JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Filmed Surfaces for Reflecting Optics*GEORG HASS

Engineer Research and Development Laboratories, Fort Belvoir, Virginia(Received May 27, 1955)

This paper gives a survey of films and film combinations used for reflecting optics. The preparation andproperties of unprotected mirror coatings, protective layers for front surface mirrors, reflection filters, andreplica mirrors are discussed.

I. INTRODUCTION

THE increasing use of reflecting optics in modernoptical equipment has stimulated a great amount

of research on coatings for first surface mirrors, andvery important contributions towards the develop-ment of mirror coatings with improved durability andincreased reflectance have been made in recent years.It is the purpose of the present paper to discuss thepreparation and properties of various films and filmcombinations used on reflecting optics for the ultra-violet, visible, and infrared regions. Today, almost allcoatings for first surface mirrors, and especially thosefor precision optics, are produced by evaporation in ahigh vacuum. The paper shall, therefore, be confined toevaporated films and their use on reflecting optics.

The first evaporated mirror coatings were preparedby Pohl and Pringsheim in 19121; but it was not untilabout 1930, when high-speed vacuum pumps andtungsten heating elements for evaporation were devel-loped, that the vacuum coating technique began re-ceiving considerable attention. Important contributionstowards the development of satisfactory techniques forevaporating metals in a high vacuum were made be-tween 1930 and 1937 by Ritschl, 2 Strong,3 Cartwright, 4

Williams,5 and Auwaerter.f The most important ad-vancement of this period was Strong's 3 discovery of apractical method for evaporating pure aluminum. Inthe following parts, some of the more recently developedfilms and film combinations for reflecting optics willbe discussed.

II. PROPERTIES OF UNPROTECTEDMIRROR COATINGS

First surface mirrors are used today in many opticaland most all infrared optical devices. Their reflectanceshould be as high as possible to keep energy losses inthe equipment low. It is, therefore, important to know

* Presented as an invited paper at the meeting of the OpticalSociety of America, in New York, New York, March 25-27, 1954.

1 R. Pohl and P. Pringsheim, Verhandl. deut. physik. Ges. 14,506 (1912).

2 R. Ritschl, Z. Physik 69, 578 (1931).3J. Strong, Phys. Rev. 43, 498 (1933); Astrophys. J. 83, 401

(1936).4 C. H. Cartwright and J. Strong, Rev. Sci. Instr. 2, 189 (1931);

C. H. Cartwright, Rev. Sci. Instr. 3, 298 (1932).5 R. C. Williams, Phys. Rev. 41, 255 (1932); Phys. Rev. 46,

146 (1934).6 M. Auwaerter, Z. tech. Phys. 18, 457 (1937); J. Appl. Phys.

10, 705 (1939).

the reflectance of all mirror metals over a rather broadspectral region. Unfortunately, the data on the re-flectance of various metals listed in handbooks are veryold, insufficient, and in some cases, incorrect by morethan 20%. Figure 1 shows the results of new measure-ments on the reflectance of the most frequently usedevaporated mirror coatings in the wavelength regionfrom 0.22 to 10u. It should be mentioned that the re-flectance of a good vacuum deposited coating is alwayshigher than that of a polished or electroplated surfaceof the metal.

Aluminum, silver, gold, copper, and rhodium areconsidered to be the most important mirror metals. Theonly material that has a high reflectance in all usefulregions, the ultraviolet, visible and infrared, is alu-minum. The reflectance of all other metals drops rapidlyin the visible or ultraviolet. In the near infrared between1 and 2 the average reflectance of silver, gold, copper,and aluminum is higher than 95% but the reflectanceof aluminum is about 2% to 3% lower than that of theother three materials. In the far infrared at 10u allfour metals have a reflectance of 98% to 99% andeven rhodium reflects about 96%.

Today, the most frequently used high reflectingcoating for first surface mirrors is vacuum depositedaluminum. It adheres better to glass and other sub-strates than the other high reflecting materials, it doesnot tarnish in normal air, and it is very easy to evapo-rate. Obviously, aluminum coatings are especially im-portant for astronomical mirrors and reflection gratingswhere high reflectance in the ultraviolet is required.The reflectance curve of aluminum shown in Fig. 1

:900 H i - _ __ _

lo

2o~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~s

Cu. and Rh as function of wavelength from 0.22 to 1y.

945

VOLUME 45, NUMBER 11 NOVEMBER, 1955

GEORG HASS

100 7sec 1

90 go 9 -- -------

80 t __ @'X.~ c

60 l _ _ -6 / _

c0 I t 2X 10'5,- H c I t 2 ) id"4 H9,5C-- _ _ 1L701__ 30- 30 ___-

20 - -- 0- _ C_-

to - - - -- - -

sco 350 400 450 500 )500 250 00 350 400 4505005WWelength in mis Wavelength In mip

FIG. 2. Influence of the evaporation conditions (speed ofevaporation and pressure) on the reflectance of 600 to 700 Athick Al films in the wavelength region from 220 to 550 mu.

does not necessarily represent the reflectance of allevaporated aluminum films. It is the reflectance ofaluminum deposited under ideal conditions and meas-ured directly after its preparation. It is well known thatthe optical properties of all evaporated films arestrongly influenced by many factors such as speed ofdeposition, pressure during the evaporation, thicknessof the coating, angle of evaporation, temperature ofthe substrate and finally, aging in air. All factors shouldbe investigated to obtain optimum conditions for filmpreparation.

Figure 2 shows the effect of evaporation or firingspeed and pressure on the reflectance of aluminumfilms. All aluminum coatings were produced withextremely pure aluminum (99.99% or better) since itwas found that films prepared with the very purematerial have higher reflectance and better corrosionresistance than those produced with cp aluminum(99.5%). Two groups of films were investigated; onedeposited at a pressure of 1 to 2X 105 mm Hg and theother evaporated at 1 to 2X 10-4 mm Hg. All filmswere just opaque (600 to 700 A thick), and condensedon glasses at a distance of 20 inches from the evapora-tion source. The time for preparing the opaque coatingis noted for each curve. It can be seen that the speedof evaporation has a great influence on the ultravioletreflectance of evaporated aluminum but very littleeffect on its reflectance at longer wavelengths. Bychanging the deposition time for an opaque coatingfrom 7 to 180 seconds at 1 to 2X 10-5 mm Hg, the re-flectance at 220 m,4 drops from 91.5% to 62%, butremains almost unchanged in the visible. The influenceof firing speed is even greater at higher pressures. At1 to 2X 10-4 mm Hg, films with very high ultravioletreflectance can still be produced by fast evaporation.The ultraviolet reflectance, however, decreases morerapidly with increasing evaporation time than it doesat lower pressures. An opaque coating deposited in180 seconds at 1.5X 10-4 mm Hg shows only 40% re-flectance at 220 mA and 89% at 550 mju. The results ofthese investigations show that fast evaporation is most

important for producing coatings with high reflectancein the ultraviolet and that it is especially importantfor evaporations performed at higher pressures such as1 to 2X 10- mm Hg. In addition, films produced byfast evaporation are more compact and adhere betterto most substrates.

Figure 3 shows the ultraviolet, visible, and infraredreflectance of two aluminum coatings prepared underextremely different evaporation conditions. One filmwas deposited in 7 seconds at X 10-5 mm Hg whilethe other was condensed in 260 seconds at 1.5X 10-4mm Hg. The extremely slow deposited film has 76% lessreflectance in the ultraviolet, 10% less reflectance in thevisible, and only about 1% less reflectance in the infra-red at 10,. This shows that the evaporation conditionshave very little effect on the infrared reflectance ofevaporated aluminum. Almost any evaporation speedwithin reasonable limits results in aluminum coatingswith more than 97% reflectance at 10,u, but it is alwaysadvisable to evaporate aluminum as fast as possiblesince fast deposited films are chemically and mechan-ically more durable.

The reflectance values of aluminum coatings pre-sented in Figs. 1-3 were measured within one hour afterthe deposition of the films. Experience has shown thatthe reflectance of a new mirror is always greater thanthat of a mirror which has been stored or been used fora considerable length of time. Most of the change inreflectance is due to the oxidation of aluminum in airand takes place within the first month. The oxide filmsformed on aluminum in air under normal conditions areonly 40 to 100 A thick.7 Such thin oxide films affect thereflectance of aluminum much more in the ultravioletthan they do at longer wavelengths. Table I shows theeffect of aging on the reflectance of two aluminummirrors produced under different conditions.

Aluminum films produced by fast evaporation at lowpressures are more compact than those deposited slowerand at higher pressures. They show, therefore, lessoxidation in air and less change in ultraviolet reflectancethan slowly deposited films. These aging effects em-

FIG. 3. Reflectance of two Al films prepared under extremelydifferent evaporation conditions in the wavelength region from0.22 to 10,u.

7 G. Iass, Z. anorg. allgem. Chem. 254, 96 (1947).

Vol. 45946

FILMED SURFACES FOR REFLECTING OPTICS

TABLE I. Effect of aging on the reflectance of two aluminumfilms produced under various conditions.

Al, 600-700 A thick, Al, 600-700 A thick,deposited in 7 sec at deposited in 60 sec at

1 X10-5 mm Hg 2 X10-4 mm Hg% reflectance % reflectance

(my) After After After After After After30 min 1 mo 1 yr 30 min 1 mo 1 yr

220 91.5 90.1 89.9 83.5 74.2 72.5360 92.4 92.0 91.9 91.0 90.3 90.1550 91.6 91.4 91.4 90.9 90.6 90.5

phasize again the importance of fast firing for preparinghigh quality aluminum mirrors. The reflectance valueslisted in Table I were measured after aging in normalair with 30 to 60% humidity. If aluminum films arestored in a desiccator or in extremely dry air, theirultraviolet reflectance decreases less than shown inTable I.

In 1924 Coblentz8 reported that the reflectance ofmetals and sulfides decreases in the ultraviolet whenexposed to ultraviolet light. Many years later Walken-horst9 and Cabrera 0 studied the influence of ultravioletlight on the oxidation of aluminum films. Both foundan increased oxidation rate when aluminum was exposedto ultraviolet light. To study the effect of ultravioletlight on the ultraviolet reflectance of evaporatedaluminum, various aluminum mirrors were placed undera 300 watt quartz-mercury burner at a distance of 6inches for many hours. The results of reflectancemeasurement at 220 mgt before and after the treatmentwith ultraviolet light are shown in Table II.

TABLE II. Effect of ultraviolet light on the reflectance of twoaluminum coatings deposited at different conditions. (Reflectancemeasured at X= 220 mr,).

"good coating" "poor coating"6 sec, 55 sec,

1 X10-5 mm Hg 1 X10-4 mm Hg

R before UV 91.5% 86.9%R after 24 hrs UV 90.3% 82.5%R after 68 hrs UV 89.5% 80.2%

Films fast deposited at low pressures are called"good coatings" and those deposited slowly at higherpressures are called "poor coatings." "Good coatings"are affected less by the ultraviolet treatment than''poor coatings." The effect of ultraviolet light on thereflectance of aluminum as shown in Table II can beexplained by an increased oxidation rate, and the effectis even greater when the exposure is performed in aroom with extremely high relative humidity such as70% to 90%.

The aluminum films discussed up to now were justthick enough to insure optically opaque coatings. Iffilms are made much thicker they develop a coarsesurface structure which scatters light and therefore

8 W. W. Coblentz, Science 60, No. 1542, 64 (1924).9W. Walkenhorst, Z. tech. Phys. 22, 14 (1941), and disser-

tation, Hannover (1940).10 N. Cabrera, Phil. Mag. 40, 175 (1949), and Cabrera, Terrien,

and Hamon, Compt. rend. 224, 1558 (1947).

2.5 - -- _

- - -- --=20 __~- i _X;__+ _ __ _ ,_

_Vaouro. IXtOosm Hgv~ti Hg o iDeposition rte: 10-15 A/sec

Vr. I X 04m Hg i

0.5 . -Deposition rote: 2to4 A/sec

400 500 600 700 800Wavelength in Millimicrons

FIG. 4. Optical constants of SiO films prepared under differentevaporation conditions in the wavelength region from 300 to800 mgc.

decreases the reflectance. This effect is rather small aslong as the deposition is performed under normal inci-dence. If, however, the incidence angle of the vaporarriving at the condensing surface is increased, a coarse,diffuse reflecting surface is formed on much thinnerfilms. The effect of vapor incidence on the reflectanceof evaporated aluminum has recently been studied byHolland.1 He showed that a diffuse reflecting surfaceis formed more easily as the vapor incidence angle isincreased.

Mirror coatings for the ultraviolet should, therefore,be condensed close to normal incidence and no thickerthan necessary.

III. PROTECTED FRONT SURFACE MIRRORS

The fact that evaporated aluminum does not tarnishin a normal atmosphere is a result of the good protectivequalities of its natural oxide film. For many mirrorapplications, however, the natural oxide film is too thinto furnish sufficient protection, especially if the mirrorrequires frequent cleaning. Therefore, various methodsfor protecting aluminum with hard and transparentprotective coatings have been developed in the lastfifteen years.

Magnesium fluoride is used in many laboratories sinceit has proved to be so successful as an antireflectioncoating. As a protective coating on aluminum, however,magnesium fluoride is not satisfactory, since it doesnot furnish sufficient chemical protection and formshard films only when evaporated onto substrates whichare at elevated temperatures (2500 C to 3000C).

A lower oxide of silicon, silicon monoxide (SiO) hasbeen found to be especially suitable for producing hardprotective coatings on evaporated aluminum mirrors.'2

SiO evaporates at a rather low temperature of about1200'C and condenses on the mirror surface in uniformand adherent layers which furnish excellent protection.

11 L. Holland, J. Opt. Soc. Am. 43, 376 (1953).12 G. Hass and N. W. Scott, J. Opt. Soc. Am. 39, 179 (1949);

G. Hass, J. Am. Ceram. Soc. 33, 353 (1950); G. Hass and C. D.Salzberg, J. Opt. Soc. Am. 44, 181 (1954).

Z.S?.2ZII I.2UZ;2

-9-EIZ,g (

. I - -I

Zs

November 195 94'7

3 )0

GEORG HASS

The optical properties of silicon monoxide films aregreatly dependent on the evaporation conditions. Fastevaporation at low pressure results in a dense film oftrue SiO while slow evaporation at about X 10-4 mmHg of air forms a strongly oxidized coating. The opticalconstants of films prepared by slow and fast evaporationare shown in Fig. 4. Films prepared at 1X 10-5 mm Hgand a deposition rate of 10 to 15 A/sec have, in thevisible, a refractive index of about 2.0 and a very strongabsorption in the ultraviolet which extends into thevisible. Coatings prepared at much lower depositionrates and at higher pressures are strongly oxidizedand show a large shift of the absorption edge towardshorter wavelength, decreased absorption in the visibleand ultraviolet, and lower indices of refraction. Frontsurface mirrors which are to be used in the visibleshould, therefore, be coated with a strongly oxidizedfilm rather than with one of true SiO. Figure 5 shows

c

, vprto ie*10 m

80

D0 A

70 - -

-Vocuum-ixuS5 mm Hg of airEvaporation time 10 min

50 Film thickness - 150 mu

40 --- VacuumIx10 mm HgEvaporation time 2min 9secFilm thickness * 125 mu

30 -

20 \ --

.2 .3 4 .5 .6 .7 .8 .9X in microns

FIG. S. Influence of thereflectance of SiO-protectedfrom 0.22 to 1.10A.

0

SiO evaporation conditions on theAl mirrors in the wavelength region

the reflectance of aluminum protected with stronglyoxidized and with true SiO from 0.22 to 1.0,u. Bothprotective coatings are effectively one-half wavelengththick at 550 mp to produce highest reflectance in thevisible. The fast evaporated true SiO coating causesstrong absorption at all wavelengths shorter than 500m/i and gives the mirror a yellow appearance. Thestrongly oxidized film material prepared by slowevaporation at 1 X 10-4 mm Hg of air causes no absorp-tion in the visible and very little in the near ultravioletfrom 300 to 400 Inp. The visible reflectance of such aSiO protected mirror measured with a photocell ad-justed to eye sensitivity is about 89% or one percentlower than that of an unprotected mirror. The stronglyoxidized films are less dense than those of true SiObecause they are deposited at a much lower rate. How-ever, they make excellent protective coatings sincethey adhere well and become harder and denser by

IC'0 1 IIIIIII

9B0

tZ 1409 mp270 -

15 -3 4 1 21

~40

10 ~~~R2 3 4 5 6 7 8 9 10 I 12 3

(4

14X In microns

Fig. 6. Infrared transmittance and reflectance of a1409 m~u thick SiO film on rocksalt.

further oxidation when exposed to air. SiO protectedmirrors can be thoroughly cleaned without damageand even withstand boiling salt water for one hourwithout change in reflectance.

Since SiO is frequently used as a protective layer andas an antireflection coating for infrared optics, itsinfrared optical properties are of considerable interest.Figure 6 shows the infrared transmittance and re-flectance of a rather thick coating of SiO on rocksalt.SiO exhibits an infrared absorption maximum at 10microns while the absorption maximum of fused quartz(SiO 2 ) lies in the neighborhood of 9. To study theeffect of the 10-micron absorption band on the re-flectance of SiO protected mirrors and on the trans-mittance of SiO coated infrared transparent materialssuch as rocksalt, aluminum and rocksalt were coatedin the same evaporation with 125 mq thick films of SiO.This is the thickness most frequently used for mirrorprotection. Figure 7 shows the transmittance of rock-salt coated with 125 mmu of SiO. In the 10,u region thetransmittance of rocksalt is decreased by 30%, about18% of which is an actual absorption loss. Figure 8shows the reflectance of aluminum coated with thesame SiO film. Here the 10 micron absorption bandhas no appreciable effect on the reflectance of aluminum.It decreases it by less than 1% which in most measure-ments is not even noticeable.

100

1 2 3 4 5 6 7 8 9 101 1 12 13 14

A in microns

FIG. 7. Infrared transmittance and reflectance of a125 mu thick SiO film on rocksalt.

948 Vol. 45

FILMED SURFACES FOR REFLECTING OPTICS

00 - - immersed mirror to make the electrical contact. When90 - - voltage is applied, an oxide film forms on the mirror8 0- -- -= = = = =- _= = = surface. The thickness of the oxide film formed depends

| - - - only on, and increases linearly with, the applied voltage2- 5d| __ by a rate of 13 A/volt.'4 To produce highest reflectance

co>4 |_Sbansdfig;1itoznespodhced in the visible, the oxidation should be performed with3 |- -. __ 110 volts. Complete oxidation is reached in about one

r20 -- M e e _ Surface fm minute. Anodized aluminum mirrors show excellentIs -- - abrasion resistance. They can hardly be scratched with02 3 4 5 6 7- 8 9 10 11 12 the sharp edge of a paper clip. Furthermore, this

Waveleogtih miWns method has the advantage that it forms completelyEicEEP Infrared reflectance of Al protected with a 125 myd thick uniform protective layers without appreciable absorp-

film o Sidr .l fomtheltion in the ultraviolet, visible, and infrared.

The fact that very thin layers of absorbing materials The protective coatings described above were madehave little effect on the infrared reflectance of metals, up of one dielectric material, MgF2, SiO, or Al203. Bybut decrease the transmittance of infrared transparent using pairs of dielectric films with alternately high andmaterials strongly, is of general importance. This is not low indices of refraction, the reflectance of a metal canonly true of SiO, but also applies to films of any infrared by increased over a rather broad spectral region.", 6

absorbing material, such as thin layers of absorbed To obtain highest reflectance, the dielectric films mustwater or grease. The effect shown here leads to the be effectively one-quarter wavelength thick and mustconclusion that in infrared equipment windows and be applied in the following sequence: metal, low indexlenses have to be cleaned more carefully and frequentlythan front surface mirrors. To explain the effect, one __ = ==

has to remember that light reflected from a high . 1.4 n2- 1.5

reflecting metal produces a standing wave pattern - -.

with a node or zero vibration at the surface, as shownin Fig. 8. The existence of such a standing wavepattern was first demonstrated by Wiener in 1890.1Absorption takes place only where there is an appreci-able amount of vibration. The absorbing film here, 94

which is small compared to the wavelength, is in the 93 -

region where the vibration is almost zero. The film is,therefore, in an inactive position for absorption and 91 2.0 2.2 2.4 26 2. 3.0 3.2 3.4 3.6

produces no noticeable effect on the reflectance. The ,Refroctiveind. fupperfil

transmitted light passing through an infrared trans- FIG. 10. Reflectance of Al coated with one reflectance increasingprsurface and is, low index-high index film pair for various index combinationsparent material has vibrations in the raendiat X = 546 mji.

therefore, affected by a thin absorbing layer.Figure 9 shows another method for producing hard film, high index film. The greater the difference in

protective layers on evaporated aluminum. This method refractive index between the two dielectric filmuses an anodic process which increases the thickness materials, the greater and broader is the reflectanceof the aluminum oxide film on aluminum in a precisely increase. Greater increases in reflectance can also becontrolled way. Figure 9 is a sketch of the arrangement obtained by using more than one film pair.used for the anodic treatment. The evaporated alu- For normal incidence, the maximum reflectance of aminum mirror is made the anode in electrolyte such metal coated with a single low index-high index filmas 3% ammonium tartrate. One or more pure aluminum pair is given by the following equation:wires are pressed against the surface of the completely

Pure aluminum wirepressed _goinst mirror

V ~~~~~~~~~~surface+ A220 v - - -

22 - - Mirror to bea- onodized

Pure aluminum

3% Tdrtaric acid + NH4OH to pH=5.5

FIG. 9. Sketch of the arrangement used for the anodictreatment of evaporated Al mirrors.

13 0. Wiener, Wiedem. Ann. 40, 203 (1890).

where

n11+1r=1n+

ri-r 2 1-r3 -rlr 2r 3 I

-rr2+rra-r2r3,

(n (- Q 2+k2 r3= IL(n+n )2 2+k2J

Figure 10 shows the calculated reflectances of alu-minum coated with one reflectance increasing film pair

4 G. Hass, J. Opt. Soc. Am. 39, 532 (1949).'5 A. F. Turner, J. Opt. Soc. Am. 36, 711 (1946).16 R. Messner, Optik 2, 228 (1947).

949November 1955

n2-nlr2= ��'

'K 2+K I

GEORG HASS

TiO, t-480A

Al t 1200A

Untreated

I TiO a t-550 .

1Al 1-1200A

25 Volts

.TiO t-550A

IA120, t650AI

Al t.750A

75 Volts

FIG. 11. Layer arrangement on mirror form coated with Aland TiO. before and after anodic oxidation with 25 volts and 75volts to illustrate the preparation of Al-AI203 -TiO2 mirrors withhighest reflectance in the visible.

for various index combinations at X= 546 m/u. Theoptical constants of aluminum used for calculatingthese curves are: n=0.76, k=5.49.17 Film pairs ofMgF2 (n2= 1.38) and ZnS(nl= 2.33) are most frequentlyused to increase the reflectance of metals. Their pro-tective qualities, however, are not very satisfactory.Therefore, a new method for increasing the reflectanceof aluminum, using film pairs of Al203 and TiO2, hasbeen developed.' 8 The method is illustrated in Fig. 11.To produce the A1203-TiO2 double layer coating forhighest reflectance in the visible, TiO2 is evaporatedonto freshly deposited, double opaque aluminum untilthe aluminum reflectance reaches a minimum at 620 me.The direct evaporated TiO2 results in a decomposed,strongly absorbing film material which may be calledTiO,. By anodic treatment with 25 volts, the decom-posed material on aluminum is re-oxidized to non-absorbing TiO2 and the film thickness is increased fromabout 480 A to 550 A. By applying a higher voltagepart of the aluminum changes to A1203 underneath theTiO2 . Seventy to seventy-five volts must be used toproduce quarter wavelength thick films of Al203 andTiO2 . The arrangement and electrolyte shown in Fig. 9can be used for the anodic treatment. Figure 12 showsthe reflectance of aluminum coated with about 480 Aof TiO, before and after anodic treatments with variousvoltages. The untreated mirror has a very low reflec-tance due to the strong absorption of the decomposedTiO2 . After applying 15 volts, part of the TiO isoxidized, and the light absorption is strongly decreased.

400 500 600 700Wavelength in millimicnons

800

FIG. 12. Reflectance of Al coated with an evaporated 480 Athick film of TiO. before and after anodic oxidation with 15 volts,30 volts, 50 volts, and 70 volts as function of wavelength from400 to 800 my.

'7 G. Hass, Optik 1, 21(1946).18 G. Hass and A. P. Bradford, J. Opt. Soc. Am. 44, 810 (1954).

.8 .9 !° IIWavelength in microns

FIG. 13. Reflectance of evaporated Al with and without tworeflectance increasing film pairs of MgF2 and CeO2 as function ofwavelength from 0.4 to 1.6/i.

With 30 volts all of the TiO is converted to non-absorbing TiO2 and Al203 has started to form under-neath the TiO2. With 70 volts a quarter wavelengththick film of Al203 is formed underneath the TiO2 andthe reflectance of a great part of the visible spectrumis higher than 95%. In practice, for producing high re-flecting mirrors, 70 to 75 volts are applied directly forabout one minute. The abrasion and corrosion resis-tance of Al203-TiO2 protected mirrors is very good.They cannot be damaged by strong rubbing with linenand can be boiled in 5% salt water for one hour withoutchange in reflectance.

Recently, a new material, cerium dioxide (CeO2), hasbeen found to be especially suitable for preparing high-index films. 9 CeO2 can be evaporated from a tungstenboat and forms extremely hard, nonabsorbing filmswith a refractive index of 2.3 to 2.4 in the visible. CeO2films can be used in combination with films of low-index materials such as MgF2, A203, and SiO forpreparing very durable reflectance increasing film pairson metals. Therefore, in the future, there is no reasonnot to combine mirror protection always with reflect-ance increase. Figure 13 shows the reflectance ofevaporated aluminum with and without two reflectanceincreasing film pairs of MgF2 and CeO2 from 0.4 to

80_ _ _ ] l-1eri transparent metal

60 De spae l

40

0__ ') -- - -200 300 400 500 600 700 800 900 1000

Wavelength in millimicrons

FIG. 14. Reflectance and construction of a single reflection filter.

'9 G. Hass and J. B. Ramsey, J. Opt. Soc. Am. 45, 408 (1955).

8030 V.1�1�_ 50 V-1tj

60

i15 V011.

0 1 I

led

950 Vol. 45

c

0

Z

FILMED SURFACES FOR REFLECTING OPTICS

SiO to R = 0%

Ge to RminI

Al Opaque

FIG. 15. Layer arrangement in Al-Ge-SiO filter mirror.

1.6 y. Over a great part of the visible spectrum, thecoated mirror has a reflectance of more than 98%,while in the near infrared from 1.0 to 1.4,u, the protectedand unprotected mirrors show the same average re-flectance of 95% to 96%.

IV. FILTER MIRRORS

In many optical devices and instruments it is de-sirable to use the reflecting element directly as a filterto eliminate unwanted wavelength regions or to isolatedesired wavelength bands. In recent years, variousfilm combinations have been designed for producingselectively reflecting mirrors. Suitable coatings forisolating wavelength bands were designed by Hadleyand Dennison.20 The construction and reflectance ofsuch a reflection filter is shown in Fig. 14. In its simplestform the reflection filter consists of an opaque highreflecting metal such as aluminum, a dielectric spacerand a semitransparent metal film. Maximum reflectanceoccurs at wavelengths for which the spacer is effectivelyan even number of quarter wavelengths thick. Theminima lie at wavelengths at which the spacer iseffectively an odd number of quarter wavelengths thick.The reflectance at the maxima is equal to that of theuncoated aluminum and the reflectance of the minimacan be reduced to zero by proper choice of the thicknessof the semitransparent metal film. Reflection filters ofthis type have been made for the ultraviolet, visible,infrared, and microwave regions. The reflection filterhere has a spacer which is effectively one-half wave-length thick at X= 380 mc. Its reflectance is, therefore,

SiO to R 0% I

AlRthru0tol5% ISiO to Rmin

Al Opaque

FIG. 17. Layer arrangement in Al-SiO-Al-SiO filter mirror.

close to 90% at 380 mAo and almost zero at 780 m4 and at250 m/.z. The half-width of the filter mirror is about125 m/u or 34% of the wavelength. Turner and Hopkin-son2 ' have shown that the half-width of such a reflectionfilter can be decreased by adding more film pairs, eachconsisting of a half wave spacer and a semitransparentmetal film. By adding two more film pairs to thisfilter they obtained the so-called" triple reflection filter"which showed a half-width of only 60 m/o and extremelylow reflectance throughout the visible and near infrared.

For many purposes mirror coatings with low visibleand unchanged high infrared reflectance are verydesirable since they can be especially useful for reducingstray light in infrared equipment. In recent years twofilm combinations for producing such "dark mirrors"were designed.2 2 In the first design aluminum is coatedwith evaporated germanium and SiO, each film ap-proximately one-quarter wavelength thick. Germaniumis used because it absorbs strongly in the visible, butbecomes nonabsorbing in the infrared. The layer ar-rangement illustrating the preparation of such filtermirrors is shown in Fig. 15. If germanium is evaporatedonto opaque aluminum, the aluminum reflectance, con-trolled at a certain wavelength in the visible, decreasesto a minimum of 30 to 40%. By adding SiO to the

1!

a.

Wavelength I, Microns

FIG. 16. Reflectance of an Al-Ge-SiO filter mirror as function ofwavelength from 0.4 to lOu. Layer deposition controlled at 430 mtZ.

L. N. Hadley and D. M. Dennison, J. Opt. Soc. Am. 37, 451(1947); 38, 483 (1948).

100 - I - --

I( I I - -I I

- - - - - -

.3 A 5 .6 7 9 2 3 4 5 67 8 9 I

wavelength in microns

FIG. 18. Reflectance of Al-SiO-Al-SiO filter mirror as functionof wavelength from 0.36 to lOp. Layer deposition controlledat 550 my.

21 A. F. Turner and H. R. Hopkinson, J. Opt. Soc. Am. 43, 819(1953).

2 Hass, Schroeder, and Turner, J. Opt. Soc. Am. 43, 326(1953). (A more complete paper on "Mirror coatings for lowvisible and high infrared reflectance" will soon be published inthis Journal).

951November 1955

I I

GEORG HASS

Al-Ge combination, the visible reflectance becomesalmost zero. The reflectance of a Al-Ge-SiO mirror inthe visible and infrared is shown in Fig. 16. By con-trolling the mirror preparation at 430 mgu the visiblereflectance is decreased to 2%. The reflectance startsto rise rapidly at 0.8/, and reaches about 80% at 1.0t,90% at 1.2,u and is as high as uncoated aluminum forall wavelengths longer than 1.5,u.

Figure 17 shows the preparation and layer arrange-ment of another "dark mirror." This film design usestwo SiO coatings, separated by a semitransparentaluminum film on top of opaque aluminum. The firstSiO film is evaporated until the aluminum reflectancereaches a minimum of about 60%. Then, aluminum isdeposited until the reflectance, after passing throughzero, reaches a value of 15 to 20%. Another film of SiOon top brings the reflectance again down to zero. Figure18 shows the reflectance of such a reflector in the visibleand infrared. The preparation of the mirror was con-trolled at X= 550 m.t. The reflectance of the mirror islow in the visible and near infrared, starts to rise rapidlyat 1.5,u and reaches 90% at 2 .8 /u. The reflectance in thefar infrared is as high as that of uncoated aluminum.In both "dark mirror" designs the region of low re-flectance can be shifted to longer and shorter wave-lengths. Many modifications of these multilayer designsare possible with respect to materials and filmthicknesses.

-Al-I ssMar -Ag _-

Glass Master Coated With Separating EFilm And Final Mirror Coatings

=Si _=-Ag

i la ss r

Replica Separated From Master

Epoxy Resin Cast On Coated Master

Finished Mirror WithSeparating Layer Removed

FiG. 19. Sketch showing the four steps for preparingmirrors with epoxy backing.

V. REPLICA MIRRORS

A simple and inexpensive method for producingreplicas of optical elements such as mirrors and prismsis of great practical interest. The electroforming processhas proved to be very suitable for making metalreplicas of trihedral prisms and mirrors.23 However, themethod is rather time consuming and expensive. In thefollowing part a recently developed method2 4 for pro-producing replica mirrors with plastic backings isdescribed. With the new technique, replicas equippedwith the final high reflecting coatings are prepareddirectly on the master mold. Epoxy resins such asCoil Seal No. 11 or Epon 828t are used to make thereplica mirror form. The epoxy casting compounds arepourable liquids which can be hardened overnight withlittle or no heat through the addition of a liquid hardener.Very little heat and very low shrinkage are developedduring the hardening process. The method for producingreplica mirrors with epoxy resins is demonstrated inFig. 19. To make a replica mirror, the negative glassform is coated with a thin layer of silver, SiO and ratherthick aluminum. A plastic mold in the form of a ringis then placed on the coated glass master and filled withthe epoxy resin. After the epoxy resin is cured orhardened, the replica is separated from the master byapplying slight pressure to the plastic ring. The silverfilm, which has poor adherence to glass, is only usedto make the separation easy. After the separation, thesilver film is removed in nitric acid, leaving the replicamirror coated with SiO protected aluminum. The alu-minum, which has been deposited rather heavily toproduce a rough surface, adheres excellently to thehardened epoxy compound. The quality of the replicamirrors can be improved by adding 50 to 100% offinely divided SiO 2 filler to the epoxy compound. Thisresults in mirrors with strongly reduced expansioncoefficients. Experiments to produce large size re-flectors with this replica method are underway. Forlarge mirrors, thin metal shells are used in combinationwith the epoxy compound as the mirror backing.

23 B. Goldberg, J. Opt. Soc. Am. 38, 409 (1948).24 G. Hass and W. W. Erbe, J. Opt. Soc. Am. 44, 669 (1954).t Produced by National Engineering Products, Inc., Wash-

ington, D. C.$ Produced by Shell Chemical Corporation.

Glass Mt

952 Vol. 45