urroundcd by a white bordcr_&ince the ob!cr."r will ha'"' an immediate rcfe,..,ncc 10 1hc whuc...,.. of whitc. lt can,howcver,bc acccptableifthcprim ismoun<cdona dark grey or C\'CR black card, and even more to if the imaged strongly illuminated .with di~tional light. This cxplai.., why projec1cd1"'n1p.ucncicscan g" ·esuchagoodaccot<ntofatubjcct of thiskind.

Cont.ct Printing

In this method 11.., tih.,,r image of the nqati•·e is placed directly in contact"iththecmulsi.onsurfattofthcprintingm:ucri.al,an upotun:: being made through the back of the nqa1;..,,, If the two surfaccsarcinclosecontact,thcrc;,,noappreciablci..ofdctail.As

~~h:~l{hc ~;.:n~ ~~,;rya ~::l~~;~~:.=t~~ ~ ·~~~

:!f~cE1~:'.:~.::-.~;::~;~~~E:i~~;:j;}~:~~~~~rc~r::~:

TIU: $C/li.\"Cli Of' Pl/01"QGll~l'/ll"

I\ nu her"""" cooi~nicnt means of c~pooing cooi~t pri111> is wirh a printing!l(>JLwhichincorporattsalight110Urcc. lnthephoto­finioh.ing in.de, automatic print""' cmplo)·i,.g dcct...,.1ic ocannen whichindicatetheexposureandgradcofprintingp.aperareustd.

ThediJ1ana:a11doiuoftlM:light110U=willh_...,.,rnceffec1on 1he oontn.Jt of the print image. A diff- light '°"""'· •uch u a pearl tlec1riclampusedatacloscdiJtancc,"'illgi'·caooftcrimage thano11cmadc"·ithalight....,n;cof>rnallan:a:1tadi11Rnccof >CYen.lfcct.

OptlaalPrl11t1111 Primingb)"Opticalproject;.,,1;,,commonlyk11own a•t•l•r1i•1 and anopiical printcru """""'"''though in facr it may be used to oblain..,tt>C<oi•eorredueedimagcs.ltoffcnanumbc:rofad,.,.nlagcs .,., . .,. oon•act priming apan from pcnnining cha11gts to be: made in1heirnageoi•c.Thcscincludcsclec1io"of1ubjcc1m.on..-,local uposurecoo11rol,eo<r«<ionofvtt1icaband•-ario><ntr;,,bof11M: kindalrud)• rrlcrttd 1oin1hiichap1..- (p.'.MI ). Tltt &l•rJn : This is cssentlally a d"'•icc fur projc<:1ing an im.age ofanqati\"COn to a1hectofprintingmatCTial. lut....Occompot>en" are •ho"·" in Fig. 3. Fo.-001,,"Cniencc the components are u1uall)· arranged rn>a •·crticalcolumn,but if,·erylargcprinuaretobc made, a boriit<>ntal arrangement is more $Crviccal>le.

Enla rgc<'I may vary in thcir•)'ltcmofillurninatiougiving><>mc·

\~"" !;=¥''-;;...'

- .....

\._. ~ f1'. 3. Ilia~'"" of'" •nlu•"·

l'/l/.\"TIXC 1Hli ,\liG,ITln:

"·hatdiffcrcn1con1ras1rauhsfromthesamcnep1i'-.: (0ttp. l65). Forl:O'flC"Cllati, ... orwhen<>nlysmaUmagnificariont•n:requin:d, a ouitabk ligln llOUroc can be: made "~th a lamp a few inches awa y from the ocpt;,.., "·ith a 1hcet of flashed opal gla.10 e"wre uniforrn illumination. F<>r 1m.oll negat;,..,. <>r with hi8h magnifi­cation11norc cfficicnt uscismadcofthelight><>urccbycmpk>)·ing aamdenterlcn1 1}'11emt.ctwttn<helarnp andthe1~1i\'e.

Thc""luiretncl!,.ofag<>odcnla'flerlcnsdiffcrinJOmercspc<:U fromthotc<1f a goodcamoralcnt.. Whilcbotharercquir<:<l m gi•·c uniform illumina1i<>n o•·cr the picture area and Aatness of field withoutdj"orti<>n,ancnlargorlcm""!uirc)Onlytobc:correctcdfor 1hor•dista11cn..lfiti•tobc:uscdforcolourpri1ning,it1h<1uldbc reasonably well corrected r,,.. chromatic aberration. The focal length;. U!ually baKd on the dlagonal of11>e ncgati,.., to be cn­la'fled. [ 11 practice a well-corn:ctcd camera lent may ocn-.: 1hc purpooc •

With oomc cnlargcn the foc .... ng """"mcnt;. coupled to the cnlarging"'°'·erncn1 (leru-caoclddtana:) bymca1Holatr•[JC':ium •)"Item. Additional fcamres that could be: useful are a tih m(l\·etnent of11M:nqi:ati•..,camcrand a swing "'°'"mcntoC1helcnstocnablc corrections 10 be: made to oon••erging \'erlicab in the ncgati'"· ~le>wcvcr,iloe sarnceffectcanbc<>b<ainedbytihingthepriming

ca.,.land1toppingdownthelent.. forblack-and-whi1c priotingasafclight fihcrisfintd ><>tha1 ii

can t.cowung in frontofthclcruwhen ii ii""!uired to check the focuswi th a shec1ofocmitivepapcrin[X>Sitiou.•"ihcrin11rtttuire­mcntofor.,.,,lourprintingaremcntie>ncd<1np.'.lH.

h larl illlTtdllll•H Theru:gati\-.: 1hooklhcfrecofduJLt andmaskedatthccdgcs'?tha1 there io no""')" light. The sc:lection of the area IO 1:"' pnmcd, scale of e11la~mcn< and fucusing can be do1>e by placmg a 1hcct ofbla11kprimi11gpaperonthecasel.Wi1hadcnscncgati•-.:i!ma}' bcadvdablct<>"""asharpncgat""Cofnormaldenoityiooblamthc best focus. ·1n;, applits ab<> '" dca~ng with olightl)" unsharp

""t'i;;:;.. ;, !iule or no control .,., . .,. cxpooun: erron by , -.ryins thetimeofdcvcloprncnt,th<:cxposurernuslbcacc11ra"'JyaSICISCd.

~:k~~:.=:~~:~~~=;~:~:~;~t2~~0·~~; the 11cgativ~. However. tobc: of practica l use thetc te<t c•poour.,

THE SC/l.' NC£ UF l'HOTO(;ll.dl'llf

mun be gi...,,, normal dt:'l·clopment and be properly fu:ed JO th.at they can be cnmincd in white light. Vis .. al peraption of Ol)Jlt~t fallsatlowintenoitie<ofillumina1;o..001hata print image which appcan satisfactory in the darkroom safcli11~1, may P""" to be lackill(indensi1yandC(Jtltra.<twhen•·""'-ed'ndaylight.

f.nlarging pl>Otomc1ers and cxpoou,.., meters adapted for the

E~~.:,:';~;;,.;:•,':t~~ :::".:~r;: !:.~.r==~cr ~~::".:1~~ the highest prinl quality. Test> print> also form a guid~ to any loa.l cxpoourc oontr-ol that may be needed (p. 247).1?ev1ce1 wed rordodgill(and1hadingshouldbckcptgcntlyoscilla1ingand held at a midway diotan« in the bcam of light JO a1 to prevent ouclineelTccts.

PraCtHlng [k..-.,lopmcru is pttfcrably baxd on lime and tcmpcratu,.., ar>d not ,.;.ualobscrvation.Ifdishesarcusa:l,thcysbouldbe"°"""'·hat

~~:§ .. ~~~~:r~~f~~~-E:~f~i; tcnd10 1tagnatc0>·cr1hcsuN«ofthepapcr.Asthcmorchcavtly cxposcda,..,a1Qfthcprintwilluoeupmorcdcvclopi"S(agcntlhcrc willbeatendcncyfordt:\"<:lopmcnttoslowup<oagrcatcrdq<rccin 1hctearu1 thanin1hoscwhicharclcoscxposcd.Somcworkcrstakc advan1agcof1hiJ as a means of reducing contra$!. Agic~tlon.can take1heformofasce...._wmo1ionofthcdi1hinl>01hdm:cnom, butit 1houldbcof anirrcgularnaturcorad.,.,.,lopmcnt'pancrn'

~.!~1~;,~~::c°':rat::n~~r~:~"':~ii~:~~f!~g~.;; :::;'~%:;i~)~~~·,'~~ :~;r ~-;:,~:~~d=-~':In:"r;:,!~~ rccha11intofthc1~.

It ;, wually recommended to we a llop bath ~\er d.-.·clopmcnt.

~~~~~:~0f~~~~~l~~~~~ thorough wathingofprints hat already been covered in the chapter on fi,;otiou and otahili<ation (p. 154).

COLOUR PHOTOGRAPHY-THE ADDITIVE PROCESS

MOST VI EWS arc illuminated by approxima!cly white

:~!~:!:~:~~, .. ~~ni: ~~~~h :~~l:~·h; t':i:~~·r~i::: ~ ~: sky. rhe eye acc.cpu thcte"' variations of while. The tcns.:uion of light, 1hade a nd colour iJ cawed by the variat;.,n in in~cnsity and quali1yofligh1 rcllected 1orhcC}·efromdiffcn:ntpo<t>011SOfthe ,.;.,.._.,

Ptrc11tq1 ~ ellct11111

\\'eha\'cscen thatwhitclightcanbcanal)..cdimot!.ccolou~ofd1e viaiblcspcctn.om. lfa portion ofa \"iC»" rcAeeu al~ tl1COC001utotucnts cqually,rhcnitwillappcarneutnlincolour(wh1lc,g,..,y,_orblack), but if it rcUccll !he oonotitucnts unequally, then it will appcu coloured.A red hrickwall,forcxamplc,absorbsmorclightatthc blue end of the 1pcctrum and conocqucntly dte rtA"':'led light i•

::::~~~~ir!:~·~~;h~~;:~:~~\~: ~!~-e~~c~~:-;!;:!:~?l •·i~i. ~cthod of colour pl>Otogn.phi· in wh.ich thc lig~t rcllecred or

=~t:~ ~~ c'!~~ ~f;Pf"'"W:,:;~g g:~ abri;~cc'7~ renderintofthcbrick"'llll· UnlOnuna!d)·,al thoughouchmcthods c:<i!t (b cnmplc, rhc Llppmann ar>d SetMck p.-) '. 1hcy arc

:!~~·;Ei;;=!~~~·.?!:~~r~~~:E:~r~:~ iotheumca11hatof1hcoriginalobjcct, bu1if!hc1pcctrumwcn: di•·ided into a numbcrofscetions,P.ytwclV<:,and 1hc average

~~=~;~; ~~mw:~f::e:~~~ •:: ~;~;~~i~~g~r;_h~f:a~l~·:

Tl/Ii SCU-XCli Q f " 1'11 0 TQG/llA l'HJ

thc~ -1io.,.of 1he 1pcctrumi1><1i,•iduallyreoorded, thcneo.rcr

thc combination a pproad><S thcitl.a l (F;,. I ) a nd the~ «>111pkx the proccM ii. Wit.at, then, ii the minimum number of _,'°'" rC<juired •o gi,·e aocepta ble C<>lour record•? It hu i-n

-~ 400 500 600 700

WAVELENGTH IN ml'

f"il. I. Sp«1tal ttlk<lion o( b<Oo~ wall (l:JUs) 0-400 soo 600 700

found tha! nomore1h.an1hrccarcllCCCIPJ1. l"bo:maincolounol che 1pectrum arc red, onn~, )"cllow, gr«n, blue and ,·iolet, and if "-.:di,idethe'J)e"trumintothrcc-tiotuandblcndthecolonn withincach-Uon,theygkcwred,gr«nandbluccolour1rcs­pecti\-.:Jy, tl1eprim.arycolonnolcolourphotography {"'" P· 263).

C OLO L" ll l'llOTQG llAl'Jlf- AVVITl l ' f : l'llO C liSS 25J

Practically ~-.:ry colour in na1\lre can be matehal by a ppropriate mixturt1ol th.,..,th rcc .. ·idcband1 oftheopcctrum.8y 1uchathrce­colour 1y1tem, therecordforthebriclt"-.ll""OUldbcrcprncn t<xl by Fig.3,andthcci•ccoulddil1inguilhliulcornodiffercnceincolonr bc1 .. ·ccn1hi1 reco«land theorigiN1 l, t·ig.l.

-}Jdj 400 500 600 700

All com mercial mcthod1 of C<>lour photography arc l>aoed on the indcpendcmrccordingof1hcrcd,grttnand\Jlucconsti1\lcll1s e>fany colour,andthcir 1ubscqucntoombinatie>n1ogivca11approxirna1c matchwithtlteoriginal.Thcrci1 a fundarncntal..,a>0nfor the ouc­ccaof1richrnrnaiiccolourphotography,as thc 1hrcc-ce>le>ur •y0!cm is ca lled.lthaol>ecnkno"'nlincc d'cbcginr1ingoftheninctccnth ccnturyth•t thcrei1 1Qrnc1hingofa 1riplcna1urein thc pc1"CCptie>n of colour by the eye.

T hc 1uggcoti<>n thatthcpc1"'Ceptionofcolourwa1buctlonatri­"01ourrttcptor 1yo1emwa1putfo....,·ardby Thoma1 \'e>ungin 1801 and thethcoryrttei,-.:dcon>idcn.blc1upp::lntomcfiftyycaro latCT by the im..,.1;,a1ioru of Clerk '.\!u .. -.:ll. Curlou•ly enough, it was wi1h thcobjttl of1upponing lhe ori-colour thcor)• of,·ision that he demonstn.t<xl 1he firs• colour photograµI• al a leeture a t <he Royal l nsti1utcinl86l. h i1imeratingtorccordthathccngagul a photographcr1omakcphotogn.phlof • 1.1.nan ribbon ...,,;ng bluc, green a1><1 red fihcn. The 1h1tt imaga """"' then p<Ojl'Ct<xl _with thrcep<0jectoncach1hrnughiu appropriatcfihcrto..,to!f1""" single imag<' on 1he ocrccn. The <eiul< ,. .. , remarkably g<><>::l for a firo<aUcmpt.

Tiii: ~<.I E,VCE QF Pl/QfQGll.IPllr

t:nronun:udyhi>dcn10l'l.\trationw;;umuch inad\':U><'.eolcolour :M:tW1ilingand1hcp...,."8raphicpla1aused"·=qni1ci1...,nsiti'·e 1og...,...,andm:lligh1. ThefuctthatinU1gcowereohtai1>edthro"gh the grttn aoKI m:I filters hu \Jttn explained br k. '.I.I. F..-an• :u bci11gducH1•heocruiti•·i1yol1hematcrial10u!ua»'iolctradia1Wu which"" ' rdltt1"'1 b)" lloe•ubjtttand tran.,nincd brd>< lihcn. The111ccenolthed~mon"ra1ionthusrcsted<>naroincidcncc"·hich happcned1ufi1thc1hrorr!

,\ few)'"""' !a1cr, in 1669, a ~rnMhblc Frc.,chmnn, Lo.ii• Duroo d" lt a1ax>11. puhliJhetla hook, /,t1C011/t1mr•l'lmo1rapMr, inwhid1 he >uggcstcd marl)" mcthod•whcrd1yrolm1ncould l>< rcpn>dL1cetl phuoographicall)'. So comprehensi\'e w~s hio work 1hal he amici· pal ed n>Ottorthe mcthodswhichha,·c•ul>se<Juc11il)·L>eenapplied He pn>J>Olled a modification ofCkrk .\laxwdl"o rncd.00 which would mllkc i1 pra<1icablc by di>p<:nsing with tl>< thrtt s>ruj<:cwn. He JU~tcdthatthe,·ie"·Jhouldbep...,."!':raphnllhruttghamou.icof

minute red. g=n and blue colour liltcn, omall C1>0ulh tod<f1· raolutWn by II>< "l""· In 1his "")"all thrtt rttords woold bcub­taincdund><u.mcplatc,cachrttorduccul'}'in11at01ala~aolonc-

1hirdol1hcplatc.

The Sep1r1t1ColourSer11n l nl895,l'rofcssorJuJrolDublit>achic•·ed1ucccs>inar>1>l1·ingthi> principle. Acr<iuaohtttofgl""hcruled adj;iccu(li11clincsofrcd, grccnandbl L1tdycs,ahout200tothc inch .Aplatc11•a1cxpooetlinn ca1ucrabchin<lnndi11cumac<with 1hi1.c1"ttl\. Frum ohcpr<>ecsied l\Cl!(ativea1>00i1i•·cgla., tran•parencywas madcb)" nm1act priming, andun~giotcringthctranspa~ncywiththc>Cll.'<nWthatthc!ine>

ofrffi,grccnand blucr.:cordcoincidcd wi1h1hc rcd,grttn and bluclinaol<he>Cll.'<ll, the•·ic"·wa•r<:o><kttdinculoursappro.<i• ""'tin1wthcuriginal.

Fig.4ilhutra1 .. ther.:pnxluctionofam:lcolo<irbyol>< J ol)"and limilar rr>cthodo. kcgU1ration of the ,icY,in1 O<ll.'<1\ wi1h 1hc M:J'"'ra1el)•prottUCdpoo.iti'"Ccanbcsucccooru11yattornpliohcdunly ifll><oerttll it ~lari•-clycoanc; but 1000001e<bat...nee thiodi<­ad,·antagc, the method is'""')" oimplc toopen.oc, and 11ith on imp'°'..,,C "'"""'it •urvi'"Cd until Tttt:lltl)" :u the JolvuM 5"tt11 />,.,.,,. Thi•ocll.'<n had 3501incs p<:rinch a11ddiff~1..,nt 1akingand \'iewing JCrccm we~ uocd. Thcexpoou~ latitude""'• nfcounc, 1ha1 ofblack.and-,d1i1c photographrandanrnurnbcrol identical copies <:<~old be nbrnined without 1hc dc1eriora1ion in colour rcndcril\gi nh erentinrnan)"Uthcrprocew:t

I l w"; '""''""•tL~ I

:::::::=f re· ·JW· """'*. "®i· ... w.r.,. .r

.cattN~~oli~ ,;:>:;;:::.,-{ - .- - - - - •

Rod L¥1

r..-.4.Rcpt<>doc<.iono/M<Olourbyoc~•'<><~·

R1ndo111$erunMethocls Thc j olyexp<:rirnen111howedthcpouibilityofcolourpho1ugraphy using a nlOlaic .C.-ttn, and cxp<:rimcnts """~ =de with powdered 8 1~;.~~:·::;:~t:~e~;:":'~~.:~:i°~:~~:·:a~"~:; ingenious one deviscdbythc Lumitrebruthc"andputonthcrnarkctinl907as the A•k><~rd"" /1•°"111. As colour clcrncim, the Lutni ~rcs uoed otarch

~~· ~:~i.:·:~~~:;_.;:~::d~~~n::~1~~i~g~:l~~:! coated with a tacky varnish. On remu•.-1ng 1hc unanached grams, a lingle layer of red, gum and blue graim ad~rcd to the gl:uo plate. The graim were then p<.-d into thin dua by rullin1 under prwm-c,andduttedwithlinelydividedearbon":·hichadhcredonly wthcnpoted 1acky\'an>osh,thus6!1inginthemtenticcsbct""""

·~ ... ·:.~lh"~;:::potliblc to rcgist~c,.;th.thcorigitutlKrttn a poo.i~i•·c trampal"CS\Cy, made from a ncptl\-e which had \Jttn exposed behind ruchatcll.'<n;thcrd"ore,afleraninsulatingvamishla)'Cr,the cmubionwascua•nldircctuntutheocrccn plate and proccsocdlO

f~~e1 ;sr~:~: ;t~;g.,~~i.~': ,:.:-,""~~c~:,~.;.;;~~~1~~~~

THI: SCU: .\Cli 01· f'H01"0Gll.UI/(

rqistralion """perfect aod doc Au<OChromc pla1c enjoyed dncn~ popularityfuraquartcrofaccnlul")·.

TlocAgfacolorplatc"·hichappcarcdaftcrthc 1914-JS,.·ar"""' similarinprinci plc,hutUOC<ldroplctsofd)"edgumanbici">tcadof Harch gr~in•. Tl"' clements were more tramp;orcut 1han the L11mi~ra"starchgrains,auda1thercwerenointcrsdccstofillwith

carbonblackthcrcsuluwcre~luminous. TheAutoc.hrornca•>dA&facoloradditive methods differed from

other m<;JAic method• in that the arnngancnt of 1hc clcmcnto (otarch grains and gum droplcu rcspecli•-cly) ,..,.. completel1· bapha>.ard. In any ouch random ammgcmcnt, no matter ho"· completely 1hc graios arc mii<ed, b)" the La"~ of chance there is, of course. bound to he oome ju~tapooition of clcmcnu of doc "'me colour. lthaobttncalcula1cJ1hatineachsquarciochofanAuto­chromcplatc1hc1"Cwo11ldloctoR"'fift)·dum[>SCO»•isti11goftwcl'"C or more grain• of the same colour and, of course, mn11y more clurnpocoruainingfc,.·crg,...ins.·n,ccfftt\i'·csi•cof1hccolour­clcrncn11 in a screen ofnu>dom distribution wo11ld thcrd"orc hc ( r"Caterthan thcsittofdocil<di,·W;lua!ckmrnts..

This disao:h";\nta!" don oot apply. of course, to ,._tri< or regular coloo.rKrccns, since there is no chanccofadjac"ncyof..,...,.,.I ckmtnt1ofthc11amccolour. l'robabl)" thebcHkno"'lland most 1Ucccuful <>fihc resular "'"""' additi'"C processes""" l)ufaycolor, pcrfcctcdaofilmabout 193~.Acclluloocacctatcfilmbaocwasgi'"C" a ooating of l>lue-d)"ed cellulose nitrate, and primetl with fine J>"'tallcl lines (20pcrmm. or!.00 per in. ) ofgrell9yinkrunning diagonallyacrouthefill1l.Thcfilm"·asledi11to a bluchba1h, which''"'"''~ tJ,,, blue oolour from tJ,,, unprotected opaccs lx:h•"Cttr the lines, a nd 1h<'nin<0agrccndycbathwh;chdy.r:d lhc ·~green. The greasy ink was then remo..~ b)" a tol•·•1U bath, lcavingthcfilmdycdwithahernatcblucandgrccnlina,40pcrrnm in all. A ....,.,nd >Cl of' greasy ink lina was then printed, at right anglatothc first, and lhc unprotected lin .. blcachcd and dyed

ra.1e1

!:li~=·:....~=. ... ~~="'~:e,:.h::!fi*.~,~:::;;; IJ•U""i<c.

!!11io~~"!.=:~ .. ~ .... ~ =~~bldc";:'!d. -~io::i threclllltt$overlap.th<R11ato1alabKJrio:_of_

Pla1<ll

~..&<~- • .,.-.. •• poia. ...... fi.lte•ol ... i.;tc1willo-

~~~~.=.._.i._., • polari<;fllt fiJ.., .djuuod10 rive

COLOL-1< 1'110"/0GllAl'/11 ADDl7/l'F.PROCl!"SS2}9

r<"<L On «:moving the grtasy ink, the tcn:cn or "'"""-" pattern consi01cdoflin .. olahcma1cgrecn and bl11coqua•ct,wi1h imcr­mcdiate thin red Jin" (fig.)). After an insulating varnish, a p>nchromaiic emulsion,...,. applied and, as in lhc Aute><:hl'Qffie

f'8.),))uf,)_'_"~'Oil0<Tia1<1>l ... OndJr«~"I""'<>. <-byr<dh .....

p•<:><ao,thccmubicm" .... expoicdthroughtl>eocrccnandp~ byr<:,·cna!.\\'i1haboutamillio11colourc!cmcnupe.-oquar<:1nch the DufaycoJor mm represented a trillmph or mechanical pMnting ThcC<>lour•cnde<ingwasgoocl,andproccsoingrelativclysimplc.

l t ntlcular ScreenM1thad1

A moot clcgam and ingenious mclhOO of additive oolour photv­~raphy-dtc /(t{/., l>o•i•#/"il<•u-..,·asdevised in 19'15,and ma rketed in 19'18 as KOOaC<>lM 16mm. cine film. The back oftlu: film lwe ,, .... rttdcd (emoo.cd lo form minute paralkl cylindrical lenses ,.-id1 .,..., running along the length or the film). The emulsion ..-as uJ><*d through 11.c film l>;uc, and the focal lrngth of the lens dcmenu'"'"" ouch that 1hc apenure ofd"' c.uncn lens wao foe.._! onthccmulsion.Theviewwasthusfocuxdonthcmiulsioninlhe UJualway,but 1hccylindricallcnticularelcmenudivided the image intoaocMcoolveryfincparalkl ba11d1,011Cbchindu.ohclcmcnt. i-ton:ovcr, the diA"..-cnt portiOl'll of the image aa... each band ""'"'made by light passing through diA"crmc »utical ocgmc:nts aa... tl>e camera lens. The three colour n:oord• "~obtained by placing ovcrthecamcralensa1Mplefil!er"'·llichdividedthelcnsapcnur<:by two vcnical linco •mo thrccapproximatdyequal an:as, coloun:d red,

711£ SC/ENCF. o~· f'llOTOCRArtlr

green and bluc (Fig.6).Tioaethrceponionsofd..,Le...ape1"\urc ,.-.,re IOcw.cdbythc~nsclerncntsonthctmulsiontotlla1 each band ""a1divi<lo:din1othrcc•tripi,r-ccording1lw:rc<l,gn:cnandbluc con•titucnuorthcvicw.Thisopcrationtsshownd'"-grammatically infig.7.

@. G 8 ,., ' ,.,, .. "'"' '• "-·· ·~ @ ~''*'"'

~It The film was processed by '"'""rsal in the norrn.ol way an<l

appcaro:dtobc:ablack-and-whitcfilm.Onnonnalpr<>jtctioni1gavc a normal black-and-white ,..,.ult. lfowe\-.,r, J,y placing ,,.,·er d>C prnjtttor ltn1 a similar triple filter to that uxd on the cameni, the op1ical path shown in fig. 7 becoma ~- The strip recordt ofthcro:d,grttnandblueconsli1uenuoftheviewan:: foc:UKd by the lenticular clements on the oorttSponding fihen on theprnjtt1orlcns,andthcpic1Un::on1hcoc:n::entsinrullcolour.

DiHd•.ntagn ofthtlddltl•e Processts

The proceuca oo far cornidcred a1-., all c~amplcs or 1hc additive proccuofirichromaticoolourphotog1"3phy,the.,...,11tialprinciple

COLOUll f'llOTO(;R,tf'llr-ADD/Tll'/; f'ROCUS Ml

ofwhichtstha1 1hrcccolour'""""""1arc~ewhichcmbodyonly the red, green and bluc oonstiH..,ntsofa ,,;.,..., ,..,.pcctivcly. The ihtcccons1in..,ntsan::1hcnadde<lt~l1otttOgi>..,1hcoriginal

'""""''· Additi,-.,proa:socosufferfromtwoseriouodd""u.,whichcanbcst bciUuslrate<ll.>ycoiui.dcringamooaicadditivcpr<>CC1t.Whi1cligh1 in a •·icw is rendered by the mosaic with no ti l,•crdcpooit. f..ach clemcn1inthcmooaicmu.,inthcory absorbtv."<>-third1 ofthclight falling on it IO as to transmit only 1ho appmprialc third of1hc

~;;,'.'.::\1~~1!;,~:~~~n~~ i;i::~1~ 2~;;:~<l;:~~1!£ ;;::'~ii!~ r~:~:g t~.~ it.Whilcthisis aocriousd1oad,·antagcn1transpan::nc1co, itmak.s thcMl<liti>-.,pro<...,qui1cuselessforpap<rprin11,uwhilc~uld

J..,rcn<ltrc<ladarkgrcy.ThcocconddiMdvantagctsthald1ennage is di,•Kle<l into clemcnu which must n«c•;orily be large compared with cmullio~ g,...im.. The pattern 1hcrd"on:: bccoma olotnnivc ar'"''""'"Y limned enlargement. Noithcrofthcocobjce•ionsapph .. to 11"' • ltcmativ<: syncm of thrce-wlour pho1ography, •he ..... /Jctirt poun, v.·hich will be coiui.dcro:d in the not chapter.

COLOUR PHOTOGRAPHY -THE SUBTRACTIVE PROCESS

T~;~~~:;~~:+.~~l':~~~~~:~~= oolour by addi<ion of red, grttn and blue corutiturn,., tl>c ahcr· n.a!i•-.:,knownasthc,.lt.r1fltliN1'«tu,d-ooby1uccasi•'Calno<p­tionofappropriatca1noun11ofrcd,gt-.:enandblucfromwhi1cligli1.

Tl>c oubtno:•i•·c procasionotcaoy '" undcntand. Ln us '"°""sider 1hccon«pt;.,,>1outlined011hcbcgi11ningofthc1•rc'·ious chaplcr. Avicwisnonnal!yillumina!cd bywhitclight,andlightand1hade and colourarcpcrtti•·edbecau..,ofvaria1ion•inat-rp!ionan,\ rcf\ccticmfromdiffercnlp>ruof!hc'·iew.Ablack-and•"'hi1epicturc is p..,.,Juced by a posi1i,·c image of black nictallic sil•·cr, which absorl.>3 light in J>roportion 10 the 101al absorption of corresponding p<>rtions nfthcvic"·· lfwcwishtorecordoolour,ll"'positi veim.o.gc mu>talw>rb!hcoorrc<:tmnountsof rcd, grccnandblucindcpen­dcn<ly. We mun !hcrcforc p""'1ucc our colour phmograph by passi11g whitcfinl lhrougha red rcoord,whichabsorl.>Othcappro­priate amount of red- and red only; the transmiued liglu muoi thcnpass!hrough agrecn·absorbingrccord,andfinolly1hrougha l.>l...,·absorl.>ing record. The dilfcrc11ces l>etw<:en the additi•·c and 1ubtrnctivcprocesscoa rcilhmrntcd inFig.l.(SccalsoPbtcl p.'.157. )

Thusd1cthr<:ercoordsmuHl.>cposiliv"',ofwhich1hcabsorl>ent ionoto.ih-cr{which al«irlllcquallythroughout ll"'1pettrmn) but d1·.,.,whichall90l"bonlyred.,gr<:enandblucrcspe<:ti\·oly.Nowad)'C tha1 aboorLo only red ,..;u 1r:1nsmi11hc two ...,maining prima<}· coloun--g""'n and blue-and ,..;u 1hcrcforc apl"'ar a blue-gr=n. Since,.bluc + g=noolourioimp<>rtan1incolourphotographyand had no name, it hal been ch.Utcned <Jll&. The gr«n record;, printcdinagt-.:en·aboorbingdyc,whichtransmi,.rcd + blue and appca'11Mf""ldinoolour.t"ina11ythcblucrccordioprinttdina bluc-al«irbingd)·c,whichisarcd + gttenlransmiuingdyc-and

COLOUR f'llOTO(;RAf'l/f-SUJ/TRACTlvt: f'R0Ct:$$ 16J

amixturcofred + grcc11io:Jtllow.Wceaneasilya<:ccp< 1hat gre<:11 + blue .. bluc-g""'n or cyan, a11d that red + blue - magcn1a, Lut "·cdonotac«ptooc ... ily1ha1red. + gycen - 1-.:ll<>w. llC1"'C'·er,itio casilydcmotUU:attd by.,...,,Jappingrcd andgycen light> from'""" lanterns, or by 1pinningarcd andgt-.:enHripcd 1op. h eanno<hc demon!tra1cdbymi1ingpigmo:n11.as!hccolou'1arenotlhcna.ddtd, but1ubtr:ae1ed.

} ... l.Th<d;if«<"""l;o'_.,"a,dJ;1;V< ond•ub.fo<liwpn><..-

Thoswhilc1hc1hrcccolo11r'1111Cdind>Cadditi~procasarclhe 1hrccthirdsofthc1pe<:uu1n,red,g""'nandblue:t!•cthrcc"?lou'1 1U<:din the1ub1racti,-.:procasan::thc thrccrcma1nmg1wo-1lurdsof 1hcipectru1n,cyan, 1nasc111aand1-.:llow, oomctimcseallcdminMS·Nll,

~J~~:~~i~l~~~~~7,~~~~~{~ try subu,.ctivc mctho<l1 can be ~n try otudying figure I. ~·or

:~,:;'~:~ktl~~= ~i~':~",::il :~:~~~~~l~cf:~l,';!~Af:•:~ oubtracti,·e method ooruUts of whi1e (R + G + B) for full '.cd rcflcctionandcyan (G + B) whcrcthc...,is11ored. ...,Ae<:1ion.lhis can be 1abulated 1huo,

s.~"""''" Full rcd n:llcction . Red (R) White (R + G + 11) l'ored. rdl<:etion 111:.ck (~cm) Cyan (G + B)

Thc1wor«0rdsan:,infac1,idcn1icalaccp<1hatthc1ubtno:1i•-.: <>n<.has thcC<)mplcmcntary10rcdallOV<:ril,i.c.cyan (G + ll) a• canbcoccnfromthcabo>"C!abulation.

TH/i SCll!XCI! o~· l'/IOTOGRAf'lfr

To obtain a•ubttal'•i•·ccolour1rarup>rcncyor print, i< i• thus necasarytoobtainfir1tthrceWack·and-whitcnega ti•'CSrea>nling thc""1,grcen a ndWuecoru1ituc:nuofavicwre>PfCti"ely. •·romthc ncg-• 1i,•.swcmu1<11CXtoblaincolour·and•"'hilcpositi•'CS,thccolour being complementary 10 that T«Ordcd. ThUI the red r«XK'd is prin1cd in cya n, the green in magmta and ihcWuc:in )'Cll""'· h 1houldbenotcdthatif""-e""'l~ng isUlcditi1notnecessary 1oi10lateand printthroughthc\Jlack..o.nd-whi1encpti•-ertt0rds; '""-enhclca they arc formed a n<I mbseq1ocmly des1roycd as an intrimicp><1ofthc~ersalprooe-.

Th<:re arc three main method• of producing appropriately coloured images, A positi,·e black..o.nd-,.·hi1c Jilv<:r imag<: can be -.l (convc<1cdin10colourcdino'1anicpigmentsorintocolourlcss 1ahsuponwhich1Uitablcd)"CSmaybe-UlllH,orad10rbed).

A oeoond method is 10 produce an image in relief, "'that image tllickncssi1 proportionaltothclogexposurerccci\·cd. lf1hcocnsiti•-e m:uerial rontairu pigmcnts in the minw coloun (a1 in the Carbro p...,.,.,..), 1hc rclidgi•'Q tru: rni.uircd positi.·c. Ahemativcly the rc!icfmaybeimprcgnatcd,.·ithdyc,whichistramfcrrcdtoanothcr Jurfacc, as in the 1))-eTr;uufcr and Technicolor pre><:e»Q. Thtse willbecoruidercd inmottdetaillatcr.

The !hird and moll imporla1\I mclhod of producing coloured imagcsis<ficMrdtorlop""nl,

ColourDue lopment

\\'hcn1hc silvcrhnlidc <>fanexposo:dmaic rialis rtdu.;cdtomclallic silvcrduringtle••cl<>pment, lhedcvel<>ping agenti1corrcspondingly <>xicfocd.Thefunc1icu1ofl\!lphi1cinadcvclopcristorcactwith<hc dC\·dopcr-oxida1ionproducut<>fonnincr1wl<>urleoscompound1,tn ptt•·cm1heircawingrap;dde1cri<>ra1ionofthede\·cl<>pcr (p.l2'9). H,,..·e,-er, it ii pcaible to ...-kc good use of the reactivity of the ox;dationproduc1tof.omcdevclopcr1. Tl>0teofpyrocatcchin,ort<> a leosu.<em,olpyrogall<>l,forCllample,will ttact withg<:latin t<> make it ilUOlub!ein hot water. A pyrocatcchinorpyrodc\'clopcr ,.;th!iulcorll{)1ulphitccanthcrcforcbcutcdasai-i1111dcvc1opcr­'hus ifanunhardcntd malcrialio d<:'\-elopcd thcrrin, the g<:latin is1amlCdimagc>o·;,..inproporti<>nto1heoilvcrimagelOnned,and subsequent """hing in hot "1'1cr ttn>O''CS the unhardened g<:latin, lu,;ng•hcd<:'\·clopingi~inrclid'.

Again,1hcoxidat;or.productsofp>nphcnylcnediamine {p.p.d.) or i11deriva1iva ,.;11 r-eact ,.;th c.ertainorganicoompoundl called

COi.OUR l'HOTOGRAf'l/f-SIJBTRACT/Vli PROCf:ss 2M

«111pl.trstogi,·edy.s,1hecol<>urufwh;,,J,dcpcnd1on1hechemical corutimtionof1hccouplcr.Thc amounlofdycfonncddcpcndion the amount of ox;diocd dC\'Clopc<, and therefore <>n !he amou11t of oih·crd<:'\·cl<>pcd.

Thusbyd.,.·elopingama1erial inap.p.d. typcd.,.·clopcrin 1hc prcocnct:<>fa coupler, ad)'C imag<: will be fonncd simuhancou>Jy

with the oilvcr imagc. If 1hc lancr is rcmo,·td, for example, by hrmcr'11'cdu.;cr(ferricyan;deand hypo) (p. 220), the dye image rcmaim, and imag<:> in cyan, mag<:ma <>r ycll<>W may be produced by d1<>icc of <\cw.loper and wuplcr. The coupkr mar be introduced ,.·iththcdC\·clopcr,<>rmaybepre$Cn!intheemul.,on.

Exposin1the Thrn RHord1

Forstill-lifcoubjee1S,the1hrccrccord1maybetake.n~ucccssivcly,":nd •'f'talint "'*cameras, inwrponting fihcn in a ohdmg panel beh~nd the camera, ha,·e been deviled. Tioc method io, l>O"'"''Cr, im­practicabk lOr gcncral photographic work.

Simu!tan(!OUS ph<>tograplu may be taken on a .,.,_,Ml camera (Fig.2),whichbya1yi1cmofp>nia!rcAee«>riandfi11Cl'1'spli1t1he

lig~i..:.~i:""m~a': ::.e;~:~~:~uhancoUI T«Or~• i1 the "'.""'*· which consiouofthrtt oupcrimpotcd a nd appropnatclyoen.,ti>cd filmoinscncd inadukolidc, •uitablyimerlu,·cd .. ;1hfil1..,..Tri­p>ckJ,.'Cre1ugg.stedascarly.,l897bydu llauron,andthoughthcy ha•'C been marketed, none lw •urvi\"Cd. There ii <>ne..,..;.,,..

~~:::;:!:~~jii;~~n;;:n,.~e~:O~;.';!;~ the

i.,., 11/F. SCIF.,\'CI: Qf. l'HQTOCRAP/11"

lbltl-l11tr Films

By coa1i11g alt la)-= on the "'me film 1upporl Iha! ,.;ngle obje<tiol> maybc0>-.:n»mc;1hcloosi11dcfini1ionistht:nnqligiblc,sincc1he 1opla1·cris..,po.ra1cdrromthebo<tomla~rbyonly•rrac1ionofa

tho""'ndth of~,, inch ( Fig.~)- The dilficuhy ;, 10 process tht thrtt

11 11 11 II II II

adjacemla)-en..,po.rately,ooastopnxlu«a differenlcoloured image in each. Mc1hods ha,.., been suggested wlicn:by tht 1hrcc la1..,roarc..,po.ratedforprocasinga"'1subsequemlyu-rcgis1cn:d, but 1hcy have I.ad no commcnoi.al '""C'*· ·n,. fim commercial application of thrtt emulsions coaled on one i,a.., "~ in1rOOuced in J935fornarrowgaug<:cincpho1ographyunderthe11.an><: K<>da­Chl"Qmc film. This ""'flQ/l«k, or i•Upal l•i1«k, u ii is more co1n­monlycallcd, pmvcdoo •ucccuful 1hatiti• 1hel,..•isor prac1icall}· allcolourfolnuonthcmarkctl<>day

ColourbrCoupler-DHelopen

Asatypiealcxample""ohallbricHydcscribethecurrcullyavailable K<>dachrome film and i1t proccsoing. TIIC film <:Ol\SU1tc.cn1iall) ofl"our..,po.ra1e coa<ingsonlhci,a..,,~hcx1rcrMlythin.Thc oimples1,,.11yis1oconsider1hemi111hc...,..~ordcrofapplication. thati.,intheorderin1o·hichlightencountcrsthtm.Scc l'latell l .

The lop layer COl'lli>lt o(a non--colour...,nsi1;...(! emulsion and thcrc!Oregivall1ebluerccordofthe..,cnc. lncolour..,noi1isingwe mettly add to the •pcc<ral ..,nsiti,•ity range; there is no koown rM!hodofptW"actorilydcscruitisinginthebluewhilcrctainingthc induced ..,ruitivity in green or red. Co!UC<jucmly,immediately under thcb!uc1C11•itivelayerioagcla1infihcrlaycrcontainingayellow d)"e. lmmedia1dy below that is an or1hocl1roma tic emuhion

L·o1.ot.11 1'11010GRAl'lll·-sut1rRACT/Vli ~l«JLl>U ll>7

""'""""'oblucandgrccn°"'"bccauscnoWuclightUp.wocdh)· the}·cllowfihcr.i<ruon:l•only1hegrccn. t•in.allyd1ebo<1omlaye< "?""'".ofancmuhiond)"Cd"·ith a•pccia!pand1roma.tied)" lhat g1va tugh red bu1 nqi:ligiblc grttn ICllll<iviiy. ll1e colour ocn­si1i,~1ies an: •ho..·n in Fig. 4•, and d1e t.ffe<lil't colouracnsitivitics ;,. Fi~. 4 ••

bp0turcthusn:col'ds1hercd,grccnandl>lucconstittic11,.i11the tl.rtt la)"trs. The appropriatdy coloured pooitivc imagcs arc produced lw the following reversal processing. The film is first dc,•rlo1~inanormaldc,·clopcrtogivcablac.k·and•""h!tcnqi:a<ivc

image 111 all three layers. The residual, unde,·elopcd """"'halide now fonns the p00iti,.., images tha1 m1111 be convcrt"<I into the appropriate d)"c im•gc in each la)"'· Arte. """'hiitg. the film is

1L1L - $.» - 1t\I l()(J $()/) d.JO :ioo

"'I' rrl)J.

f'6. 4. (o). Sp<<""'ll"nu of <mu1,;,,.,, oo«I <>11 in1<s<>I u;po<~ ~In• (•) 1:1r..-1 ;V< ,.,.,;,;,-;,;.. ~·;1h r<llow ~lttr l•y« b<IW«n lop

aod •n;J,He<mul>ionl•r<"·

rue. SCH' :.'CE 01' /'lf OTOGllA PHr

unif<>rmly cxposM 1hrough !he ~ "' red light, making only the rul·scrui1ivc {bonom) !aycrdC\-clop.:oble. h isdC\"CLopcd in a~·an

colourdc.·clopcr,whichproduccsapooitiv"cyanimagcin 1hchot. mmla)"Cr,togctherwith1hedcvcloprncmof1hcwholeofthc1il \"Cf in 1hat la)"Cf. The film i1 ne><t cxpoood rrom ai-.., to blue ligln which, bcauscof1he)..,llowfillcr,affcct1011ly the top la)"'· The ...,.idualsih·erh.alidc in that la)"Crisdcvcloped in a ~Uowcolour dc...,Lopcr. Only 1he silver COl'""ponding 10 a pooiti,.., image in !he middle (grttn·scnli1ivc) b)"Crrctnai1111obcdC\"'°""4i· Sand· "'~chcd bctwttn !WO comple1dyblutcncd layu1it cannot easily bcligh110gged,ooi1isdC\-clopedinalOggingrnagcntacolour dC\·cLopcr.All1hesilvcrisnowdc...,loped,andisrcrnoval("'Sethcr withthe~llowfihcrlaycr)i>y Farmcr'1rcducer,lc;wing1hercquisi tc

1hrccpositi,·cirnagcsin1heminuooolounrqii>1crcdtol'ormacolour traruparcney.

TIIC1ypcofmaterialdcscribcdi1capablcofgoodcolourrcpro­duc1ion, and the d)'CI formed rrom p...ien1-day couple,. and dc.·clopc,. arc rcasonably 11able 10 light, temperature and moisture. However, the processing is so eomi>l<:x that only eon tinuornly ronningplant,andupcnsi,·c con1rolapparatuocancruurcsucccss. It is quite impta.:tical>le for an ama teur or professional photo­grnpher to process an intrgra l lripack film "'ilho\L\ inwrporatcd eour-lcrs

ColourFllmwlthCouplen lnth1 Emul1lon

Processing muhi·laycr rcvc,....I materials WQuld bc si mplified if 1hcappropria1c couplen """'introduced in the three emulsion• There a re 1wo1<:rious prac1ic..ldifficultics 1othis.lnthelintplaccit is not cuy 10 find couplcn free from dcle!crioiu action on the emulslollS.Secondly,itisdiff><uhtoprcvcntaoouplerfromwandcr· ing bydiffutionfromit1ownla)'erintoadjaccntla)·en. Whenthat happcns, theprimary eolou,.ofaviewarercool"dcdpa«iallyinthe wrongoolour,andcolourrcnderingisruin<xl.

Theproblerni.werefintool\'alinthcAgfal\cueproceosorl937; the oouplen were made non•,..andering by oombining the acti,.., coupler molecule with a la.'X", heavy, inen molecule. Other rnethod1ofP""-cn1ing,..anderingba\•ebttntooombine1heooupler ,.·ith the gelatin ofd1eemuWon,and {as in !he Eluaoolo<, Ekta · chrorrw: and Kodacolor proccoses) to dino[,.., <he coupler in a solvent non-mi>c:ible"·itb water, and then disperse it 1hroughou1

COi.OUR l'llOTOGHAl'/ll'-SU6'/'HACT/f"l.' l'RQCESS 269

the emulsion in minutedroplc!S. Whenoouplc,.arepr<*;nt, the rC>."C,....] procedure is quicker and simpler. ,\fter dC\..,Joping 1he

~~k:tt:~~e ~"!~:~;: ii;l~~.::~=e~~!~e~ ~~~~ :::= " 'hich forms t!1e required oolour in each la)·cr. Removal of 1il,..,r and~l)o..•.fihcrlayerlea,uapo:iiti,..,imag..oomi"ingofthrce d )·e laya~ .

The mcthod1so farde><:ribedsi,·cnwlourpie1urci"thcformofa tronspa.rency !lrn1 i• viewed by direct 1m11sn~im:d lis ht, or by proj«HOn on to a scrren. Kod•chromc w<.,ongrnnlly rntrod uced fornm0<eur cincpho1ography•nd latern•a mi11i • turefi lmfor ot ill photogrnphers. ltundoubwllyhad thceffectofre-awakcn ing imerat in projection viewing. Re-chri.icncd 'colou r slides' in.,ead of lantern slides, 1hemagie la n1cm huagain l>ccomc immenscly popular-under the name 'olide projector'. Current models of slide projecto,.includerruo.guineloadingandrcrnotc slide-changingand f<>cusing or the projector lens. Coupled ,..; ,ha rruo.gnctie upc reoonler, the combined effect of picture a nd lmmd can be quite imprcssi,·c

COLOUR PHOTOGRAPHY COLOUR PRINTS AND COLOUR MASKING

SO fAR, we ha\'C considered <>nly colour tr:uupo.rencia.

~~C::~'!h;";. ~~~.;ci,~~~~~:;'CV;~ .. ~ .. ~: re6ec1ing~•uchaspapcr.Wcha•'Calr<:ady1ttntliatl'oo-d11•

:::~·:=e::i~~~~t~::ip='&~'·~=:y~~~'~ya;;"~ dlC)" h.:tsc, un1il r<:ecml)"• l>e<:n 100 complex to 1.., applicable to

~~c;~:~:::er':.:1=!?'~.'.":~.,c ~~:~1".{;:r;';~";~'~;~~ mcn!ed gdatill la1·cn "·hid> au •ulJSCqucmly •uperimposcd. 'l11c Carbro procas, "'hich was highly popular with advanced amatcu"' andprofeaion.ol•,i1brieflydcscribcd°"a 1ypicaleumplc

~:r:·~ ::;,:~,~~,.i~:~~·~:~:;~~~~~~1~;:·;~::.~~;. ~:~.·~~;'.t1· 1~;;~; ~,:J ",:f,~~la~~~~o u~',:~~~~;:·:~; ;1~';';,"'~.,::~~~ .. ';;~~~!7~ mi nu• colour (cyan, rnagc ntn or ydlo,~)-a nd 5'n•itiscd ~ ooa ~ing inasolu lionofacid<lichromMc,fcm qanideand hronude. l'he li11uesclet1<:df<>tcachprimi>.ofCOu™',complcme1l!aryinrolour 10 ·~~~c·~~;;~::~~:·;, both intcrcsting and complu. The rerricyamdc and loromidediffuseinto1hc pri11tand O-"idise1hc1il,·crimagc"'

::;:.n1:7r':11~-~:i;:;::~; ~1~~!z;y;;i;~: ~:;;o;:::C"d~.:!: bad into the tiullC,and itrc...,xidiKd 1ofemc)"lnidc by 1hc dic:hroma.1c. T hclancristhcrcb)• r<cluccdtoachromiumtahwhic:h = ~ ,rcia;~~:::h:r ·: .. "";~1!.".' ,~r~:"'...~~~~.rr;;;s';;; bromidcprint,thc1i•ucitoq1oecgcedon 1oa,..,.xed ooll uloidand

'~

COLOUR l'llQrQGN,H'llr- CoLO!,I< /•1</.VF3 ~71

,.·;uhcdinwannwatcr.Theooluhlcgcla1inditoolw:sandallm.-.1hc papcrtotloata••'ay, lcavingar.:lic:r imagcorh.o.rdgclatinpigmcn­tcd with the appropriate colour on the ooUuloid. Thit image is tr.uuferrcdi;yoquccgceingon1ogclatin...;oatcdpapcr andpccling off the celluloid. Three ouch imag..., ill cyan, magem3 and )"llow, corrctponding1orcd,grccnandblucr«on;l1, an:1ranofcrrcdin rqiil!cr on to the ... me ~nal p.>pcr '"Pl"'"·

D11 Tr1111f1rl'roc111

Otherproccoses,1uch..,1hcT<:ehnicolorproct:s1rormotion·pictu..., r.1 .... ,andthe l)j·cTn.11ofcrproccnrorpl'pcrprin1J,arcl>asedond1.., •mbibition. ln<SM:nce,1hcbuerttHW$uo{p<ep.>ringth=i-'ti•·es fromseparationncgativesl>y priming on filmcoa1ed "'i1hycllo"·· d)'M,unl.ardcrte<lcmubioo1. T hi1-1nxfi!miscxpooedthroughthe ~.and..,theycllowd)'Cabtorlls1hcbluc!i.ght1owhich1hc

emulsion itseouiti•·e, the image it0<11>fined to the emulsion la)·er ncar11oe~.Thcmatrixfilmisde.'Clopclinatanningde.·elopcr

(p.~) andoubsequenl"'ashingin"·armwatcrrcJnO\·esthe )·cllow dycandu nhardcnedgcl,lcavingarclierirn3gcinhardcnedgclatin llte th= posili\•e r.:licfs arc ooakcd in d)'CS of 1he ~ppropriate min1uroluurandtqucegeedi1uoronl~ctwithapapc r coatedwi1h

1nonlantedgcla t in,when 1loeclyei•1rand'~rrcdrrom!l oerclicr i 1nagc 10 the paper. The •uccessi•·c "'" "(e rencc, in regiS1cr, of the th ree dyc i mageog i ,·aacolourpr i ,, t.The rnil! riccoca n,ofcom>c,~u;ed

toprod ncea mun lx:r of•uch D)•c T r:u1'fcrprinli.

ReHru lColourl'rlntl fromTran1par1nci11

Although the pro<...,_orprin1.makingoo fardcocril"'d """com· parati•·cly limple mlltcrialt, they require imrieate and •kilful ma nipula1ion. Forpop11lar ama1cur<X1lourpho1ographya process 1ha1rcquirQliulcetfonontl1CJ>artoftl1Cuscri>ncceuary.Thefint prin< procas ofthit type " "31 offered tO the ,\m<:rican public as ~ l inicolor p<itoU in 19-12; tllC name was "'bscqucnoly changed to Kodachrom< prin11. They am•iH of cnlargcornc11u rro1n Koda. chrornc1ran1parcncictproduco:<IORa Kodachroonc-•)·pccmulsion l)"llCm, coaled on "'hi1c opaqllC ~- At the pmc time a de luxe procen known .., Kooa•'achromc """ in1rodu"'1, "'hich, by clabor.uecolourcorrcction in•-olvingma1king1<:ehn1qu~ (p.272 .. produced,..,,... fai1hful dnplM:a1ct. In 1hcse method1 a double

T11i· scti'Jo' C/; QF l'/IQTQt;RAl'l/T

re•·cn.al pr"(M;W io u~, cmployi11g ..,.ntia!ly the oam<: 'YI"' or rc\·cnal emulsions 001 clur and <>n white relke1ing haoeo. That prin<:iple is ...,,,.. the basil of oeveral commercial oolour J>Tint p~.llowc•ott,ifpoo.ili•·eprinuonlyarerequittd,as.implttand

more direct method woukl be IO use a colour ncgati•..: instead of a pooiti.-., trarup1iren<:yao 1hcintcrmediatcrooord.

ll•llli'lt·postt!Yt Colour·printl'roetss The lint popular J>fOC""'to produce colour nega•i•'CS •<>}~eld paper colotlr prinu ,,.._ marke1"'1 in America ul>dcr the name of K<>da· ook>r (thcna.meofthcol:.oletelenticularO<ldi1i'..:proccss,-p.259), asamateurrollfilmin 1943,a11dasF.k1aook>r (aAa1fiJmforprora­lional """) i11 1948. Unlike Kodachrome film Koda"""" and Ek1a­colo< {and al1<> Ektachrome, a ...,.,..,...,1 film !'or the profess.iona.I 1ransparcncia) incorporatecouple,.in1hcfilmand in the paper. The couplen are diuolvo:I in an organic ooh..,,,t non-miscible with "·atcr, and dispcrted a1 minute globul<S in 1he liquid cmuls.ion bcforeco;iting.lnthatwa)',COUple,.arc]m"'"ntcdfromwandcring {p.266. )

Afterexpoourc, the film is dc•·clopo:I in acolourd.,....:lopcr, the oxidation producu of which combine "'ilh the couplers to gi,.., minuscolou" in each la)'cr. Thesil•·crand s.iJ,..,rha\ideare then remov<:<ll>yF•rmer'1rcduccr,lca,·ingadyeimagcwhichisnegati,·c Mr"tlardsb!ackand,.·hiteandasregard11hccolo11"oflheorigi'.'al (that is, complcmc11rnry). When prim<:<I on 10 a •imilar emul51on coating on paper, itgivaaposi tiv.:colourprint.Othcrnegativc­j>Ollili\'e colour prinl pr"(M;CW:S (Agfawlor, Gevacolor and Pakolor, for example) ha,·c incorporated couµlc" of doe Agfa type. The ncgati''C· posi ti,·ccolourprocessutili•ingmuhi-laycremuls.ionshas nowhccome"·cll•atabli1ho:landhaslargclysup<:rscdcd1hoocl>aocd on separation 11egati,·e1. h olfe,.acons.idcrabledegrce ofcon1rol and can yield rQulu eq•~il lo the hat producM l>y the Carbro ,_. DtfteU : CalourMu kinl Acolourtraruparmcyorprintcouldapproachtheideal<>nlyifthe 1ransmiss.ioruof1hethrte1ub1ractiV<:d)'CS"'CS'Cideal,cachal:.orhing Ofleoltheprimarycolotlnandbeingcomplttdytraruparen1to•he other 1..-0. In practice 1ueh d)'CS .,.., unknown. Yellow d)'CS, in g<cncral, come cl.,..,.t 10 the ideal, u 1hcy absolh blue and freely

COlOUR PllOTQ<;R~l'l/f-CQLOUR Pl/INT$ l7J

1ransmit !{re<:oand red; magcntad)CS a!way1absorl>oome [,luealong withthcgrcen;cyandy<S a bsorl>acoruidcrahtcamoumofgrccn and blue I.I well as ,..,.1. The unwamo:I absorption in the green andbluead•'Crselyalfecucolourrcndcringl>yreproducingthettd• muehl>righterthanthtdegr:adOOgrce"'andblutt.Thcmagenta image is thus equiV"•lcn1 10 an ideal 1nagcn1a (grcen..,.bsorbi"g) image combin(<i with an un.,cantOO weaker yellow (blue-.o.bsorhing) image; a nd the cyan image is equiva.lcnt to an ideal cyan (red­absorbing) image combined ,.·i1h un,.canted )'Cllow ( l>lue-.al:.orbing) al>dm.igenta(grcen .. l:.orbing) imap.

The 1ochnique of compc"5aling for unwanto:I ~btorptions is known as "':3'king, and is frequently carried out by.combining a ,.-.aJ< poo.it""'• made from 011e ocpara1ion 11egati,..,, >nth anod1er separation nega<i\'C bcl'o«: making the colour pooili\'C. Thus a positi>'C of the correct eo11u·ut made from 1hc green separ.uion ....gati,·e "vuld, if combined with the l>lue separation negati,·e, 1t1btrac1 from the reoulling }'ellow pooi•i>'C a )·dlow deposit cor· re>ponding exactly 10 tl!C unwanted yellow image in dlC magenta pooiti\'<. Thatcorr«uford1eblueal.>oorptionofthemagcntad)'e. focorrectforlheun""""cdblueandgrttnabtorptionofthecyan

di-c,posi1i,..,.fromthcn:docpara1i<>nnega1i,·emmtbccombino:I "'ilhboththeblueandthegrccnoepam1io11nega1i,'CS.Thcmasking proccd11reisudifficultinprac1ice asi tiscompluinthcory,and thoughal>riefaccounth"*bceni11cludo:lforthoocreaderswhoarc intere>lo:I in colour reproduction, it i• turn<:<:cssary 10 know more than that thedcparturc fmm die ideal abtorptionsof a,•ailablc dycstuffo introduee. crro,., ll'hich can, howC>'Cr, l>e curc<I by complex photographic proocdurc

Maskin1byColourldCoupl1r1

/\1imµlcand mootingcniousmethodofmaokinghu bcenintro­duccd in•o the EktaWlor and Kodacolor 11egaiivc-positi,•e pro­cew:s, a11d aJ it i1 much cuier to undcmand 1han the 1raditional process a dcscriptl<>n may clarify both thedcfoct and iucure by muking

lnnormalcolourphotography,thecoupler-thcorganicchemical "·hichcombinawiththcde-.'Clopcrorida1ionproduct10form1hc d)'c-is colour!..,. In tl>e coloured coupler tochni<Jue, couplcn arcUICdinlhe>1C3<Ul\'CmUerial thatare1hemsel\'acolou~ ooastoahoorbthooecoiotlnll'hichtl>efinaLdycat..orl•undCStr· ably, Fori1U""RCC, thccyand)..:ahoorbooomegrcenand blaclighl, whereas it lhoukl only ahoorb ..,.[light, ~ow 1uppooe a c,can

'Ull:. >Ul:. .\l..J, O ~ f'//010GJl.o/>!ll"

oouplcr i1 nude to alotorl> the same aonount of green aold blue lightasthcfinalcyandi-.:. ThcdC•-.:lof'Cd I01.)-.:rwill thcn have the s:irncalotorp1ionforgttenandbluc a lJO\-.:r- IJO<h"·hcrcthccoupb ha1 l>ecn com·cr1ed illl<>C)"•ndycand ""here i1 hao no< d<angcd Thc,,·holcla)'Cr, in fa.:1,appc:an a1 1fi1 "·ercmadc"·i1hapttkct C)"11td1·cha••ingnoalotorplionfor~11alldbluc,butwithafihcr

m·cr it wlUch aboorbitgtten a nd blue untfonnly O\"ct ohc nhok

Only '"""ooJou,.,.1 coupkn a n: ntt<lcd, a cyan coupler 1h.a1 i< pink bccawc io aboorbs oomc gtten a11d Wue like the q-an d)c, and a ma~lta couplcr11.at i< 1·cllo,.· bccaU1<: i1 aboorbit some blue like 1hc magcma drc. Af1cr dC'-.:lopmcn1, 1hcn:IOre. a colour ncgali•·cofthis trpe io equinlcnt 1<>onc madcwi1h pcrfc<:1 dY"'. butwithaunilOrmora"ge"ain a llO\-.:r,corttSpondingtothccolour of1hccouplcro.. lllat.,ainon 1hcncga1i•-.:c:ancasil1· becompcn­oa1cdinpr-intingcl"'potiti•·cb)·10rnc l uChrnc1hod H adjuS1ing1hc colourofthcpr-in1ingligh1.

Expos11,. of Cala11r P~lnt1

ltis ofinlerC>tl;ricAy tonotchowacoluurncgati•·cisprimcd. lnblad.and-whi1cphmography1hc rnajorconccmofthcphotu­graphcr is io :1chiC\"C a11 acccptablc reprOOuctiun of10ncs. In mnkingacolourprint,hciJ fae<:d11;ththcadditionalnttd1oachic.-c acccptablccolourrcnderi11g.The1n1ki1bynomcat\Saoimpleonc as i1 i• complicated by ccr1ain dmracler;Jtia in the behaviour or the eye. The one that chiefly con«rn• the colour printer is the ability of the ci-c IO adju•t ilJ colour ..,nsiti,•ity by n significant arnoum IO l!•c prev~iling illumin•tion. This mean• that " prim in \d\ich tloc colours ha,-., l>ttn halancc<l tu look right in da)"light "·i!lnotappcarsowcllbalan~lwhcn,·icwc<lintung>tcnlighting.

Thcrcatt h•'OapproKhcs topri111ingcolourncgati•-cs in current prac~i«. Onccomi1to ofgi•·ing three scp.>ra1cc:<posurcs uoing blue, ttd and g1tt11 fihc1 '1 and is CORl«JUCntly labelled the tri-r.olour rntthod.Colourbalanccisachi.,.·edby•"1ryingthcrcla1i•-.:ex· poturcs through eoch filter on a11 empirical bnisumil thci,..1 rcsuhi1achic-·ed. ' l1icothcrme1hodcotUisuofadjUJtingthcprinting ligh1wi1hfihcnwhicbttduceiuintcn>ityinoenain..egionsofthc spn:1n.1m. Si»ce ~he printing light ••ill Looks substantially • .. ·hi1c·, ithaobcenl.abcllcdthc'whi1clight'mcthod.ThcfihC<1inthi1casc arc UJually inocn<'d in the li1h1 path bet""""" the lamp and the oondcnten ... ·hc,..,asinthctri...olour"'<thodthcfihcrsarcplaccd immcdiatclyinfrunrofthcenlargcrlCN.

Th<ba.u"'•colowrtV<..ol61m. (•)0.;p...!colowJ<ai<.

~~~"!:'~~~·:;:.r:.). •<>dfu>o<lrtdopn>cn11ho«>

=t?'=!;,.,~11a.::;."!..d~~"~~ l~~.'.':;:..~-bl<ad1<d •<>d&.olftd by&u.bonl.a•-int:-ly

~~,._,,J>r,,_,,;tt<dlifht.tl>eW«i.,._.,.pro<tu«lht:~colow

CMpler 23.

CINEMATOGRAPHY

THE FIRST l'MOTOGR,\PHS in tcn<led to depict "'"''cmcnt we..., those taken by Ead11-card )..[uybridgc in 1877. He wed a number ofeamcras to photogr.oph a running hone. His

locomotion pictures <lemo1utnncd th.a< pa.intcN had millakcn the nl<>'·cmenU of a galloping horse. [n later upcrimenu, he wed as many as l"or1ycamcras in rapid •-ion 10 phoi"@:taph the movc­menll of men and animals. Some a\lempll ""Cf'e also m.adc 10 vie..· 1hc pictures in a manner which rewmp(*:d the original ,,_,,.. men1. H"""C\"Cr, the lint an.im.atcd piaures "-ere thooe dcmon­""''cdby 1'homatEdiJonin l889onaninstrumentwh.ichhcca!lcd the Kinc1oocopc. His objeet ,....,. to enhance the appeal of his tteently inve111cd phonograph by adding mo1ion pic1urcs.. These finl picturdtook 1ltc formofaoontinuo.uspir.olon acylindC1", buthcoooncbangcd tu.uingacontinuous.,ripoffilm,pcrfonu<:d at the si<ld, which was rllO\'e<l through the camera with toothe<l s1>roekct1. l'rim1 from thi1 film wen: then viewed dtrough die KinctOICOl>C by 1he *""'"means.

On1hco1her1ideofthcAtlanticand atroughlythcpmc1imc, Vricsc-G1ttnprojccte<lhisfintmovingpictun:,for1hcbtncfit,so1he >torygoco,ofa London policeman " 'hohappcne<l tobtpassingthe invcntor't1tudio.Ashispictun:wasaprojccte<lonc,Frit:$C·G1ttn may rightly be titled thc'fa1ha-ofcinematography'.But 1ltcidcaof 'movits' ,....,. ,.cry much in 1hc air at tha1 time and numc:rouo pa.tct>U "-ere taken out forappa.r.u.u which"""' ne>'tt manufac· turtd.

11 .. -asgi,·cn10thc l.wni b-cbrothmtoproducethcfir'lt'cinc· matognr.phic' " "ith periodic ""PP"-~ of the film, permitting

CMpler 23.

CINEMATOGRAPHY

THE Fiil.ST l'MOTOGRM'l-LS intended to depict mc"'cmcm wettthooctaken by Ead11-card l>luybridgcin 1877. !·le used a number of cameras to photograph a running hone. His

locomotion picturadcmono<rated th.a< pa.intcri had millakcn the mo••cmenuofagalloping horse. In latcrupcrimenu, ltc used as many&$ l"or<y cameras in rapid •-ion tO photograph the mov<:­menu of men and animah. Some auemplS "-ere aho made 10 vie..· the pictures in a manner which reoomp(*:d the original ,,,.,.,,.. men1. H~, the fir>< animated pictures "-ere thooe dcmon­>1ra1ed by 1'homat Edjj()n in l889onaninotrumcntwhichhcca!Jcd the Kinc1oocopc. His object_,...,. to cnh.ancc 1he appeal of his rncendy invc1>1cd phonograph by adding motion pic1ura.. These fimpic1uratook1heformofaoontinuo<Uspiralonacylinder, bu1l~ooonchangcdtuU>ingacontinuoU11tripoffilm,pcrfora1cd at the 1ida, which was moved through 1he camera with toothed llH'<>Ckct1. Prim• from 1hi1 film wen: then viewed tltrough 1he Kinetoscope by d ie .,.me mea ns.

On1hco1her1ideof theAtlanticanda1 roughlythe..,.mc1imc, Fricsc-Grttnprojcctedhisfirstmovingpictutt,for1hcbcnefit,so1hc >tOry goa, ofa London policeman who happtned to be paaing the invcmor't1tu.dio.A•hispictuuwa>aprojcctedone,Frieic-Grttn may rightly be titled the 'fa1htt of cinematography'. But the ida of 'mcwia' "'al ,.cry much in 1hc air at <ha< 1ime and numcro<U patct>U "-ere 1akcn oot forappara1us which ";is ne>'CI" manufac· tu.-t:d.

h";isgi,·cnt0thcl.urniCrcbrothmtoproducethcfirs1'cine· rn.:uographic' 1o·ith periodic ""PP"'~ of the film, permitting

111£ ~'C/£,\'(.'£ Of' J•/IQTOGR.lf"ll 1

ohematdy the taking o(,ic.,.., the pnnting of1he politi'-.s and th~r proj«tion. lli.ey were abo the firit 10 give public perlOo-­manca. In fact, theytu<:<:ecded in ol><aining near perftt1ion at 1hevcryoutM01Qliasbcenthowninreccntproj«;1iomof!l"'ir fimfilnu.

Th e P1,.l1t ence afVlsian

Cincma1ography relia on a cha,.,.ctcriltic of the visual pr"""'° whcrcbytheappcaranceofanimagercccivcdl>ythcr<:tinapcrsim for" ~ricfjam-.·a!, after the actual light otimulu• is retnoV"<L By vicwinga •uccelfiunofimagaatfrequentimcrv~l•,1hcimprcssion ofoo1uinuow vision it obtained. In practice a fr.:<1uency of 16 framaJ><:rscooudissati•factoryandiswuallyaduptedfor oilcnt filmt, At lowerspeedoa dcgrtt ufftitlcritKCn. Sou"d filmoare ukcnatZff.p.s.tn.providealong<:r1rackfordietoundrtt0rding.

ln '11r111iglu' filmmg, the spttd of the camera and that of the projtttnt Mre kept theP.me, but cincmatogn.phy nfl'cn a ,.,.ri.able time ha$C and can be used either tn reduce the rateofmo\..,mcnt 0<toincrcai.ei1.Thisprtr\·eda"'1Jrccofcntcnainmeminthecarly cinc:ma,alongwi1hothertrid;ssuchasn:-.~ngmuvcmcnt,thc uoc of 1>n>j«;ted background> and double printing, How<:>-.,r, i1 hastincc P""'cd an extrcmely>caluableaid in the11udyofmo•·e· mcntandgnMth

for nnrmal cinematography the imermincm 1110>-.,mcm of the film !hrough 1he camera i• nbtained with claws worked by cams which engage thepcrforationsof1hefilm. A ro1aryscclorshutter, oy nchronised with the m0>·cmcnt of the dn"1, givco an CXI""""' ofabo<nl/'!lloccondwhilc1hefilmi•atrcol, imcl'C"p<ingtheimage while the film io being rnQ\'ed for lhe nexr cxpoturc. A tomcwha1 simil1&t•}1tcm itctnplo)'ed furprojecting1hcfihn,theintermi11cn1 "'°'<cme1>1 often [)Cingobtaincd by" device knnwn at a '~faltese croa.',

FHmSlaandl'lctvr1Farm1t

Siandardfilm is35mm_ "oidcwi•hfourpcrl'or111tim111ocachfrarnc or picture. filn11ofnar..,....,.width{chiclly intended foramaieu1 use) ha,.., appeared insiu:sfromZ~to8mm., bu• thosccum:mly in use are 16and 8mm. Filmofup tn70mm.wideluubccn used in lhcpr(lfcssionalcincmaforspccial wideocrecneffttts.

The '"'Y considerable improvements in fine-gr111in cmuloions <wer lite l:ut two or tlorcedttades has made it J""'ible to obtain

C11"£.llATOGRAP!lr

a good ocrccnimagc from8mm. filmw:iththercoult1hat n<:ady all ama1eurcincmatography it bQcd on ihisgall(':. At theoame 1ime, l6mm. film is now rcg:orded as .., ,isf .. tory for 1tthnical filming, ducurnct1tariei and ir11tructiunal films. llu: Bmm. ,.·idth ""'originally introduced by Kodak "" a 16mm. width ''""''d film tubccxpoocdin1w-01taga.Thcfintttagcc•puocdhalfthcpicture wid1h and 1hefilm was\hen reloaded in 1hcc.~mcraand exposed forthco1herhalf.Aftcrp.-occooing,1hefilm"'aoslitdowntl"'middle antljoinedtornakeadoublelengthoffilm.Si11glc-widd18nun. film (Super-8),g; ,.;ngalargcrpicturcacoisnowcomingimouoc

·n 1e 11ormal picture format is that of a horiw1U•I rectangle "·ith o r~tioof1•33 10 1 l>etwecn wid1h and height. II)' using special lcn"" which coml"'"" the image ina horizontal dircc1ion, it is poosiblctoindudeawidcpictureona 11andardframe.Suchlcn_,, arekn""·nQ1JM-pllkandbyusingaprojtt1ionlcnsoftimi!ar d"'r111c1crinia, the «:umpn:socd" image isllretched tn iu j>ropcr widthonll..,ICl'CCn. lntl><Cincmau:opcproca1.awid thtnheigh1 ra1ioof2·$ 10 l is obtained in this way.

lluk-and-Wltlt•FHms

lntheprofcsoionalcinc:mai1hasal .. ·aytbcentl><pnc1icetoshootthe picture with a nc:gati,·e film and to print the ncg~ti>·aonnbtaincd on toa potitivc film. Both the cxpoturc and processing arc care• fully controlled 10 produce a n"!(ati\•ewhichyiddsa sati•factcry image on a potiti,·e material of fixed characl~riltia. (p. 210) . Thcproccuingnf.cincfi\mi!usua!!ycarriednut111 •pee1al !abora­t<>ri<:1e<J uippctl wnhconunuousproccssing machina. Ruoh print1 arc made for viewing and edi<ing {or cut<ing) 1hc ocquenccs tn make "P a complere film . The negative is then cu1 '" nutch the edited potitiveandanyrequirodnumberufprajcction prints can then[)Cprin1ed. lt is frequcndy1hepractice 1omakcanumbcrof duplicatencga1ivafromamastcrpotitivcfordi1tributionona world-wide balis. Appropriate tides and .ound track can then be addedforlanguagedifl'ttcnccs.

The \llC of black.and·whitc film in amarcur photography bas been chieHy baKd on n:-.~ film. Because the amateur normally onlyrcqulr<:1onccopy, itall'ordsconsidcrablceconomy sincethe rilm used in the camer111 becumco, after ...,,,..,,....1p<OC..,ing,1he potiti,.., film for projtt•ion . The ~crsal ~roe?" offc": thc.addi-

~:.~~~~c:i~i 1~~£2f~~~'.~:~;!:i~t~~~~

~80 Tiii:. SCJE,VCE Of' l'llQTQCRAl'll f

"'"'°''edbyablcaehingbathkavingtbcundev<:lopedandomalLc:r gnii1Uwhichare""poscdanddC\'clopedtoformapooiti,..,ima~, -p.13!>.

Colour Film• Many t)'t1enuha,·cbttn tried"'"' thcycarsindudingbothadd11i,·c andoubtnoc!i,•cproceoseo..1'0ncof1bcadditive tyoterntJ>rovcd cornrnerdally suce<:ttful and all modern proc<:OOCt are baoed on subtnocti,..,colourfilms. Thccurrcntpnocticci•to 1hootwitheither 3 revcrsalcolourfolmandfrom thiJ to1nrikciepano1ionncgativct forimbibi1io11printing•uchasthatofthcTcchniwlorproc.,,.. (p 271), or IO 111ea negative colour film which i1 printed nn tna l"'li1ivc colour film.

Amateur ciiu:matography is done abiw»t cxchuively Oii rcve""'I colourfihno.. l nfae1,thefirstcnmmcrcially1ucccut'ul1ubtracti''C colour film- Kndachromc--"""' introduad IOr amateur rrlO'"io

So11ndRtcoNllng

f;,.i.v.,;,,1>1<,.;.1oh (IQl) • .. ri•bk d<mi1y (.;,1W) ............ ~~

h i1 intcrQtingtor>01e that Edi11C>noriginally1houghtnf1nntio11 jliCtu,..,. uanadditionalappealtn .,,und recording, buo i1 was not until1<>tnethirty yea,.. later, when thccincm~"-at "''Cll·cttablished, tlla1 a [>r.tCtical method of adding 1<>und 10 motion pictur.,. wa• devised.

Som..trccordingforfilmscanbccithcropticalor1nagnctic, At the preic:nt time it iou<ual toprovide anOJl<ieal 1rackon the projec1;o11pri11<,hutforsomcpurpoocs,such ... 11c...,.;ipbon;c oound, multiple magnetic trad<• are added. Magnetic tracb are alsouscdwhcni tisnoqui rcdtoadd1<>undtonarrowpugefihn•

Optical1<>und tracbarcnftwodiffcrentforms;variablcdens>ty, ouch a1 d1at of ll'tstcrn Electric, and ,·ariable area {R.C.A.), ( t'ig. !). Howcver,currcmpracticei>1ouscavariahlearea track °''projection prinuev<:n ifthcva~ablcdenoity t}'ltcm it1ised for

~i::..::~~~~~~~ :;>.~!"'/"..:'uonnd t~~=:~u~::'~k}:'C::-~"~ f ig.2

STEREO-PHOTOGRAPHY

THE APPLICATION ofa '1cro:oscope 10 obtain thr. imprcs· sion of solidity from '"''Odrawin!f1 pre-dated the inw;mion ofphotography,butr«0'tl11u~<thai1hefirs1Hcrco­

phot<:igraphtwettnuodeasearlyul841.Thcoeearlytt""'°'"""" made eithtt " 'ilh one """""" from two viewpoinu <)<" with IWO carncraspl.a<:cdi.idcbyside.Th<:fi,.111erc<>-Umcrah.a•·ingt""O lcn1t:1wasin>"1'ntcdbyOavidB,..,w1lcrill l8'l9 , andJ1.,..,.,.wcreon di•play at the Great Exhibition in 1651 and later in 1he Inter­national Exhibi1io11 in Pari s in 18S~. 'Ilic combined publicity , .,.ultedina1r<:1Mndouscmhusiasmforthiofonnofpho1ography. Unfortunately, it rapidly became• kindofvulgarpttp->howand by the aid of the ttniury it was virtually banned from polilc drawing rooms!

,\coruidenblc~i""1ofttcro:<>-photographyoccumxlwithtl>c

arri•'31 e>fmodcm colour photography amt a number of excellently madestett<>-<:arncras""'"''P"'"" themarketaswcll asavariety of i lerco-a<tachmcnt• which <:<>uld bc fined over single-lens cameras. At the >a.me time n numl>er of stereo-viewers were marketed Fin>lly, polari•ing filters were applied to the colour projec1ionol 00.h "ill and movie picturQ

ln >pi tcofthe•'O>rio;lfonunesolst..,...,,.photographyasapopular medium olphocography, it still offers a utcful recording rntdium "'hettspatial rclatioiuhiJ>'l>etwccndifTcrcntOhje<:uauim1xN-unt. Thui. it eanl>eofvalucinrtt0rding1urgical opcrations,arch>co­logica! surveys, photogrammetry and a1 a display medium in a<kerli•ing and oommcr<:e.

Stem.-photogn.phy i1 bas..d on the binocular nature of vilKm Thc$1:parationofoureycspro,.;d .. lll"'ith tWOasJICCtsof •IOlid

"'

The MekingofSt1ru-pllatograpU

~rr~::~2;~:~:;~~:~T;::~:~:·~~ onthis~,ohnuldbeoolidlysuppnrtedonas.'and.lhcca=ra 1houldl>elevcl (la1erally)thoughit1naybeultedupordown. TheU1Cofagrcatcrlatcralscparationhasthecffc.:1ofe1t.a~erating depthandocctu"Sinacrial1u"''C}"Phol<>g~Ph.y(p. Z96),Fordooe­up1 ofsmall objccts a smaller scparahnn 11u"'<lto0\..,<00D>C

parallax.

!$4 Tift.' SCIENCE Of' P//OTOGR,IP/lr

Tbcupoouramustl.><id<:nticalandlh<:lc,..aperturcohouldloc ttopped down wO\ci<:ntly '° gi>-c a«q><Jlbk lh.a.r'J'I><* in all plana or the 1ubj«t be:ing photoi!:raphcd. The lighung ol tbc 1ubjcc1 should be 1ucb as to awNd aboenoe oldetail in either 1had<>w or

higF~;g~~=~~rapl,. 10 be made with a hand-1..,Jd ca""'ra or or action 1ubjcc1>, it is nca:ssary '°....,a ,,...,.len~ ~,,,.,ra_or an

~~;~~c~',~~:~:7;'g~!h,~,."i=:~i~,:·:~~f~;,:,i<J;~ g,~~· 1).

Viewi ng Systems

~n~C:,~;:~.;:;;::i:: .. \;~ :.:~ t;;'C:~!1:!:':n;~ll .. ~~ 11~,namelytbc,·ic--ingoftwoocpanucpho•oSr:aphlto obutinthccff.,.,tof1hcthirddimension.

Somcpeoplcarcableto\-ic..·apairofRc«:"'"pl""°llraph1"·i1h· out the aid ora11crcosoope.mcrclybyrcluingcon_1rol0>"C1" the C)Uoothat a double imagctSseen. Ata certain J><llnlwh<:n '""'

~r:!'," ;~"::~1:i~1~~;,'':.~"".:~· ;:"'~~··b;"i~j:~.·:r:; cardbct"-ccnthcc1·Qooth.atcacheyccanonlyocethcphoto-

~~~;:~.r,;~~ ~;;n~r ";'~P~~ ,:.oa~:,:~·i~y~~c~~: ;~~~:~ "';~::.P'.~~",8;~]~~;;;:1 ~:i=;i~o':'::":j::;rs~;~:j~,ion io by the

::;~:E~E~EE~~~fa~~a·~~:~1

:E~~!~1

~:~~~ t;:';~;.,;:~.~·:ii·n~n~:;...i~11 b;i;~ ~.,.,':"~f .~~~~; ~~,J.':;Jl~l;..1: :~::n:=,:,., ~.~~~~ ..:rccn "nee a

THE APPLICATIONS OF PHOTOGRAPHY

P MOTOGR1\l'!H' IS normally used t~ a<1gmcn.t ""d cx.1c11d \'ioual oblcrva toons. Not only dOCI 1l g•VC a pcrrnancn1, dctailc<lrccordofC\-cntsandplaccsforcuminallon atlc11urc,

but it rccord1 Kena ,.·hich arc t<>odimor C\'Cml which arc too rapid or1ooolowfor thc C)·ctopc-rcci'"C,rapond110radiatiotutowhicb doc eye i1 inocnoiti,.,,, a nd by mc:uu of cincmatogn.phy can •pco:d

~i::,:~n~ac=i::!:~~~~"'.!':;'.!""~:u~~t:C~~ ,·aluablc rnol, and rcw of their activiiico do ntM make use of pho1<>­graphy in oi.., cw 0thcr of its aspcc1S. l ndtf:d, in....,.., f..,J<b, •uch :u •.J>CC•J'(lf!;nphy, u1~nomy and "'diograph)', photoSraphy pla}~ a11"'1tf1ralar1d.,...,n toalpari.

Speetrogr1plt7

Thcscicnccof1pcc1rogra phy, forcxamplc,dcpend10r1 thcfact 1ha1 whcn 1ulls1ancQ a rcraised1osuchatcmperatu...,thallhcyb«omc incandaccntsases,thclightcmi11cd itnotacominuou11pcctrum, bu1COJUisuofanu1»bcrofindi,;dualli...,.orba,.d1whooc,..,.vc-

2£.~f2¥oo~i¥I~~~~;J; con«:nuation, i1 it aloo pcmiblc to calcola1~ 1hc proportion olcach clcmcm prcocnt ( •·ig. I) by""""uring,.·11bamicrode.ui1omo:tcr 1bcdcrui1it1ofthclincs.

~~:§n:l~:~~f~I.~~~~~:E1~~~]~~i~ "'

Tiii! SCIA.' .NCll UI' /'//OTOCllA/'/lr

dime•. M'"''"'"er, many of the important Jines arc in the U.V. '° whichthecyeitin>Cn•itiV<:. Thcteclmiq...,ofquantitati•-.,analysis bys~trogn.phyi1kno,.·n u sJ>t<rntlwotiu/""4ly•is,andin10mc f1eld1 (i'.oreumplc,thc a n<llysisofalLoys) haslargely 1upcneded the oJ<n.,-.,r and mo..., cumbn1ome chem;,,,.J ttthniques. In many casesitwilldetccl cle111Cnll"'hcn thcirconccntrationsarctoolow forchemicaldcte<:tion.

I l1 1 1\1\ ,,-.,.l . OPOCTllOCH~L•><AL-.Sp«U'Of1•moc•fcopp<r.1incor.dbr>".n,. bru<;, oun•o -.U11>..,.1yol coPP«on~•;,,.,1,. .. . i.o ....,,.;..,,m•il

q ... n1i1;.,ot o<h<t<l<mtnU

Astronomle.lPhoto1r.phy [nas1 ronomy,dirnctvitualol>Ser>·a1ionhasbc<:nalrnostcompletely rcplacedbyphotography.111ercaretwnrc;uonsforthi1.0ncisthat a phntograph is a permanent record which can lle mc.:isur<:d and clu.ckedatanytimc.Theothcristhatcv<:rythingwhichthccyccan oce in any gi•·cn tclel<Xlpe ca n lie photographed in a $-minute expoourcJOthatCX]IOIUtt:sof4·5hourswi11 ,.,...,a!obj«:tspcrh.ap:i 50timcs lessbright1hanthotcocenbythceyc. Thetclescopic carncra1hmex1cndsthcdcp1hofpenetrationofspacci>ythcsquar<: rootof50 (p. 39), that i1, by about 7 times. The cfT«:ti•·enc;s of photograp~1y is probably. not quite as great as indicated by tl~ese figutt:sow!llg1<>thchnn111osityofthecarth'1atmospherc, but nit diflicult togi•·e really a«:urate estimates 1incc most photograph> arc made wi1h blue-sensitiV<: plat ...... nd for good rc;uon. The obocurationbythcglowortbeokyitlessforthelighttowhlch1uch plata arc ..::nsi ii,..::, •hough in oome special cases a pl:uc with a 1Utrictrdrangcor..::mitivityinthcdceprcde>.npcnttn.1cfarther 1hrough.1hccosmicdust.l'late1oloihcrwlour..::nsi1ivitie1canabo g;,..,uscfulinform.:uion;inrn.-rcd pho1ography,forinstancc, has ,..,.,·co.lrd1tart0tbe1'wileinvi1ible.

TI>e brighier 1tart appear larger in pho1ographo 1h.an ~h.c fainter onesbecaU>Cofthcoid,... .. )'lteatter oflightinthe..::nsmvelayer and this phcnorncn.on ;, made u..:: of in estimating the .... ,.ui.i1e or brigh1ne&Sola..,.r (Fiji;'.. :Z).Ourinfonnationaboutthena1Urcol

"fl/E Al'PLICA"f/ONS 01' l'llOTQCRAl'llr

fiA. 2. ·~1.,.;., ~1·, 1hc ·~h;rlf>OOI' r.tOOI• (~,... of 1fH- """' b<->0oirul•i"r•l0<b..l• ckM,.·n)i.1hte<mm;,.nH8IH\.,., ,.,w• r from th< ea,.)o, T h< ino.n;tudd o(•un<mnd;n~ "'",,..de"'" m;....i f"'m <h< •; .. , oloh<0~~· .11 ... 1 lt'~-•-'1'.i,..,

1tartisob1ainrd from photograph1of1heir1pee1ra. From these we candeterminenotonly1hcchemicaloompooitionofthcou1crla)·cn (muchuintcm:s1rial1peetmgraphy) bu1al101hcirtempcratutt:s, and 01hcr oondi1ioru 1uch as prcssurc, or"''""· in 10me cases, ""l>Cd>Cr 1hey ha•.., • mag11nic field like the earth. The displa<:e­""'"'of the lina in d1e 1pce1ra of ncbulu from llu. "·a,·e-lengtho thcyhaveonthcurthtdlousthatthcdiotantnch<>lacarc.IDO"·ing away from us. Pholography.bolhdirttt andofspcctra,haogiven usagooddcalofinfornuuionabouttbe1urfaceconditionsand <hcmical oompooitionoftht. atmo1pbett:sofplancn.

Phatagraph)'lnSpaee The dc,·clopmcm ofrockeu p<>l<"crful enough 10 escape from the carth'sgravityhaoaloocxtcndedthefrontierforphotography. CarnerasQf1•;11ne5'>rtoranothcrl<"e,.., in,.allcdinallthcfirStrockct prohes and sciu L>ack the firSt views of the earth from "outer space'. Wi1hmannedrockct5,1he crcw u .. cameras,hoth>1illand cine, much as anyone might do from the window of a plane. (Fig . 3). Howe>·cr,1clcvi•ioncameraoarcu•cd togi,·edircct,·i•ual impressions and arc al5'> used in di'1a!lt p':"hes where 1he:e. is noproopectof,..,covcringtherocket.lndu•cascthctelevJS<on images a,.., photographed 10 pro,·ide pcrmanc!lt records ouch as thooe shown in Figs. 4and'.:>

Photographyoutsidetheear<h"s atmosphereolfcrs manyad,·an-1ages to the astronomer and l<"hcn the first >.~ 1clli1e labo1·a1orics a,.., inor\J i1,wcrnaycxpcct further progress in thiofocld

r,i.,,.;,;oo•p001<>s"Phof ,noon"• ou.-f•«>o:ntboekbyR•ni<r81,.,-, >«Of>d>b<foreimpaco

Fit, ~· l'M<ograph of 1he >u.-fa« of ~fan ••ken from a d~tafl<< of7,800 mil« bv Mann<r i, onJ •d• y<d bod"'~~;:!', from• diston« ofr.<o•ly<f(),000,000

Radiogra11h1

Thescno;tivi1yofpho1ographicma1erialo10X-rays (towhichthccyc isin•cnsiti,·c) mak<:> it i:><miblcto pn><luc. shadow photographs of objc.:tJ placed in a beam of X-rays. X·ra)., arc electromagnetic •·ibrationo which differ from light only in wa,..,·leng1h. Their rdaii,-dy •hort "·a,·e-lcng1h allows them to pcncu-ate many maoeriab whi<h are opaque to ligh1 . The "'a"e·length• of '"isiblc light manifest thcmscl\'<:> a. differenc<:> in C(llour; th<»c da...,d under the general !errn X-rays are chao-acteriscd by increasing pcnetrationastheyde<reascinlength

Resistance to the pa""'ge ofX.rays is an atomic phenomenon, and increa..,s with atomic weigh! and with Che number of aloms per unit ,-olumc. A gas will therefore offer lcso re•i"ancc than a liquidor solidofthesamea\'erageaton•icweightbecauscthcreare fewcra1om•p<:runit,·olumc,andorganic substane<:Ssuchasn<:>h orwoodwillbelcM opaquetoX-rays thanl>oneormetalbccausc the a,·erage atomic weight is less. Aluminium is relati,·ely trans­parent to X-ra).,, compan:d ""ith othc,.mc1ab, becauscofitolow atomicwcight,whilethen,..,,t<>paque 1ubstancccasilya'·ail3blei s lead, which io therefore uoed for protection against X-radia1ion.

Ashadowpho1ographtoX·ra:-., {calledaradiograph) mayre,·eal the internal otructureofa material of heterogeneous comf"'sition Radiograph•ofthehuma1>bodyshow!hel>onestructureand may re,·ealfracturcd,d iscasedormal·formcdl>oncs.Theymayalooshoiv abnormal condit ioru in non·bony Portions of the hocly if the><: showdifferenc<:>inopacitytoX-rays;inthi• waydiscasedrondition ofthe lungsmaybediagno:>scdand'ma>S" radiographyofthech<:>t i• now carried out periodically for school c~ildren to detect signs of tuherculosis (Fig. 6). Special tcchniqu<:> arc used<<> examine organ• which normally are not sharply differentiated on radio­graph•; forcxarnple, the alirnentarycanalcan besharplydcfincd after the patient has eaten a meal containing barium sulphate, aninertmaterial\'eryopaqueto X. rays,becauscofthehighatomic ""ight of barium. Similarly blood \""5Sels may be outlined by injcc1ingintothcmanorganics11bstane<:cont ainingiodine, which absorbs X-ray•.

Ofrecentyearsradiographyhasbeenapplicdtoacon•iderable extentininduscry.hprovid<:>anon-dcstructi,·crne!hodofteltinga manufac1ured article; for example, it is widdy u>ed for testing castings for defecis •uch as porosity, blow-holes, cracks, slag in­dusiono,e!c. (Fig.7) andfortcstingforfauhs inwelds.TheCJ<amina• tion of light alloys does not require very penetrating X-ra).,, but

r."'; ... l ..... ,,,

.'~ . -

• .. · .. . ~: . _, ... - ... · .. ··. . .. - ... , .. , . . . . ' .. ... ' t '~ I t. ' II ' : t • • • ' t i I t tt' I I I I .. t • ·.• ' . " ... .. , .. , .. . . .. , ... _ ... : ..

•

..... ..~ , . .. .. : ........ 9 · - • •. - .. ~ · • . . , . . .

. .. ,;,.&. X·••Y diff•><IX>n potl<m ol ,;.JKon • .,i.;.i... C-"v: Rt-~ ~.., <! Mot C.-.i "-"'"" ~. w....i.;,.

stctl castingo or welds do. In such caoa x.,..,.ys ofshot-1 wa•·e­lcng1hs produoodbylargcretsofhighkilovoltagcar<:u..,.),andi!ttl up 10 twelve incha thick can nowadays be cumincd. X-,.,.)"Sol Ulrt:....,ly • ll0<1 ,.,_,-e-lengtht, known as p.mma "')'t, a"'- emitted byn.dio-active1ubs1a11ces,and1Mrtttt1tproduc1ionofartif.cially radio-acri,..,elcmcnts,•uchascobah,Wbttnincrcas.inglyapplied to indu111ial ndiography

X·R11Crr1tali\111lysl1 X-rays arc '""nsrnitted or rcAcC<ed from cry>lallinc oubstauccs in ccrlai " well-defined directic>n• only, the angt., dcpc ndingo11 the

Tllli Af'l'LICATIONS Qf f'/IQTOCllAl'/11

""'-lcngtholX-ra)-.,•hcopacingandarrangcmcn1ofatom1in1h< cryst.al1truc1urc, and1heoricntationofthctry1la l.1'husif (Ninthc original cxpcrimentQJ> X ·ra)" Cl')"Olal diffraction) a bt:lmof1<·~i1t X-rayo (thatis,witharangeofwav<:-lengtlu.) ioallow~tofallona

t')'Wll, a pholoS"'Phic pla<eorfilm placed oomcdiHanccawa}' ,.·;u ,..,,,'Cala pancmofopois,afttTd"'"Ck>pmcnt (l'ig.8). •"or each of thc..:1po11 thcoricntationofthccryotal, andconscquemlythe oricnta1ionofit1 imen>al struc<L<ft., haolxenjust right roc1<1mc p:micular wa,·c-lcngth of X· ra)'S to i>c JIO"'Crfully rctltticd or ""nsmi tttd.Thercarcmany ,·ariationoor1his ttt:hnique,10mc \"try ingcniow, all dcvioe<l '°""ardo detem•in ing the arra,.gcmem <>f atomlincrysmllincmattriat..The mcthodhaorcvca!td1hccry>tal · line1tructurcof10 many malerials tl~" it can"""' t.c used for 1he ana l)'lil olf""'·dcnofunknowncompooition.

P•ottmictOV'lll•l l'hotomictography, d><: pboo>graphy oflhc i1nage producal bya microocope, is of imme,... •<due in 1uch lcie11ca as medicine, sur­gery, boto.ni·, :.oologr and mc:tallu.-gy. Although quite dabon.tc opp:oraius is marketed, good photomicrngraphs can be made b)" a.caching a simple camen. <o a microscope. The abili ty to •eewd fincdctailismeaourtdasthcnurnt.croflineipcrrnillimc:trcwhich cant.craoh·cdasoeparatccntilics,andiscallcd 1hcm.t1i111powo {1>.Z37). Whilc1hcrc ionolimit tothcdcgr-ecofmagr1ificationor <he image in pho1omicrography,thcrcisalimit (dcp<:ndinl(ontht raolving J>O"'tr of the opt ical sy>lcm) to the ainount of detail which can be rc<okcd, and magnifica tion beyond thi1 limi1 (tha t j" rnagnif1ta1;on which reveals no more detail) is k110wn"" '"'/")' ""'l~ifi<dtia<i. The rao!ving powtTdcpcnd.,ofoou.,.., on 1hcdcsign alldcons1ruc1ionofthclcrusystcm, but howa-crcla-croncioa t e>ptia thercisalimit to 1hcrcsolvingpow1'1"ofan optical rnic...,_ ocopc,dcpcndingonthewa,·c-lcngihofthelight """1-onccould notupec11oraolvclineiwh.icharcmuchcloocrtogc1htTthana ""'~-leng1hofligh1. The oma!ltT the ,,.,.,·c-lengih, thcgttau:r.•hc p<m1blcrCllOlvingJ>O"tt,alldinord1'1"1oobtainhighrnolu110n U.V. pho 1omicrography is often U>Cd.

A r=nt imi-tant ad'""™"" in photom;.rography h;u been the U>C ofa beam oftltt1rons instead oflighl or U.V. radi.uion•. F.k<:tro,..alfttt pl10tographicmaterialsinmuch1htoamcwayas light. Like light, a otream of clttu•o'."' bthavcs.i111<>mc rCSpttl> aothough i1consi•11 ofdioerc1cparocla,and1nothcrrapccto ao ir it were tra\•tlling like wa,u. The wave·le,,gdo or Rn dtt:tron

fi,1. 9. •c•cno>< ""'"°""A"'. l.odh·idu•l """"""'°' ..t , i.,.

Turnip \ "tllc"" ~1-i< 1·iou1 >< l80.000. C-u1r R . . !/.,u, ... • ..i u. w.11 .... ,c .. ,.,;,1 1,,,_ • .,.,,c ... mq,.

beam, howc•·cr. it m"ch smnll cr thi111 that of light (t~• "'I' as agai n11 ~50rnl'forg=nlight-lf thcclectronshaveb<:<:nacceleratcdl>ya WkV electric r.cld), and hence the theoretical maximum resolving power should be much greater. An electron beam cann<>t be rcfrac1cdbyalcn1,butit canbcdcHectcdhyanelectricchargc ora magnet. By pas.inga J..,amofclectronsthroughaouitably oha~ magnet it may be brought to a focus. Such magnetic ten.a, uocdmuchatgluslensa•rcina"optical microocopc,gi'·eclcctron microocopcs capablc ofcx1rcmeraolution.\\'careno1y.:tablcte> make magnetic len..,, with apcnu...,. any"•hcre near u high u th01Cofglaoskntct,001bat"'Cha>"CrlOl y<:tapproachcd1hcthco­rctical maximum raolution an electron microocopc will gi,..,, but wbcreu an optical micl'OOCOpe Cll n uxfully magnify oomc Z,000 diamc...n, "" can now obtain a magnifie11tion of up to 100,000 on an electron micl'OOCOp<' (Fig. 9). It i<C>"Cn poosiblc toraol•"C individual molecula wt..:n tbeoe arc large.

Mlgh-Spe•~ Photo1raphJ l..igh1..,.,rccs1uchaothcclectricoparkand1hegasdischargclamp mayhavcadurationofulinlcatone-millionthofa"""'°"d, and can 'frcc1C' themo11t rapid motion. Mia:h •pttd photographs ha.ve bccnustdinotud)·ingtheprnpagationofft..mc,thcdischa.rgcand beha•ioorofprojectilQ,&cronautica,theHightofinxctsandbirds, themotion>ofa1hlc1oand•porumen,andmanyJimilarproblemJ (Fig.10).

Crcatingenui,yhaobttnU.p<:"<lcdoi•dC>·isinghighspccdcine camtru, oomeolwhich can u.pme•t the rate often million fram<S per oecond, each frame being cxpoocd for one hundrcdth-mil­lionthola 0tt0nd.Atthi< ra1etheaction0tudicd is"°"·eddown 600,000timco.Thi<timcmagnificationi<farg,..,a1.:rthanisneces­oary <>r doirable for 1he 1tudy ofmQ11t problcms oihigh •J>ttd action, and a commtroial high 01...,.) cine camera ialr.ing 16mm.

~-.,. 1(1. ~l•gu;.,.. ,;11< INlktol<;k;"ll~"pt.,"' (•i<......i~ .. ·;..)., , ..... d•y<>f?.ooor1 . pu0«0. Spouk ~°"''P~ od<n

l>y ~;;,1~~:."'''

TllE SC/li1'' Cf; OF l'llQ/()C/!M'llr

film at 3,000 rl'2IDCO per sccond, gi•ing a tirn<: magnification of '200, i1 ll"llerally ad""luate. This amen ha, bttn incre.asingl)' applied to tbc im·cstigation of high 1pttd rnuhincry, armament, dot~work tntth.anism,electricalgcarand..,for1h. Thcme!l>Od is particularly,.,.Juableas, apart from11robo•copy (,.·bich an be applied only'". ~petiti•·e motion), cincm.otogr.iphy io our only "':"aruormagn1ry1ng time and examining at lcitu"' action other· ,,.,..,toor.ipidtobcp•"cc"·cd.

By low l pee<l dncmatography-k"O"'" ao 11""/D~st 1llw1ography­action1 which a rc too 1low to be propcrly observcd and co.or<linaw!

~~To'.~:st.~~·~; ~~~~,:.cc~;.:;:; !:;,~x~i~r;;;;·: ~~~~~;1d~~: andprojcclatnormal 1pecdof l6or24framo1>erSttOnd.Thetimc rcduclionis•·irtuallyun!i1nited,and1hcmcd>0<lha1bccnappli•:d<o the1hKlyorpl.a.ntgrowth,cloudronn.otioi1,croo'°''problcmsand the like

PHtq rammt trJ

The pooilion ofdi1Te..,11t paru of an image <>ti a pho•ogr.ophic ncgativciodcfinedby1hcpositicmoftbccor...,.pcmdi11gp;ir1Softhc rubject and the optia! propcrtia or the ca mer-~ lens, modified by anydimcnsionalch.angC1inthenega1ive.Byusingncpti•·cmatcrial orhighdimcnsiona!S1abili ty (glassor1pcciallycured film) and an accur-~tely calibrated camera lcm, it is poail>le 10 deduce from meaoumnw11ofthcimagethepa1hof1ho lighl r-~y1fro1nvarious par11ofthc 1ubjcct.Bytakingtwophotograph1of 1hosarne subjcc1 frorn•wodiffc..,tttaccura tel)·lmo"·npoinisor,·ic"·,thcformand di=miot,. of the object phowgraphcd can be determined from theintenection ofrai..,from corrcspondingportionsofthcobj.ect SuchphotographicrncmunuioniocallM~1r•,,,_iryar>dhas

bttnappliWtOtcrrestrialandacrialsun"C)'work,architcc1uraland qtronomial pbotognph)'• In ground '"""Y work, for aamplc, aftcrpn:liminarytriangulaliontogi•"Canaccu,..,terlatummcasurc­mc1>1, the theodoli<e may be "'Placed by 1hc """'""'for rurthcr uten1ioi,.. Calculation of mcasu..,mcnu may then be made a1 lcisu.-efromthe..,ricsofphotogr.oplu.

llutit lli nacrialsurvcythatphotogrammetryhasp..,.."Cdi...,lfof greatotvaluc. lt '"'"'firstappliedtolargcarcas,p:1rticularlyin Jiff.cult "'°'""ainous coun\ry. llccau"' it is mo"' prcc;..,, more rapid and less cos1ly, in "'"'C co\!ntrio it ha1 now completely replactdclaMi~al" '"'•Ymet h0<b. lnacrial survcy( •'ig. 11),ascrios

rw. u. "";.,·,;...,~ ~ .... .m..ot_ ... ,...,. p1>o>ognopi..in

on oeMol "'"'"'· C.."'-" 11.....WV ,.__"'JI' LU.

Fit· 12. Th< Wild A.8 S•<r«> Plow:> u,..J in n\&P mo~ln1. "-"'.>' 11,,,,,;,q A--~1/,IJ.

Tll f: SCll!.NCl1 QF /'l/QTOGRAl'llr

g;~'.·~:·!.f.~ ~~t:;::c':;t~ ·r~~;.~,!:n:~ ~~g~~ ;~r,~t ~~ 1he camera oo as to produoe a scria of ph04<>gnphs 11andardiicd oofarasl..,ightand angJcofcameraarco:>nccrM<l.Thescc.anbc oomt.i11ed ina .......,,ic1ogi•--eanacc1>raternapolcl"'ttgion. The O\"erlappingport;.m,l'Cpf'Cfempho1ographsoftl1eoam.ea""'i.akcn fromdiffcrent-':""nts,_andvicwedin.•$1cr'""'"100pC11>c1errain •PP""-" 10 be m relief. Since the dostancc bct"·ccn 1uc.ccsaivc photogr.tphsol1he1amcan:a isfargru•erthan thcdil1anccbct-

~:.:.~·:~l~E"£~~a~~~~iF:~.~~! rnapo. lndttd, instruments l>a•"C bttn dC"o"iscd wi1h whi<:h an <>]>(T"tor may ~u~tieally c.a,.,..., a rdid" map from a block of pla>li<:, by 1racmg he1gh11 ofa 11creoocopic pair.

On a smaller oeale, pholognm1nctty has been of gttat value in

~~e~~a:.:;:i~: ~.~~~~~~~~.o~::::,i~i~~~o ~[:!~;r: '"~;!.~~ti~n .::::~,!".'~~~n«,!a>f~~ !':,;~p'::=~~e:t::~: ;~rc~~~~~~ .. :~.m;~:~;;~ ~~n,.,:nac~i~!h~h~~~~~::~:~~:; by making we of 1hado .. 1. Moreover, dioturbcd ooil doa not return mill <>rigi 11al compacrncu, even alier cen tu ria, oo that

f ~~a~S~:~~:~~,~~~,~~:~~:~;~~b~=~:~~i~~;:~~:·~~~~ ~~:~:'.~~~;~:i~~~E:~~:~~~~:~T,~~-~·t~:~~ andadmi~iltrauon. l ! ioa lt\0$1uscful10ol in gwgraphyand gwlogy

~hc~n~.:;,,~~~~fi~:=~i~!,.:"'i.:n 1':°:t!~!d;';.in~ra~:fat?~~ di•iributi<>n, and ofooil erOOon.

Jnfra-redattialphotographyh-.btt11uscdtophotographobjec11 o•·cr phenomenal diltan-=-up 10 hundredoofmilcs-thanb '" 1he abiliiyofthc longerwa,~lengths 1openc1ra1eccn ain l)'pcsof atmospheric haze. Howe""'• in1pitcofoom.espccuocularoUC<:cs>es, 1"Ctul11ha'"CPffl'~disappoiming. l'cnctrationde1"'1><ison1he si%ll of the hue pan;.,Je, and the conditions under which infra­ttd is moot cffccti\'C are not f~ucntly met in pr.1c1icc. Hue P"llctra1ionbyinfr.1-ttd,in1pi1cofpopularopinion,ha$bccn

concioclydcscribcdasnotgootlcnoughtousc,notbadcnoughto di..,ard . lfowe-·er, infra-red aerial photogrnphy had a useful wartime applica tion in the detection ofcamouAagc by otrongly di ffe...,n1iatingbelwe(:11gn:en f<>liagcand amatchi nggn:enpa int. T he pai nt <>ri.ginally used reHec lcd li n lcor no ;,.fra · rcd, whereas thcchloroph)•l in foli'lgerellcc11 it1trongly.

LegaJandFore n1lcPhotoguphy

The val ue of the camera for providing rcoords and evidence in policcworkrcquiranocmpha1il;thc..:encof thccrimeoracciMnt, thevic1im,d"'weapon, 1hcconvic1cdpcroon_.,llarercc<>rdsof vi1alimportancc. Bullincc modern forcn1icocicnccdepend1on th_c cxtraclion of the maximum infonna1ion from microscopic spcc1-mcn1, U.V. and e1«1ron microscopy, radiography, X-n.y crystal .,..11-si<, spectrograph;., analyM.., and other modern 1cchniqua ha'·callbttnp<ascdint0scn·ice.

Pbotogra.phyhasbccnmooturd"ulinthecxami,..<ionofdocu­mcn111uspcc1cdolforgcry,oblitcra1cdby1hc~ordamagttl byfireorothermeant.lnfra-ttdphologn.phy,pho1ogn.phyby U.V.radia<ion,photographyofthc,-isuaJHuoraccnccproduttdby U.V.radiation,photogn.phythrough..,lccti•'Cfihenarcallvaluablc

tools. Uni.., i nk• i<lcniical wi1h the 01·iginal ha,·c lxe11 used for forgery, obliteration, etc., it is m0$\ unlikely 11>a1 a ,·isual ma tch to the eye wi ll shm,· nodiffo...,11ce mhler 01 hor conditions of eMmina­tion (•'igs. 14 and U).

Ph1t1gr111hri11llucltarR11Hrch

Thesil,-cr halide of a pho~ographic emulsion is "'.rl<lcr<:d dt">..:lq>­able i>y~ngthroughncharg«luomicpart1clcs.theimag<: taking the fonn ofa rowofsikcrgraim marking the P"th of the P"n;clc. Special emulsion1ofhigh1e1uiti\'ityand low gel <:ontcn< (togi\..:tightpackingofthegrains)a...,DOWca!>"bleofrccording ele<:tron tracU, and indeed the !rack olany charged P"nicle of anyen<:rgy(fig.16). l'loooographythusprovidcsavaluatile (and in many""!pectstuperior) ahemati\"tdcte<:tion tooltntheionioa­tionchamber, 1hc Gcig<:rcounttt, cw the \\'iloon ck>ud chamh<r.

F"v. I~. """"""••......._o.,.JhdiooooiV<<i«•~olor>dio1horiu0>01<>m lt10phoLOf'>phO:emu""°".The1<'00rline••'- the,.><k<ol ro..talpha p.utkl<••nd thedotL«lwavyli""''howdo:<"On l<Hb c.,,,,,, lk. fl. //, 11,,,, A"Md Htu."l IM«•..,;,1 , 11-.

and io pbying a rr>t>$t imponan1 part in the mOOlt fundamental of a ll studies--1he11n1ctu rcof the uuclcu1oftheatom.

Phota-Recardlng

Thi•i1almmchofappli"'1photogra1>hy"hichenableste<:hnicia11 s tomakevi1iblc"""'1ll1of im·i1iblcchangcsinclcctricalormcchani• cal quamitics. S1rai11 gauges, or1imilar<leviccs, arc fix"'1 to the obju1beinge><am<ne<l,andthC1Carcoo11nectcd"ia finewircs,tna ca1hode-rayO<Cil10$00pcotamirrorgalvnnmnclcr.IYhet1!heyare fcdin101hcru<Mdingin11n1ment,lhc'signals'madcbythcchanges in the ol>jcct arc grcatly magnifi.,.L and 1hcn 1,..,n.rormcd into ,·isiblc images or''"'""'"· Where ca1hode·ra)' oooillosonp<s arc uscd,thc 1,..,uappearson1he1erccn,usually asaoon1tanlly"""'ing line of light, and thi1 ii pho1ograph"'1 f<><cumination. Where mim>r gal•·anome1cn arc used, • lalenl image ii formed <>1> plM>IO­graphic P"l""r l>y mca1Hofhigh-in1cnsi1y btaonsoflight actuated by 1hc changes in ,i,. objut (•"ig. 17). In both ca.cs 1he image appeanon •he photognph;., matcrio.1 aoa w•vy line, frnm which the

' ' :I .'·

Hz . 18. Ot<illog<>1~• '""" <:<>rd;"ll·

requirulinformationc.on be'rcadofr', h i1po11ibletorecordand measurephenomenal.utingnon>ore1han a miUion1hofa<ttt>nd.

Phato11n1itive R11l1l

The Fr<:nchman, Niepoe, was probably doc first to apply the principle or a photoscnsiti•-.: ruin wt""' in 1818 he devised a technique in ""h;eh bitumen, coated on 1o a m(i.al plate,,..,.. upoocd t0 light. ' lne unexf'<*'d pot<io"' or•he hi1umcn were then d;... ooi,•M from the mcu.l in oil of la,..,ndtt oolution, leaving bitumen protcclingccrtainarnsofthcmcu.l.Theplatewasthenimrncno;I in acid which etched the unprotected m(tal. l'\iq>tt ......J this

Tiff: Al'Pl.IC.lr/O ,VS 01' /'1107-QC/l, I Plll

etched image in a printi"g pr<:OS. At the time d>e O)'Stcm had •·ery linlepl'<\Ctical ,.,.lueand was temporarily forgom:n in doe succeos ofthed•guC<r-ypeproc...._

'.\ lochlate<intheninctccn1hcenturyimen:s1 inphotoocnsitive ~"""'r<:awakenedinitsapplica1ionintheprintingworld m the l'"'P""..._'ion or h.alf·!Olle blocks. Bui it i1 compar.ui,..,ly

f-;,.19.So .. llp«<ioioo<0mpoooo1>m&d<l>)' 1>Mto-"'U>1p,-<><en. 1 '1"' x"I' i>in«n oion<1.-u.C..,,.1J; .ll~.t l>w.a.r-.t1.U.

f"",.-. 2(1. Silicon ~itt«o«oaio-..ihul'd"""-ofmic,....,;mia1u .. cl...,;,.ofohe ki<MI -.. .• ..,,i..nsh~"h"'h"" ....... o.°'••·""- c-.."1-...,..,,;1.M.

Tl/Ii SC/liNCB 01' f'llQTOGNA~llr

r"C<:cntly <ho< a new kind ofpho!OM:noiti,·c ...,.;.,, oonsisting ofa11 ultra-,ioJc1sens;ti\·crcsininanorga11icooh"cnt,has~napplicd

to ptteision engineering and the making of microminiaturc dtt­tronic circuiu {•'igs. l9 a11d ZO),

l npnctiee,targc-<calcdrawingsarel""'P"~ofthcparutol>c

made, and thae arc phot"!l'.raphicatly .muced on to a high-raoho-1ion mau:ri.al l"or priming on t0 llu: raist-eo<otcd mc,...I. After dc-...,lopmcnt of the image, the """""'cd mcoal;. etched a...,,)., ka•inga,..,ryaceunucpattcm.

PflotoeraphJl• lducat1111

The""' of 16mm. film and film 11 rip IOr oraining purpooes recei,..-.J a powerful imp<:tu> during ti"' last""'• and the value of ouch mc1hods in cduca1ion ""' well establUhcd. They arc 11ow ~. withorwi1houtJOO•nclrccord,inochoob,uni'"n.iti<:oandhospitals. and in induotry Rlld commerce. They are <»ed for training new employees in factory methods, for •raining 1upcrvioory•talf, for explainingoafctyand huhh prccaution.s, Hoalesaids for demon· .,ratingandadvcrti1ng,forinstruc1iononcrec1ionofplantand for propaganda generally.

The importa!!Ge of photography in !ICWSJ><lpcr illuotration a11d t.ool<.Sha.resulwliniloctt:chnologyofphotomccha11ies.Todaya graph ic record of an event taki11g place thouoandsofmiles """)' canhepro<luccd within an hournrtwoofiuoccurrcnce.

ln the world ofcntcr1ai11mcnt it has given uo the vast cinema industryandi111clcvUio11i1011coftheba•ic mcdiumsforrccordinK the image which i1 la1er1ra11smincd and l'CC(:ivcd dt:etronically.

Finally, l>ut by no mcansleaotimportant,phol"lf"'phyprovid .. onc ofthcmootpopularpraemdayhobhies,

DOCUMENT COPYING

I~;:~~ ::=::~~~~~:.;~p~~~; ·t~r:l~g~~~~~~ documcm O:Op)"illg. The •~tcm to "'hich plouwgraphy is ~ for thUpurpooc,ho-."C>·cr,iono1gcncrallyrcaliocd.,\boutafif1hofthc world"• productK>i> of phowgraphic paper i1 <osed fur documcm copyiug,andthcpotctotial1uc {a11d1hcrcfurcthcprobo.blcfu1ure uoc) ofpho1ograph)·for1hi1 purf'<>"'C i1fargrca1erthaniupr=m application.

huhu<ryhastlioe<»·crcdthat theplio1ographicoop)·ingofdocu­mcnt1 is a m0<11 cnicicnt li<crary lautivc. It is hy no mean• a new idea. Over a conwry ago, at doe dawn of the plmtographiccra, Fox T albot rccommcuded it 1\9 l>cing more uccur'~te, more rapid and cheaper <h:m <><her mcthods, and the"'"'" is \n;c toda)". Photographic copico of documen" may he of many difrcrcm typeo ; tlteymaydifrcrin1heprocasulCtl {1i h·cr.dyc-lincorbluc­l"i'u ). in 1hc •u pporl (ftlmorpaper), inoiZ<:, in 1>ennancnceof imagc,andineaJcandcheiopn.,..ofprodue1ion.Theuscotowhich photographic copi<:< nre put also fall into 1wo fairly wcll·dcfined c:ucgories,a11tlna1Urallyithul>ec11foundthat spcciflc1ypcsof oopiesarcmo...•ui1ablefor•1>ceificrequircmc11<s

Copies rnay be re<Juircd mercl)· for ttorage against p<mible refercnccalsomcfu<nrcda<c,forcnrnple,oopiesofchequcowhich ha•·c~throughabank,rcccipuandcor1Upondcn~in.buoi1~css )..,.,..,., ncw•paper r<:eords. bool<.S and 1namo1Cripll m !!bran .. , andooon. •·,,.. ... charchivalpurpooaropico1houldbc,..permanc11t a.s poosiWe, knd 1hctn1Cl>'<:1 tocasyJt<>ngc, a"d be rclati,..,ly in­cxp<:ru.i,..,; microropies on film balC foll 1hi1 sp<:cifi"":•KH> ":""' >a<i.tactorilr. Ahcrna1i,•d)", copies may be rc<Juircd fOr 1111mcd1atc smdy,a11dbeof 1ranoicntimpor,..."".".onlr.·1~tis,thc)"ma)'he <C<Juircdforfrc<Jumlrcfcn:neeforahmllcdp<:riod,as,l"orcnmplc. oopia or patents. ocicmifoc pap<:n, '°"'1uhr, lqial clocumcnu.

~

TllE SCIE,\'CE QF l'llQ 'rQGlUl'l/T

in in"'""'"" work for birth and death ccnificata, ou,.,,·cy rcporu, lea.a, propoll"I fornu, 11ati1tia,<:.llwcyanca, liuncaande,..,Ju.o.­tions. Copies on p;iper, ol the P.mc order of 1i•c as the original, would best ouit th it purP'l"'=, and"""''""' pcnnancru:., of image would not \Jc nec...,.ry. ·11...., two main categories .. ;n,..,,.. br: cono.idered.

Mlcr1phot1graphJ

Microcopj•ingitabranchofmicrophotography(thcproductionof microocopic phowgraphicimagei) whW:hhasbttna"=>urceoflludy andam,,..,mcn1 1ophotographc,.f0r 1hcbeotpar1olau:n1ury.,\J carl)·as1639,afcwmomhoafterthcimroductionolthc0aguer­n:otype pro<:e1$, a Manchel.tcr optician, J. 8. Dancer, made micro­photographt, but owinii: to the limitations of Daguerre"• process, thcycouldbeutefully•·iewedonlyatWdia.rrw:1c,.magnification. In 18S1 ScottAttherin1roduced1hcwc1 collodionproccss,andthe following )'car Oanccr applied the pro<:e1$ 1uccaofully to micro. photogra phy. He produced com1nercially groupo ofpon ..... i1t and documents in a cittle of ,'. in. diameter, and "=>Id them mountocl on microocopc t lidco. Similnr resu lts wcr<: ooon produced by a school of British miCl'(lphotographc,., who also made graticul"' andmicn;ooea lcs fortelCO<X1peandmicrooropcc)'cpieu:sanddiffrac­'io11 gratings.

~W. I. 14!. l'<lliclewn<byO..gronbypir«"' pootd.,ins<h< ...... J Puia,1870."""'n

ong;...i.; .. R(pot.l;n1a.,,._ntJpo.rtJ1<.,.

fif.2.\\"b""""~Spec•

~~:~:it~ l.ody1oroo1>fo1h<n.ooo - (in<iu<Un1 ...,._..;,..) ol tM• <OCT<lop<dia.

Aflor the pioneer .,.. . .,,.kin thio country, tnicrophotograph•wcr<: 100nprodu~ i111hcU.S.•ndfrancc,1herorcmostpra<ti1ioner

in Fr•nccbeingRcnCOagron,who•boul IOOOdidacon•idcrablc bmincss inrnicroponrait1 nnd ••icw•, WherUI prcviou• micro­photograpltl required a rnicn;ooeope ror view ing, l)agron mounted bi$atonccndofa1inygla,.rod,1heothcre11dofwhichwa•ground toalenstoformachupvicvi-,,r.Suchmicrophotographs""""'"'ld

=~~~: 1::i1~:r~i~~~;;~i;~~n ";;~c; ;~: ~::~·~:::,, ~~ §£~;,:~§~?.~~:~:t?:~~S~(~§ ~~~~~~~g~~¥i2 ··ig.2.

MitrOCOPJlll :\licrocopyingfor""'nmcrcialpurpooctre<JUir"lthecheapes<a"d simplcstmcthodofprodtlcingadc.:iua1cruulu. l l1e<x>o<ofma1erial1 a nd P""'essing chemicals decreases proirressively with image size, l>u11hcCQ<1tofcop1·ingand,·iewingapparntus,andal>o,·eall!ahour C0<1<o,i ncttasevc1")"rap id lyaswcapproi.chcxtttmdy>1nall micn>­oopio. There must l>e an image 1iu ttp...,..,ming a oo mpromisc wlsctt the pr<><:<:S<$ is cheapest a11d moot cH;.;ient. h is fOund that rcductron of normal size documenu on to l6mm. or 3Smm. film, aCC<>«lingtottquitt11>C1ltsandsiuoforiginal,isal>ou1du:opti­mum. TOO..""&$ 1hc additional adv:antage Ilia! oontin"°"' pro­~ing madtin .. w«<: alttady in existen"" for prof"""°'1al and amaieur cine pho10graphy.

l6mm. Do<umtnl C•f'.Ji"IJ.-The first large-scale '"" of micro· copying washy hanking ho'™'> in 1928, for ...,cording cheques which pass through clearing !IOtlSCt. A mtthanis.cd camera 11hich copied on 10 l6mm, film was used. In 1945 an apparatus w;u

marke1cd which ph04ogniphed both1idaofthechC<J"" 1imuhan­eouolyatthen11eof400pttminutc, 1hcfrontand l>ackoflO,OOO chc.:iues l>eing copied 01110 lOOft. of 16mm. film.

The outb...,ak of war iaw an enormoUI incrcasc in document ~.opy ing for security reaoon•, and although hank, arc otill majnr

IJ<ICt.M£XT COl'lllo"G

uscnof l6mm. microoopying, it hasbtt" adop1cd by mauymher commef'Cial hou""' imd l>y oome Go....,rnmcru dcpartmcnu. (•'ig 3).

C>mm<:Tcial app;mUWI fo< copying on 10 16mm. film and for .,ewjng1ticmicroc:opics1ho""Sahighdqn:cofspccialiseddcvel· opmem, culmin.:uing in dcllr. modcb which can be lllCd for both ODpyingandrcadillf 16mm.photogniphiccopi .. ohowadvanu.gcs <WC1"0thcr methodo ofdocunu:nl copying in rapidity, accuracy, occ.urityagain11 fraud, and induraltility;mor<:O'•cr, lessfirchazard anachcstosafctyfilm1hanto paper copies. The economic adva n­tages of microcopying arc even mote pro!Ound. The amounl or sctui1isedmaicrial l11Cdpcrdocument i1trivial;andthesaving in stonogcopaceis98pc<Ct"l'lt.

Thcsmallvolumeofmicroc:opicsas•gaimtfulloizcpapc"COprCS abo introdUCCI economi .. in fr<:ight,cspecia lly in airtra,..,I wlw:n ~hose tO l>e di1pa1ched o•=c:onsidcraWc dUtances. To utilise 1hi1od~amage,theAirgraphprooc.swas .. tal>li•hcdduringthcwar, "'hereby lenen on a 1rnndard form were rnicrocopicd on 16mm. film, Aown to the dcstinatinn, and an enla rged print delivered 1hrough 1hclocalpottal .. rvi"".This1ystcmwascx1c11si\'Cly uscd

T/11' ~· r.;IHJ(C£ OF l'llOTOGRA.1'/lr

by our tmopo; the Americans used a sim.ilar1ystem "'hieh 1hey called V-ma.il. Well (IVCr 300,000,000 leucn were thut transmitted by thcllritishArmyaLonc,andabout700,000,000bytheAmcrican Army.

""'"'· V.0.-0l c.,,...,.-Whilc l6mm. copying is moot 1Uitablc forbulkmicro-filmingof=ute1ofn--=orle#1tandardrecords, 35mm. i1 n--= suita ble for copying a wide range of1ubjocu of different oii<CI, fOr cxamplc,engillccringdrawing>, invoi.,.,.,decdl, lcdgc11, book1 and nc .. ~papen. Again spo:cialiied, photngraphing and viewing apparatut h"' been cvol,·cd. The camera• arc 1nore fluible though Im effi<:icnt in output than 16mm. micl'O<Xlpying • pparatiu {Fig. 5) . A speedy and complete !6mm. or 35mm. microfilm ocrvice is nowadays available to thoK who do not wish iopurchateapparatiuor1odo thci r own proc'*ing.

Anahcrnati,•e1om.icro-copying<>n10filmistha1ofmakingamas1er nepti\-.:ontoahighraolutionfilmfromwhichptinulOrrefloction viewing can be made. TIUs •)~cm i- been widely uploilM in the United State. in l<:\-.:ral forms. In one IJ"l<Cm, publications are reproduced on 5 x 3in. filing cards, achofwhich mllyoontain as many .. sixty paget. Tu., negati,..,. arc utu.ally made on l6mm. film and after being cut a nd aw:mblcd in gmupo printed by con· tact on 1oa 1po:cial printing paper. AJwell asbeingoflinc-gmin, highrcsolulio!landhighcontra.ot,\hcpaperincorporatcsanan.'i• ha!ationbarytalaycr. Largcrsizcshavcbccnuscdtoprovidccop1cs of books having up IO two hundred pages. Thcprintl arc viewed bymagnificationorwithareflcction-projcctor.

An ingcnioiu 1ys1em dcvisal by the Kodak Company i1 that of doc Mi11icard 1ystcm. In this, the cards, 32 X l6mm, ill 1izc, arc on filmuthey alooincorporate tllecqui""len< ofa punch card 1ystcm. Documenu arc reduced sixty diamctcri on <>ne ltalf of the card while tllen1hcr contains clear and opaquedou.,..hich can be ieannM 1>1>o«M:lcctrically for rapid filing, toning or retrieval.

llor111•l-SlaC1111IH Therc arcl<:\-.:rall)'ltenuforpreparingnormal-lizecopia,tllemoot generally used being camera-copying on to norm.al photngraphic papet'. Thi1 was the fi11l mcclianisal method of copying documenlS, and after 25 yca11' ''"·~ice it is still predominant. It is used almoot CJCdwiv<=ly by Government departmcnu, a nd requcsu for docu­mcnu from Probate Registry , Science Library, British M<Ueum,

r.,. ), Mkro-~lt .....,hU., mini 3~ "'"" ~1 .... °""""' l:MM LU

Ordnance Su.,,·cy, T°"·n Planning, Air ~ l inistl)', \\'ar Off.cc and ooon, ar<:invariab!y met by•uppl)'ing 1bi111·peofoop)I.

Thedocumcnt iscopicdon1oarollofbromi<lcpaper1hrougha ku• and prism, the latter correcting th.c !•11-m· riglu im•e11ion (Fig. 6), The copy i• ucgati\'e-whue pron1n41 0~1 a black back­ground- "'hich ;, quite legible, but when a pooni\•eoopy >! re•

Tlll·:sc11;,,cf: Qf'l'llQTOGHAl'/11

quil'Cd (as when h.1.lf-1one illw1n11ion1 are induded or "'he11 muhiplc<>:'J'i .. are needed), lh~ negati,·ecan again be<Xlpied to gl\·c a pos1tivereplicaof1hcongiMI. "ll1e 1ystcm iod .. iglOCd for ~ and npidity of working, and opcnta untlcr normal office ligh!ing;liulephotogn.phieskilliorequil'Cdandl'Cduccdor cnlaTOd copia can be produoed if dcsil'Cd. The system may be <X1uplcdwithotherCOp)-ingprocasabyproducingaprin1on translucempapcr,tobeu>ed asaninlcnnediatefo<obtainingdye· !in<: or Wuc-prin1 <Xlpia.

°'ffav, I.JM Dr•u.U11.- Di.uo or fcrmpnmiatc papen are nonn.allyu>edlOrbulkcopyingofcnginceringdra .. ·ings,an;hi­'""tural plans and similar documents, btu it io common pracliu 1oproducesuchcopiafromama11ercopyproduccdbythc sil'·ct procelll.Thc~tercopycouldbeproduecdbyacamera<Xlpyon tnnsl~centp;ipcror~1io«l1racinglincn,lrutopecialio«lappa­ra<us11 marketed for copying on tO t...lf-plate film, whi.ch can be readily filcd, .arul the oa.mc appanmo may be u>Cd for producing enfargcd <Xll"CI thc...,fT(lm up 10 60 x <!Oin. (Fig . 7). T his >)">tern

""' cvol,·ed during lhc "-ar, when man)' ind we rial concern• rcahocd 1ha.11hcirmojoratsctlayindn>«ingoff1<ertt0nh,arulthatcopia were csxn1ial for securi1y. Oiher applica1;oru Wt:rc at onoc d;... "°'"'<ed-hwuoftcnnecttl.ll.rytotnn,.t:rrccordsO\"'ncas,or10 scndascriaofdn"ings101hadowfac1ori .. inthis<X1untry.:. lajor 1his-carried t...lf-platt:lilm<X1pi .. ofdrawingsof1hcirequipmcn1,

l>OCU.t/£,\1 co1•n.\C

'° tha1 af1erac1io11, "'pairseould be carricd ou1 at aa.. Such copi .. art: more aecunle, and rasicr and cheaper 10 produce and file 1han1racings.

Pert...i:-1hcgrea1at llOOJ>Cfor phowgnphi.cdocumcnt copying in !licfu1ureliesinthc<>«liMryoflioc,whcrcdicamountofworlmay 1>0t justify the upcnx ol a 1pccial;,.,t copying camer.o. S..me­si ze ropi .. can be obtained by Uling die aclwil documents a.

f~.t.ll<lk•"'l'!'i"f.

ncga1i,· .. if1heyarefairly1ran1lucen1andprintedonone>idconl1·, d•crauhingcopiabcinginrcvcne(whitconablackb;ickgT<1und). \\'hen Ilic document to he copied is pri111cdon hoth1ida,oris fairlyopaquc,itcar11101convenit:11tl)'bcu!t:<lasancga1ivc,and an ingenious method of copying known as llt}tt~ Copyi"ll i> used

lldU~ Copyin1.- Thc d<)(.umcnt \(>be <Xlpied is laid face upwards andscnsi1i,•cphm"llraphic1Mperl~idovcri1.facedo"'nwards. The paperioexroscdfrornabo,·c,\!1T<1ughthcb:ockof\!1crellcxcopying pa1>er,using(by1hcimcrrooi1io11ofa)'cllowf1her)or1lythe!ong .. 1 "'"''e-lcng1h1o"'hichthel"'peri1ocnsitive.asthcscraysa...,Je11 scattcrcdandabto<l>e<lbythccmulsion.Thcpor<ion1ofthescnsi-1iWpapcroppoai1e1heblackprimarcu]l<ISCdonly101ht:incidcnt Jight{afterahlorptionand...,tlectionbythepapcroupp<>r<of1hc scnsiti\'cm.aterial ),Linet:1hcblacki11kpre•:cn11backrc!\cction. The p<>rlioru opposite the \\'hitc parts oftlic origina.l d<)(.umenl ...,...,i,..,thcpmeCJl~U"' 1>lu1anuca.due1omultipleinter­rdlec1ion1betwttnthcwhi1cdocumcntand 1hcwhi1e papcrhasc of1he scruiti'"' material (f ig. 8). The uposure is rather critical, andiladjusted1<>that1hccxposurecorresponding•olhcblad:. portions produces a low dcnti1y on the P"J>CT• whcn thc added cxposurcgi\«,.n by 1hc "'hilt: port;orugiw:o,withtlichigh<XllltraS\ of1hcrcflcxpapcrcmultion,asa1isCaaotyhighdt:nsi<y.Themcthod is r.opid,Limpleand"""""""c butgi,'Cflatcrally,..,.-cned, white­on-black copicl. Thio i1 co~ted byusingrhcrcfl<::<copyasa

THI! SC/ll/'fCE QF PHUTQC#Af'/11'

z.eg:ui,·c, and priming on to high contrast papcr "' gi•'(; ,.. many oopiaasan:rtquin:d.

A/,.,_,;,,. MttM.b.-Of n:o:nt years much cf!Ort t... b«n cx­pcndcd in ~ising ahcma•i•'(; method..., the nonnal sih"tt prouu. ln g<:1>en.lthaemc1ho:><h avoidthelcngihy"'(;tp~ngofthe "ormal •ikcr J>mCCS:' (which uoually entail•"°'"" pho1ographic •kill),andoomen:qum:onlythcsimplestof«tuipmcntfOrcxpoourc and processing.

D;.., Rr}k.:.-TI.e rorurao1 of diazo paper is normally too low'° allowiu "'cceafol use for rcftcx copying. An ing<:nioUJ modifica­tionwhichO\-crwmcs th is dcfcct isthcu..,ofadiazolaycronlilm base ft!tachcd loa rcmo\"ablegrid consisting of a cl05e mcsl1 of narrow clear holes on a ycllo'v background, When expooed for rcAcxcopyingthroughthcgrid,the)atter ~tta1harptl>adow

th~ugh thenon-scatceringdiazom.atcrial oo thatonly thcportiom

hehmdthcclearholesarcbleachcd. LightrcAcctedfromthcwhitc portionsofthcoriginalishighlyscattcn:d,andblcachesthe1<Cnsitive material prottttcd by the grid. The differentiation ihuo becomes from clear white to near m.axii:num density (fig. 9). After cxpoturc, the grid isnrippcdandthediazofilm p!'OCQledandusedlOrsub­""IUC:nl copies. Although till$ material is rclati.-ely upcnsive, ~~""nt.copiaondiazopapera«:ehcapanddtyprocasing m bright light fa•'Oun the met~ for document copying where imag-e pcrmanenceisnotcritieallyimponant.

Sol""'/ TrMlftr MttMd's.-ll>e principle of doe oolv.::nt tr:a.tufer med.00 has been described in Chapter 19 (p. 138). In iii appliea­tion todoc:umcntcopying, the1ensitiv.::papercontainingdevelop­ingag<:nis 11 cxposcdtogivea latera!lyrcvcrsedimag.:: (forexample, \Jy reAex copying), and after "-cuing with an alkaline sulphite

DOCUMHXT CQl'TIXG

oolution,isoqueeg<:cdint0contaetwi1han:o:ivi"ll'paper"·hicl'I containt hypo a.nd a unall cot>0cntra1ion of finely divided ~l~-cr When tl.e latter 11 ocpa.,.tcd, it bean a positi•'(; image of the ongmal docurncn1,Fig.lOU.O..-.adiagrammaticrepretcn1ationofatypieal

developingmadllru:.lmagesarcheingtr:a.nsf~tobothsidesof theoentr:a.lrcccivi"ll'paper.Anadvantage of d>emcthodisrheease of pmccaing, as tlu: copy is produced ocmi-dry, and requires no further 1rca1mcn1.

A modifieation of the method combines both ncgativ.:: and rccq>­tion layers on the same paper suppon, the negati\'(; layer being •uboequen1ly removed bywannwaterwashing.

~ft Gt/atilt T•~oufa MdAod.-AiCCODd modification of the normal

~;v~~f~~~'~f~a!::S'~Yi:r~~~:;::~~Ec;;~~ whichaboreaetswithconstttuentsin1hcemulsion layertogwca pigment. On oquecgtting tlu: WCI developed ncptiv.:: inlO contact with ordinary paper the unhardened (uncxpoocd) pigmented ponionl of the g<:latin layer pani.al!y adhere to thcordin.o.ry non­ocnsitioed paper•ogi~alatcrallyco1TCCtpositiv.::copy. U~toStx

~~1l""E:.'.dKE~Ew~~E;: r:;:~:~~~i::~t~~~~=~~:;;;~~;~~E used i1completelyfogged,and1howathcllerschclerTttt (p:206l

:~n~,;:·~~x~~~·d~s~~~·~:i;::;;:,.:~l~:\~~~~i,;~ :;v;.:

;:;.::::;i'1:.~':'.::-.'.:.i~~==:n~~-=~~~ d<•linf..i1b(l'r)prac<ic<ot{Th)1""°'J.

(;EXERAL

n.FouJji,q<i.-..f/,._,.41'v,2v<>b.(t'o<al l'r<tt, 19&5). l; Th,l't. f....._..,f/,,..,.,."""'7Mory,2nded.byT.lt.Jo~•ndC.C.IHJlin~

(MO<llon•ndM<><pn 1960). S; Th c,..,.,.110.'"1</,,,....,..,i.;r:•P.otliuiG"*f"'l°'ef•"**'""'A.4wMM

A._,_,, by,.,. JL S. C..oeybo<kx ( fo~'""'" J'r< ... ~<bed. 1962). T; l't. ll~l#J VI'-.•.µ..,. by J. M. Ad<<, 11•n>bl<d by Y.potcon (Col~ml>i• Un~

vcni1y1'..., • .,0.<forJ Unkc,.i1yf'r<>o, l ~~). S. n. ll~l«J ,/ l'l•••t'.µ..,• by I!. el A. Cctruhdm (0>.ford Univm.i<f rr...,

1mi.1.. "f"' If!•' M-' !/ f'IM't'.µ..,• (llfo<d, ~th ed. 1%8). T; J'r. f' ..... ¥iv:itt .11....uls ul,,_.,,.,,i.yc.B../'>cbl.110 (V•nNooor>nd,6ob

ed.1962).S;Th,P.. ,,....<#Iv. T,,_, .- f'rothu, b<ing on f:osl;.!, ed.iOon ol i.. T .. .-,.. ,,_ ,..,..,.,byl-1'.Cltt<(/'lun.an,1%4).T.; Pr,"Ih.

,._,...,,"'-".._,,vo1o.1er2,byP.~t..inf•nl:t\lli>h«li..,., ol i,,. cw.,;.,._,..,_... (1nd ed..) "'""U«<I by IC M. 11-"r (l"ou .. 1.ain""'-.l9J.8andl'JIOOJ,S;Th, l'r.

n.~<{..,,...,..,.,,.,_,,.,rded. byC.E..K. )tttt(Ma.cmWan. l!IMJ. S; Th. ~;.. -,.: LMt<S-S<l-l, 1967: (FouJ J'r<to. 196'). $;Th.

C.-• 1-, by A, Lockett (Piunan & CrttnwoO<I, ~lb ed. 1962). L; l'r Ln;nlol'ito,.t'<l't!,byR.Kinplake(Bornco,l!l63).T;Th,l'r. f'~Of"~t,byA.Cox(foe>ll'r< ... llthed.1966).1.; T h, J'r, r"'..,..,.i.;.o,..,,,by,.,,R.Grecnl<:•f( M•<mill•n, 19~). S; Th. n,. .. ,....,i...,,,,bfC.ll.Ncbl<ue(Foun<oin l._ 191,;,i. l.; T h. l'r.

C-•G"""'(fooolf'Hoo).L;l'r C--: 1"' 1•4: _ IMJ_,_,llwJ.nll'o,-IMJ-ru. by W. I t

1'. ... nualond A. M"'hcton (Fo<ol I'.- 19~). L; l'r. ,,._ ~ C.... 1o ... 111-. C--. by T. L J. ll<~ol<y (S<"""'"

196,).L;l'r.

,..,..,,..,.. ,..,...._ T"""'""" by T. T . Boice< (Ame<. Pho<. P,,b. Co, 2..t «l.l!MS). T;l'<;Th.

~~~!J·~~.~~ ~~11!~~& l'~~·~; (f o<ol..,,_19&4). T;Pr.Th.

1.A1't:NT IM ACF, AND PHOTOGR AP HIC l'.FFE CTS

li>Jo,,.., ""f~• efC.-. T~, loy W. ~. ll<'1 (f <><al p,,.., )<d cd.1961), T:Th.Pr.

li>/<J-M.,...J,byJ .F.Ounn( f o<mb;n J'r<-.19:18). T ;l'r,

flX ,.T IOX "XD "FT ER-TRE,.T MENT

n.....,.,.,: •1>-,,..;-T""'"'rw,byC. l.J-- (Fo<a.I Pr-, l61hcd. 196&). T; "l"lo,l'r.

~a...;,..r,-~,by J. Sou1hwortho...iT. LJ. llo:Dtky ~ p;,.,...,, "11 ed. 1'»1). L; Th. l'r.

d T•_<tf,...,.,.~CM.oiw.>. by D. H. O.Johno...iG. T.J. Hckl (O..p­manf/l lloll. 196,). T;Tb,l'r

S-i.......,.: O\< T~i.fwef.11,.... ...... P~ .<f""'iolt, by 1. Loki •nd .\!, Uu bola, """'lated •nd •d•pl<d by£. Y. Teol f1om 1hc t·,..nch cd Mon""l doS.n.;<om<1n. (focal 1'1<><. 19))). T; Th, Pt.

s..,;_,M,,_ri<,,by l\:.M.ilot1»by (G...:o-,l9J7) . L;Tli.1'1 r;-.IS....,_.._,,byYu.N.Govo.<hovili;oDdT.M.l.ev<obe'l(f"<><>IP"'"·

196lJ.S;Th.l'r.

COLOU R SENS ITIV I T Y "ND f lLTEllS

S,..,.M.-,!/"",.._,,...,l'>ouu,by Y.._N.Go.olci-.k;;(t"<><>IV.-) l!IM).S;Tlo.

K°""'W.-F'ohm,1f>d<d.(Kodll,196LJ.T;Tb,P,.. F"'-",._.;u,by ll. 0.-f/I H.~kuo<i (FoalPr-.ltGi). L; P,.,

l'OS ITI\'£ l'ROCEU

C-,,U.. d ,.!/Pr;w.,fllE.o/_,;.,;,byO.R.Ctoy(Poall'ttll,1962). L; 1'1.

C OLOUR PllOTO GR,.Pl\Y

/l~"?efCM-/'Mlor<¢Jv,by J.S.F<ied,,..n(A"'<•·l'hot.l'ub. Go.,l9+1) . T . .11.-1 tfc.low PAo'°f'o/'h.>, by E. S. l!oml>a<k (fou 11 10in l'«oa, 19&1). L

l'r,Tb . ~ f{c.I.. P/oM>trOll!J. by R. ~I. E••M W, T. ll•n°"" ac.d W. L

6t<W<t(O.•l""'"'fllllall,19J3).S;Th. n.:;.-.... f{~. by R. W. G. llunc (f <W.1n1ain l'ttll, 19)1), T; Th,

~- u c-r.. fll r-_., by"· F. Torloo- (Foun••in p,..., 1962}.

1-o'• .11"'"1~, by !'. Ho1>11<Uf/I R. OU<tt ..... w(Focal f'r-. 1961). T; P,.,Th..

1'-I.MM-tfdHIW!'lwMrW.)..,i..(Kodd).T;l'r.Th..

DOCU~IENT COPYING

~"'' ~...JR.,,._~"'-'• by II. R. Vo,,.,. (FO\mto;n l't< ... 19'1). T ; F•.

C.-• ~ .,./ R.,_iwN-, by 0. R. Croy (F<><ol ""- 1964). !.; !'t, Th.

A.5.,..optt<!,119 '"'"''"'""""'l<-"9 Ahooop<io<io(~h~ 78 ~ ......... -iop..,121 MMl&.l ... bo1h.ISI

~~::;.";;.in "-<ld;,;,~-.,.... .... 2» "-<l<li1i>"<pn>« ... 200 ""-pii<><o.10 "':dol.,.....,y,:196 Af1<..,lf .. n;.,..oo "'-fa N<u< 1"""""- 268 llgl.cok>tp!>t<,2S6 Agi<ot;,,..ofdeV<lop«.l:li A;.fnophm•ll.:JO!I ..um ... .11.,-,1• "'"'"""' ;""' thiooulpho1<, 1.\0 ..,,.....,.pi.;ci. .... 2m ""'a.c.••1t• l28 ""'ih>lobodU,..106 ,.,. •• ,_.tpi..tco,141 ......... , .. _.,.,,211 ......,._.ic.1plloooc{nploy.286

~'.'::...'!. .... 11,

"'""'"' -·h~e& "'"""h_pn>« ... 2» 1!.S.•ptttl.179 11,,;.,m..,lplo•"·H, ll» il><ylO<Oatin •• 24,l0),20 Rin<><ul•, ,;Mon.182 Bla<·k!i5hrni11g ,WJ Bl••}.,,IOS

::!,':~'.:',.";.~~ s. ........ v..N,282 ~h • .- ......... ...,..210 -s~r.u;-. •3.161 _...,.,..._.9:2 - ,<lf«u,1'3 -.poP<T.168 _1_..,u --r-.1'9 Cmdmn.m..,lploO;lc<cU.213 Colliet00<ffic""'1,l66

g:::;:::,:-n ::::==.~:..~"O'·Z6S

-.ITll<•.16

~=~=rk,W ~:.;;.~.)] C.!bonpt~14 C.•bro"'°""'.H.2&1.n11 C.<oly>i~ 12'.I, 14(1

~;!;2~:::: ~I -, ;D!<tpt<l&ll.,n,110, 113 -,..,...,,...1"'°"""'"• •IJI Chcmi<al,eh>nF,8) -.wml""'nd~8) -.<l<•"<lop.n<o1,I~

=;:;i,:-~-.:;,!!.,m Cblori<l<.popen.H

=tl~,.._24

c;,,,,...._,2.,,

g::-... ~J;..~1:: 0.ydcncff<e<,70) C.,.1U.c,cmu1Don.IOI - ,l<n..,_%_)S

W1':i~~·1 ioJ Collod;....p.-o« ... 18 c.,ioo,.<1<v<1op"'<••,:!Gt - ,dn<m"'°''"phy,280 -,mooldnc.212 -,pct<cnl•g<t<l\eclk>M,231 -,ph<>lofnophy.odd;<i.-.p<oa-.2)1 -,-. ... .,, ... ,;...,,._:N;? -.prino;.,.,,,_.,.,214 -.-~,_;m.,2n -,pru...,2ro -.~p.; • ..,211 -.f'<'......al.,...,......,.,,$,212 -.....,;oM<y.lSI -.'<"'P<"'''""·.4(1,160 Coloutm ..... pl<n.,21, Gompo•u>d~ el><0>0;ol, 8) Coo«n1n1i<>ftoffu0<,l"9 Gonw:tpt;ni;ng.241 c;;..,. ... filt<'l,193 -,ind<>.pt;n1iogpopct .. 2ff

- ,n<goti>'<on>t<•i•l>. IH ConV<r<ihle len>. 53, » Copy;"i:lintdr•wing .. 312 Com<«•J>OOU«,ln Corte<tioofilcort,192 Conplers,129.265, 272 Couple•-dcvelop:rs,267 C.y>1• l l• 11;.,e, 115 Cy<lk « nnponnd>.123

,,.,...,11,.<,307 o.,..,,,L. J .. 11.,li D•gun-typ:,14

g:"';..G{b!:~ •. l&J D.,,,,s;,11""'1h7.14,17 D•yti<lht,42 !J<ruito.,.tn,163 Ll<n>ioy, diffo1<, 166 -·""'"'""""'"'·162 - ,••n;geofpopu.IGS,161 - ,n:lle<tion,16'> - ,>pe<nlar, 166 Ll<pth-0ff><kl,64,11,7S Ll<pthoffocu>.63

~.':=i:::;~;,'iN3, 126 - ,•giL>tion,134 -,foot< g.>in,23~ - .1ow •• ,;,.;,y,234 -.~l.Q'>129 -,p..,, ,....,;,., ,128

~ ~E7~2:7~'· 135

-,tannin;g,2~ Dev<lopins•S"'"•l l 6

§'g~~~r~ =: ~:~~~\~;,,:~:w - ,phyoiul,130,H5

=::f;:'~~nr<, 133 rn • .,,,P";""·312 - , - ,..,n,., 314 Diehn>i<fog,Ul,2Zi Dilfra<tionofligho,35 Di!fn1<,Jtn>ioy,l66

i);ir'.:!:~~liRh< 35

Oige>oion,<m..W00,\19 DINope«l,179 o;~ip-OOiti>'<P'intiRf,315

l>~p:.-.iooofliKht,31 O.,.,t.,ing, emn4i-0m, 100 Do<um<:tlt <cpyinr. 305 Donblede<:oonpooition,89,JJI

~l=...~~D-,2~.2'» Duf.."""""-2~ Duplkatin1 >b1>,eopyinJ ,316 Dux-0ehromep<in1>.270 l)y<<OUplcrs,265 l)y<Tr•ruferP'<><«•,2&1,2 71

~·;;;,::..i~-:-r:;;,2/:Hf;ll l\dg<t!f<eu,13t FAWn,T,,._,,277 Effi<;ie,.,.y,ohnttcrs,82 Ektocolorfolm,268,272,21' Ek<tn>nnti<n>O<Op:,2'1'1 Ele<tro"k,«n'l<lu<1ivi1y,l lS - ,fl>oh,4(;

2?l~J:.~ss -. artt .... ip:ning,\19 - ,oo>ting,101

=: ::.::~~~~~95 =: ~~%~ati<,188 -, panchn>n>>ti<,139 -, t;p:ni,,g,91 - ·'""'l"i"81Xl""'"·2"° -, oup:,.o-0>1ing,1"4 F.nl"'g". 248

~~;.:~::,!~,~~ 81

f:xh1u,.ion, fi,ing b>tl>, 151 E•?-OfU..,,209

=: :.::t;;;;o.27•

-, fla>h,+1,46 -. l•tioude,172 -. -t«,212 - ,n><>dn!.oion,100

E;·,:~;::1;::::;;,;11;

l'«ri<chloride,219 f'«ro .. •u!phate, 18 fil• ..,.,ntory gr•in.s, 131, li5

~ .. ~~~.;~~~ JO}, 2+1

-,dayl~lk>adl"i:,72 -,roll,21,73 fil1<r(light),..,,.1 .. ,.,1~ - ,--ti-On,!92

~~:~~~.160 - ,no:utno.lde,,,;ty,191 -.p<>l><i>ing, 36,198

~::;:,~'::~"l:,~~'f'""' m. m - ,m«hani>m,lt3 -,rapid,!~ -, time,li8 -,t<n>p<•>lnre,1;1 Fi>e<, <on«n"ation,H9 t'ixing bath, ~id. 1;1

f:~~l~1~: :~J f'la•h,elc<<n>nie,4(; - , 11·n<hron..,,<ioo,•;,47 f'luo.e>e<ntl•mp<,49 f/nun,b<r>.~ foullenf'h,32,00 f'-1-plan<>hull<•,81 . ....... i.,,..,2,76 t\>11,aeri11,225

=: ~;,::~-;;!· 158, l73 F0<<noicph01ogr1phy,299 f'"'T..i...,1"1,16, 193 frUu·Gr ..... flW-d,217

Gam.,.,l7s C.mn>•·•im<CU<;'e,177 G0>iiKhtpap:r,2'1,198

~':-1,;?2~2,105,2()1! Gnoino.fi!.n><ntary,131,l i S -,•il>'<t,h•lid,,9; - .,J«di>tribution,96 c,.;.1 • ...,,226 -,denrity,n:l•oion,228 -,.,.•>un:m<nt,228 (danul•rity,229 Guidennmh<roy>1em,39,215 Gun><r·~l-01' tbemy, 117

ll•ndll<un·e.,15'6

~::~~H!..88 ::;:!:ri"~~::.:15 11 .. ui..1,s;,;o1u,26,206 iligblighoo.p-00u«,2lt lligh •i-d ph-0<-0grophy, 29~ llwd,. ..J Drij/idJ.211 Huygeo'>v.'1''<ttttory.2; ll)'<lroquioono,126,129 llyp:rf0<aldi1to-,66 ll)·po-... k>diumthiooul?h•t< H1·podimin>ti-0n, l.\.t

:="~'"''·'l ln<i.dentlight""""'"'""'"'·213

t~~;;~~~ ~t~· 130

=:~:::,l:~"''Y•298 -, pb-O<-Ogr>phy,299 - ,..,Nitivity,18!1 lnorgoni<chemi>try, 123 lnt<f"'ltripack,266 lnt<ruifi<ation,222 lnttnri<y>eale>en,Jtom<l«,l&I lnt<rfe..,ru;c,light,3; ln«'m;ot<n<y<ff«t,21)\

::1:di~.~!?. 106, 235 Johnoon •<:o-.,tnproc..,,?5+ Jolyocr.,<n,2~

Keller·llori>npro«,.,2~ Kd,·intt"'P<'"'""'"'' le,tl Kin<t-Oo<op<, ~77 K-Odocolo<, •dd;,;,., pro«,., 2~9 -,.b1~1"t'•P'<'<'*'· 266,2f;il,272,

K-Od>ohro....,film,i"O<dng,2f.6

K~~·;;,!1.~-fi"' · n Kotav•<hn>m<prin1>.27l

[.ompo,fluo<d«n0,49 - ,highefli< .. n<y,47

~·.:::~:;;i-0·:.izo;·i i, 4? 1 .. 1<ntimag;,llO,'lOl

-,dH,,ibution,1!9 -,...,b-;mag<, 121 l..a•i"•d<,cxpOOu«,112 Lawof.\la"A"ion,128

~~: ~~rne•a-r.,.,, 68

-··"""""'"'·,',() -,anon>0•p/lk,279

::: :;:!~~'~ 5+

=: ~~:~.~1.~: ~. ~ ::: ~:~~~'~· 5! - ,fla .. faww.~

=:~~.~:::·;; -, perimman«,66 -,..,,.,i,.;,\$'.po~ .... 67 -, •huu<r,81

:::::~~~~:~ -,•upplem<nto•y,60 -, wKleo"llk,~ kn•kul>t><to<n,259 Ught,ob>orption,28 -, diff .. <iion,35 -, diffuoe,28 -, di•pe<>ion,3) -,in«ri<ttn«.~ -,in'"'"""l"'"'law,39 -,..,i.,; .. d,26 - ,H:ctilin.e>•P..,P"g><Kon,26 - ,..,fkction,28

=: :~~~:t:· 29 I;ppn>anpro<.,..,36,251 !.ocolr«lu<<r,220 l.owacti,·i<yde-tioper,23+ l.um>!..:b.o•l:><.,,277

~~:;:;'.'u'.::'h: eube, 163

,\!,Q.<l<>"<loper,129 ,\!ae~;. Jines. l3i M<#o•,Dr.P.,d,2!,91,11 3 Magnc,;umfl.,h,+3 ~l..i.ing,<Olou .,212

- ,«>lou..,d<ouplen.,273 ~lau.ad;ography, 291 M~l,Clnk,253 M0>>U«m<no,<kn•;1y,162

-,gninio..,..,228 M..,,D<.C.K.,99

~~;~~'T~~~' lamp, 43, +9

Me<O(juinon<,l:W ~He...,.,,l')'ing, 308

~:~::t:'.;';,";"'"'·229 Mi<ropho"'*"'pliy,306 Mkro-prin,..310 Mini<••d>r>«m.310 Mini<olorl"i""'·271

~:~~f:~'~nomc<e•, ~I Motionpic•u .. folm,278 Multiple<oaoing, !O~ M•)'ffiJf<,f.'.,277

:.'.g~~~;:~z~ •«• •men•, 211

- ,pe.fcct,113 -,m><.,;,,l,conua ... 11+ -,-,rnJ;nary,187 -,-,ot!hoehromaoi<,188 -,-, poochromai;e,!09 -.-.•pe«l,2!0 Neso1;,.,,.~,;,..,P"""'"•1s1

l'«uot•ldem;tyfilt<r,197 ,l',,. .... ,1 ...... 25 ,l'i<pc<,;\i«plww<,14,:)()2 -, St.Vi<.,.,16 Non-<udingfilm,22, 106 Nud<><n:o<>r<h,300

~~~~~~:~~"ti~~~rrs;i· 23. 212 Pa..,r,bale,243 -,..,.1ing,ma<hin<,l01 - ,den<ity,<>nge,168

;.'.!1~:~~~~ 21, 196

P>r>phenyl<n«liamin<(p.p.d.), 2&1

P,d'«tnep1iv<,113

;:;;~~:.'!':t.i.f::.21a v. .. pe.;:tiV<,60

~~=.;,!.116 Pl>otogrammetry,296 l'l>otognph;e,omuloion,91 -,filt<t>,29 - .r<duc1ion,221 -, op«<l,177

~ .. ~~~~'.i:~Ut>'<f, 296

-,infr.,r<d,299 --,hip•p«<l.~5

- ,legal•ndfot<O!ie,299 - ,nucl<ar,300 -,time-l•po<,296 -,uh .. -viol<t,299 Photoly>i•,lll

:::::::~.0~r· 12

-,inf"inting,2,',() Photometry,%! Photomie~raphy,293 l'llolo-""°nling,l(ll Pho<ootn,;o;.,,.,.;.,,:w:? Phyoi<•l<hango,85 -,devel<>pm<nt,111,l:W,1+5 Pinhol<<>n><,.a,27,56 Plate~!i"11ma<hin<:,102 l'>lorfaed!ight,%

::::::i·.t·~~~!~~39 p.,.;,;_.,,,.-,,.,.,;.i.,23 P<>•.,.;um,<ya nid<,18 -,dichro""'"'•219 -,fo,.;cyuid<,219 -.l'<tmang>.-..<e,218 -,p<nulph•t<,121 l'<><en<iometry,Hl !'r«ipioaoion,90

::=..;:;r;,,.~:~~r·· 128

Prin•,5rainin.e.,, 231 -,pro«QinJ,2,',() Ptinti"ll,241 -,<olour,210 -.<onto<1,247 -,opti<al,248

:: ~~:;;:.::~::.!:~ 243

- , -,max;mumbl>ek,::46 Ptoc.,.;ng,i.;..,.;.,o,,.,,,.,l66 -,prin<1,2!>0 - ,roV<rW, I~ -,sol"""'"""*•,138 i'r<:>j«tion,•<e.=,284 Ptopo•bon>.l~u«<,22l

Qu.anoutnth<ory,25 Qµiooo><·<hiofulplio<ein<e,,.;li<•,223

R~;.,.,..,,;,..,<lem<n"-292 R~iography,290 R•ng<fond<t.78 Rapidfi•ation,l!>O R«iP'« ity <un'<<,203 - ,fa;Jut<,46,199 R«lue<r,photog .. phi<,217

~:~~:'n';:~~.!~1. m R<00<1iondt,,.;tomete<,l67 Rdl«:tKond<n,ity,166 Rell«<ante,.,,76

;:r;:'i.'~~·J'3 Reli<fimog<, I~ R<plenHb<r,135 Re>o(v;ngpov..·«,16,237,~l

R<n .. iner,d.-.. lopc"128 Reo<n•loolwrprint,27! -,pIO<<:O•ing,1~ -,-,colout,269

~~~i"i1~'i <•me•>,l0 //.,.IPJ.iro1••{'ki<S«it!J,W

Sofelight>t.-een•,198 s.fo,yt...e,23,105 ::.:i:;d,171 S..11-A.W.,!6,306 Serc<n,(oolw<),g<o>n><"i<,i~ - ,lonoieulat,2W -, ,.,,,fom, 2~5 -·"'P"""'2~ Seleniurn<ell,212

~~~~:~:~?.. 1

1~1 -,tim< ><•l<,160

t~:~::"'e:i<'f, l~l -, dioph .. gm,81 -,dlle;en<y,62

:::::.:::••,81

- , oyn<h_..,,,.,. wi1h ~wh. 0, i7

§;;JP -.-.de<lriQol-..l""tl.;'J, LIS -.-.lifh•'""'""'·n -,-,>OlubiUty,HO

=::i~Sl6 ::;::;~~18 -.~,Ill -,oulphOk,911 siOJcp.v;.c ..... m Sodhimtll'-'!pha1<,l7,l"6 Sodium•·apo<1rlamp,'ll Sollc<latininnol<r~SI) Soluiulion,:!07 Solubi~1y, ptt>d..:1, 1"6 -,.;1 ....... 1 ... 1..0 Sol•-<n<<l<-.lop<n,2~

-,1 ... """'oopy;..,,,!4 -,trt.••'"pn><d'i•J.1$8 Soul>dtt<OrdiRf,280 'Spo«'pl>o1<>srophy,288 Sf>«'"'l<nerarcu,....,~ Si>«tr«h<nti<• La .. Jyalo,186 SP«'""f"""' l89 Sp«tNgr>ph, 189 Spe<"Ol"Phy, 26) Sp«1,..m,vl>lbl<,U

~~!:::.~ ... ~~· lt:.tl Spee<!,161,111,210

~~,~~il7f =::·t::=ioi.f7tM:•1,l7i .::;;;~:;:'~M:n1,179 StabWiuboo>, lolf, I)) -,p.-O<d<,Jl6 Su.r>dudundlr:,$8 s1 ...... phci..,....phy.21:1 51<...,.pn>jt<<iooo,2&1 s .. ._.t&t s..;::~ brit;h ...... rans<. 110, 209.

- .mov<-<lt,21) S..t..la1<mi"'"*"•l71 Sub"""tl""colour,-262 -..,..l<m,2S5 Sub4ta.:tl•-....t._..,n1 Suptt«»tloe,lll'I S..pe.~ti<>nal«<L-.t?J s..Pt ... , .... .;,,...1"11 S..pplt<n<nurylr:,...60

To ...... d.nle>ptt.129,264 T"1'--.ll.D.,Sl ToehDi<>Ju...,a,tii Toebfticolor...-,:1&1,180 Tolr:pbow.1<,...s1 Temp.-r>1utt,oolo<",'12 -,d.nlopmon~l'2 -.fu..o ..... 1)1

T ... inJ.pl>o<ovapbicmo1<..W.,!0$ Timo,d.nlopm<n1,ll2 -.s ....... 1"" n ........ .,.. piloqnphy. Z!l6 , ...... ~ .... .;"'""'""160 l'Ofl<ttptt><IU<lion,170,181,2"6 Trit;h";";;'"' colour r<prodoctioao,

1'1ipock,71il Tu.,..t<nlompo,'11 - ,iodlnrlompo,"7

Ul<<>·•·i<J<1,71i,U -, lom,,.,'19 -. phol-Ofl"Phy,299 - .plto<0rniuog.-•phy,2'9J - ,l<<Wll•i1y,189

Voltncy,114,12'1

~~:-.i:;:;~,2~ Vi<w6nder, 7~ v;,;w.._.,..,,,,u l'or/,11.,ISJ

w..i.;,,.,1!>'1 \\'01.,i.o.....00p,~

""d-p«!Ngr>m,!il \\'••«lllod.,.,~(8 \,-.............. i.a w-...ru.a..1<,,..S1 Wood .. Jlaa,'19

X·rort.2'° -.<.,....!&nolpi>.29?

r-,,._·"'

The Author

Born in 1901 of Yorkshire parentage, the late Harry Baines spent his youth in Nottingham, where he studied chemistry under the late Professor F. S. Kipping. He took a London external B.Sc. with First Class Hons. in chemis­try in 1921 , and M.Sc. in 1923, in which year he entered the photo­graphic industry as research chemist to Thos. Illingworth & Co. Ltd. In 1938, he was appointed Chief Chemist and Deputy Director of Research, Kodak Ltd., Harrow, and in 1953, left the Re­search Laboratory to join the Head Office staff at Kingsway, London. His main contributions to photographic science were the devising of a number of new methods of analysis (which formed the basis of a thesis for D.Sc. (Land.) in 1928), advances in emulsion making technology, and the elucidation of the mechanism of fixation. He took considerable interest in the work of scientific societies, having been Vice-President of the Royal In­stitute of Chemistry, and of the Society of Chemical Industry; he was also closely connected with the Royal Photographic Society and was the only living person to have had two periods of Presidency.

E. S. Born back has practised photo­graphy for more than thirty years and has also been writing books about it for the last twenty years.

plan, and what would undoubtedly have been a greatly enhanced version remained unwritten. Nevertheless, the publishers have felt that Dr. Baines's book could not be allowed to die with him. The task of preparing a new edition was entrusted to E. S. Bomback, an author and editor with considerable ex­perience in practical photography and a one-time member of the Kodak Research Laboratories. Mr. Bomback has been able to add a great deal of additional information by a process of careful editing. New material includes chapters on Light Sources in Photography, Photographic Exposure, Printing the Negative, Cine­matography and Stereophotography, while other chapters, notably those on lenses and cameras, have been con­siderably expanded. As a clear and brilliant exposition of a complicated, and to the non-scientist, often baffling subject, The Science of Photography was an immediate suc­cess and went into several reprints after its first publication. Expanded and brought up to date this new edition is sure to be a standard text-book for many years to come.

Printed in Great Britain

The Science of Photography

This is an up to date and revised edition of a book which has been widely ac­claimed as a standard textbook and 'classic' in photographic literature. The reason for its success is simple. It is a book on the science of photography which can be understood by the ordinary reader, yet which has sacri­ficed nothing in accuracy or avoided any of the knotty problems. Dr. Baines was, indeed, one of those rare scien­tists with a gift for being able to express himself equally well as a speaker and a writer.

The extremely favourable press re­ception of the first edition was a source of some embarrassment to an author who was of a modest nature. He began. almost at once, to collect notes for a new edition, one which he felt would justify the description of 'The Science of Photography' as a 'classic' textbook. He sought advice and criticism from many sources and amassed a collection of references. But his illness and untimely death in 1963 prevented the realisation of his

(continued on back flap)

Cover Picture: \ Electron micrograph ( x 25,000) showing development centres and the etching out of silver bromide to provide the material for the developed silver.

£6.75 net in U.K. only