Tungsten-halogen lamps and regenerativemechanisms

J.R. Coaton, M.Sc, Ph.D., M.lnst.P., M.C.I.B.S., C. Eng., F.I.E.E., andJ.R. Fitzpatrick B.Sc, Ph.D., M.lnst.P.

Indexing terms: Filament lamps

Abstract: The presence of iodine, bromine or chlorine in a tungsten-halogen lamp provides only an envelopecleaning mechanism; longer filament life and/or higher luminous-efficacy entirely result from the smaller sizeand physical changes that are permitted in the absence of envelope blackening. Since the introduction of thefirst practical lamp, patented in 1958, much progress has been made in the technology of tungsten-halogenlamps leading to the development of a wide range of sources. In recent times some attention has been givento the tungsten-fluorine cycle, which provides a self-healing filament mechanism. Experimental lamps employ-ing this principle show up to a 45% increase in life.

1 Early innovations

The first reference to an incandescent lamp containing ahalogen as a means of providing a bulb cleaning mechanismdates back to 1881,1 and claims,

1 An electric incandescent lamp containing a rarefiedatmosphere of chlorine.

2 In an electric lamp, a carbon conductor adapted to berendered incandescent, in combination with an inclosing-globe containing a rarefied atmosphere of chlorine, substan-tially as described.In testimony whereof I hereunto set my hand this 12th dayof December, 1881.

Edwin A. Scribner

This application was filed on 26th January 1882 (withoutmodel). With the benefit of modern technology, plus almosta hundred years of accumulated experience, it is easy to seethat such a device would run into serious problems. Thechlorine vapour, even if it did not react to form stablecompounds with elements leached out of the glass bulb,would almost certainly attack the cooler parts of the incan-descent carbon filament; nevertheless, Scribner recognisedthe basic requirements for a bulb cleaning mechanism in anincandescent lamp, albeit that the materials and technologywere not available at the time to develop the idea.

Much of the subsequent development on incandescentlamps was concentrated on filament materials, culminatingin swaged and drawn tungsten wire. This opened up thepossibility of making coiled filaments, allowing good use ofnonreactive gas fillings. Following these innovations,J.A.M. Van Liempt filed a patent on the 22nd January1930 describing an incandescent lamp using a carbon, tung-sten or rhenium filament, and containing a halogen(s), inwhich a cyclic transport process operated to return evap-orated material to the filament.2 The examples he gave ofthe embodiment of this invention were tungsten-filamentincandescent lamps containing chlorine or bromine havingthe cooler parts of the filament coated with a protectivelayer of refractory material, such as silica or alumina. Apartfrom the problem associated with the loss of these halogens

by reaction with constituents in the glass components,defect-free continuous protective films are extremely diffi-cult to form, and may adversely affect filament stability —again, a very significant innovation but ahead of the tech-nology to support it.

The use of iodine had certainly been suggested, but doesnot appear to have been specifically investigated for use inlamps designed to operate with a cyclic-transport process.However, on the 3rd August 1955 an American patentapplication was filed specifying an incandescent lamp madefrom a tubular, fused-quartz envelope, with the filamentpositioned along the central axis, charged to at least0-01 id moles per cm3 of bulb volume with iodine, andhaving an inert gasfilling.3 This design obviated the previousproblems associated with the more reactive halogens, bro-mine and chlorine, and removed the troublesome source ofelements available in glass bulbs, which could compete forthe halogen and interrupt the cyclic transport mechanism.Such 'linear' lamps (Fig. 1) are usually reckoned to be thefirst practical application of the cyclic bulb cleaning mech-anism, and the first to become commercially available. Thisinnovation initiated renewed interest in incandescent lamps,and during the last twenty years a whole new technologyhas been established, embracing lamp making materials,expecially tungsten and fused quartz, and the physics and

Paper 524A, first received 28th June and in revised form 10thOctober 1979Dr. Coaton arid Dr. Fitzpatrick are with the Lamp Research andEngineering Department, Thorn Lighting Ltd., Melton Rd., LeicesterLE4 7PD, England

142

0143- 702X/80/030142 + 07 $1.50/0

Fig. 1 Typical linear tungsten-halogen lamps

IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

chemistry of cyclic transport processes, all of which has ledto the development of a wide range of tungsten-halogenlamps, 4—10 000 W, for a great variety of applications.

2 Chemical and physical mechanisms

Various aspects of the incandescent tungsten-halogen lamphave been described in previous publications,4'5 but never-theless, the principles of the tungsten transport cycle willbe reviewed to establish a clear understanding of the mech-anism before discussing more recent developments.

A conventional gas-filled incandescent filament lamploses tungsten by evaporation; in most cases the life of thelamp is inversely proportional to the evaporation rate of thefilament.6 Ultimate failure is usually caused by the forma-tion of a local hot spot which eventually fuses when thelamp is switched on.7'8 Tungsten atoms or molecules leav-ing the incandescent surface of the filament may eithertransfer to neighbouring parts of the filament, or escapefrom the immediate vicinity of the filament where they aretransported by gas convection, forming a metallic film onselective parts of the internal surfaces. This graduallyincreasing tungsten layer absorbs radiation (light and heat)which sets a limit on the minimum size of bulb for a givenpower dissipation. Now, if a halogen is added to the gas-filling, and provided certain temperature conditions are sat-isfied, a chemical-transport cycle is established which pre-vents tungsten from depositing on the inside surfaces of thelamp envelope.

The all-important function of the tungsten-halogentransport cycle is to keep the bulb wall clean; the otheradvantages of tungsten-halogen lamps result from this singlebasic benefit.

In a successful tungsten-halogen lamp, the bulb wall doesnot accumulate a radiation absorbing tungsten film and inconsequence its minimum size can be greatly reduced. Gen-erally, the envelope is made from fused silica (quartz) orglass containing a high content of silica; such envelopes aremechanically strong and can safely operate with a highinternal gasfilling pressure. Also, the reduced envelope vol-ume allows the economic use of the more expensive densergases, krypton and xenon in place of argon; both the highpressure and denser gas filling reduce the rate at whichevaporated tungsten can diffuse from the incandescent fila-ment (effectively reducing the evaporation rate), therebyincreasing filament life.

In summary, the essential mechanism of a tungsten-halogen lamp is,

(a) The tungsten transport cycle prevents envelopeblackening; this greatly reduces envelope size (typicallyby 90%).

(b) Small, strong envelopes allow a substantial increasein gasfilling density by increasing the filling pressure (typi-cally by 2—3 times) and/or by the use of krypton or xenon.This effectively reduces filament evaporation and increasesfilament life, or luminous efficacy (typically double lifeplus 10% increase in efficacy).The tungsten-halogen transport cycle using iodine or bro-mine has been extensively reported and in its simplest formcan be expressed as follows:9'10

W + nX ^ WXn

where X is the halogenTungsten evaporated from the incandescent filament dif-fuses through the inert gasfilling, and as it moves down thetemperature gradient from filament to envelope, there is an

IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

increasing probability that it will combine with the halogento form a tungsten halide. The envelope temperature mustbe sufficiently high to keep these halides in the gas phaseand when the halides diffuse up the temperature gradienttowards the incandescent filament there is an increasingprobability that they will dissociate, releasing tungsten andthe halogen. Inside a lamp a dynamic equilibrium is attainedbetween the many competing processes and any successfulsystem must maintain virtually zero concentration of tung-sten at the envelope wall. While this chemical transportcycle counteracts the radial diffusion of tungsten it doesnot prevent axial transportation, and in consequence hot-spots still develop, and although filament life is extended,the ultimate failure mechanism remains unchanged.

Models have been proposed for tungsten-halogen trans-port cycles based upon thermodynamic equilibrium condit-ions, but many ignore the kinetics of the reactions,11 andshould only be used as a guide to the species that couldexist. A definitive, proven model has yet to be established,even for the simplest tungsten-iodine cycle, since this almostcertainly involves significant reactions with the residualoxygen, carbon, hydrogen etc. within the lamp. Qualitat-ively, the activity of the tungsten transport cycle is inverselyrelated to the atomic weight of the halogen, with iodinebeing the least active and fluorine the most active. Oxygentends to increase the activity via a tungsten oxyhalide cycle,whereas hydrogen combines with the halogen in the cooler,regions of the lamp, and generally decreases the activity ofthe cycle. Many ingenious chemical transport systems havebeen developed to tailor the activity of the cycle to the life,luminous efficacy and power loading of the device, includ-ing additions of iodine, bromine, chlorine, hydrogen halides,hydro-carbon halides, metal halides and phosphonitrilichalides.12

Considering the physical process in more detail, thereexists an optimum diameter for the envelope of a lineartungsten-halogen lamp where the power losses to the gas-filling are reduced to a minimum.13 This diameter corre-sponds to the dimension of the so-called Langmuir sheath,(a gas sheath surrounding the incandescent filament whereheat is lost by conduction only), which can be calculatedfrom the equation

= K(V/p)2/*(TB/TF-TB/3

where

b = sheath diametera = filament diameterrj = average viscosity of the gas within the sheathp = average density of the gas within the sheathTB = envelope temperatureTF = filament temperatureK — a constant approximately = &jg1/3 where g is

the acceleration due to gravity

Although this construction produces the most efficientform of tungsten-halogen lamp many long life ratings(>2000h) have the disadvantage that they are not universalburning and must be restricted in burning angle. The reasonfor this restriction is that most long life lamps contain iod-ine, which is the least reactive but heaviest halogen, and if alinear lamp is tilted by more than about 4° from the hori-zontal position the heavy iodine separates within the gas-filling causing a deficiency of iodine at the upper end of thelamp. This leads to envelope blackening, and in extremecases causes the envelope to soften and fail. The precise

143

mechanism of halogen separation is complicated in detail,'but fascinating to study, since it contradicts the obviousexplanation and can unexpectedly show maximum separ-ation at only a few degrees from the horizontal, rather thanthe vertical position. This can be explained in terms ofthermal diffusion, causing a slight inhomogeneity in thehalogen concentration across the temperature gradient fromfilament to envelope, the axial convection velocity andmutual diffusion of the halogen and inert gasfilling. This issimilar to the separation mechanism for isotopes in aClusius and Dickel column, and the combination of theseeffects can give a powerful pumping action, separating theiodine within the gasfilling.x* The separation is stronglydependent upon the gasfilling density and burning angle. Itcan only be avoided by matching the molecular weights ofthe inert gasfilling and halogen to prevent thermal diffusion,which presents considerable practical difficulties in long lifelamps since it necessitates using the more active halogens,15

e.g. Br with Xe or Cl with Kr (Fig. 2). Of course, in somealternative embodiments of tungsten-halogen lamps,- vig-orous gas convection can occur which prevents separation,but with some slight reduction in the efficiency of thesource.

Fig. 2 Linear tungsten-halogen lamps containing bromine (addedasPNBrJ

Having an inert gasfilling of argon, krypton or xenon, after 1000 h'burning at 15° from the horizontalMolecular weights Ar: Br2, 40: 160, gives severe separation - topof tube has severe blackeningMolecular weights Kr: Br2, 84: 160, separation still evident - someblackening at top of tubeMolecular weight Xe: Br2, 131: 160, separation only slight - noblackening — this lamp is universal burning

3 Recent innovations

During the development of tungsten-halogen lamps manytechnological advances have been made in both construc-tional details and processing techniques; this has broadenedthe range of applications. Initially, it was thought that twinfilament lamps would be impractical because it was arguedthat tungsten would be transported from the cold to theincandescent filament. However, radio-chemical tracerexperiments soon showed that this difficulty could beavoided by locating the filaments so that they were effec-tively isolated within a convection cell. This innovation ledto the development of twin filament motor car headlamps;one form of the popular 12 V 60/55 W lamp embodies anovel flat molybednum-frame construction, rather than theusual round lead-in wires, which enables the filaments to be

mechanically clamped.16 This avoids the difficult tungstento molybdenum spot-weld, and is suited to high-speed pro-duction techniques (Fig. 3). In this case the molybdenumfilament shield is attached to the frame by a laser weld,giving the whole construction exceptional mechanicalstrength and resistance to impact. Twin filament lamps havealso been developed for colour television studios; in thisapplication, changes in light intensity can only be achievedover a limited range by voltage reduction, because thiscauses unacceptable changes in filament colour temperature.This problem has been obviated by incorporating twofilaments in a single envelope, having power ratings of\\ and 2j kW, giving the option of three levels of intensityat the same colour temperature.

Fig. 3 Twin filament motor car headlamp

Flat frame allows mechanical clamping of filaments to avoid em-brittlement at the weldThe shield is attached by a laser weld

A major problem exists when making hermetic sealsbetween metals and fused silica. This is because the expan-sion coefficient of fused silica is about an order less thanthat of the most suitable metal, molybdenum. The techniqueadopted in sealing electrical lead-throughs into fused silicais to embed a thin feather edged molybdenum foil into thevitreous material. The foil in this form is sufficiently ductileto deform, but not fracture, under the tensile stresses pro-duced by differences in expansion. Early high current sealsrequired an elaborate technique of first making individualseals and joining these to a large disc for multiple lead-throughs. A technique has recently been developed wheremultiple foils can be pressed in a single seal which has a cur-rent capacity of 48 A; this greatly simplifies the constructionof high current lamps (Fig. 4).

144 IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

t.\

n * !i ll

L i 1

!1 i

1!I

Fig. 4 Improved high current molybdenum foil seals

Originally, these were constructed as two individual legs sealed toa fused quartz discModern seals can be made as a single pressed seal, containing multiplefoils, and having a current carrying capacity of 48 A

At an early stage in the evolution of tungsten-halogenlamps it was realised that the compact filament, high source-brightness and extended life of these lamps offered con-siderable advantages over conventional incandescent lampsfor use in slide and cine projectors. Many projection lampshave been developed over the years but the combination oflamp and mirror has offered a unique advantage in providingboth light source and means of focusing onto the film gate.Such combined lamp and prefocused optical systems arenow available in several forms. Fig. 5 shows a 12 V 100W'Super 8 mm' projection lamp in a dichroic coated mirror.This mirror is coated with 23 layers of alternately high andlow refractive index material and is designed to reflect thevisible radiation but transmit the infra-red, reducing thetemperature at the film gate.

Many other innovations could be mentioned which haveextended the range of tungsten-halogen lamp applications,but it must be admitted that after twenty years of intensiveresearch and development, the performance of tungsten-halogen lamps has reached a plateau. The reason for this issimple; tungsten-halogen lamps containing iodine, bromineand chlorine have a transport mechanism which counter-acts the effect of radial diffusion of tungsten; however, it.does not prevent axial diffusion, so that eventually a localdefect occurs which burns out. This is a fundamental limi-tation which can only be overcome by counteracting bothradial and axial tungsten diffusion via a truly regenerativetransport cycle.

4 Regenerative transport cycle

Whilst there is no net loss of metal from the filament of atungsten-halogen lamp, axial transport occurs along the fila-

Fig. 5 Typical tungsten-halogen 'Super 8' projector lamp andmirror combination

The dichroic coated mirror reflects the light onto the film gate, buttransmits the infra-red radiation

ment. and tungsten returned by the halogen transport cycleis preferentially deposited at the cooler parts;17 Hence, thepredominant failure mechanism remains unchanged. Thereason for this is that the tungsten iodides, bromides andchlorides dissociate well below the normal operating tem-perature of the filament; however, tungsten fluoride (WF6)is stable up to ~ 3400 K, dissociating only at the filamentsurface, preferentially healing the hot-spots. This mechan-ism was first demonstrated by Schroder, when he passed acurrent through a non-uniform diameter tungsten wire in aninert atmosphere containing a low pressure of tungstenhexafluoride gas.18' 19>2° After a short period of burning heobserved that the diameter and temperature of the wirebecame uniform. From the thermodynamic considerationshe concluded that the dissociation of tungsten hexafluoridehad a much stronger dependance on temperature than thevapour pressure of tungsten. Hence, if a hot spot exists on afilament running in this atmosphere, the dissociation oftungsten hexafluoride, and resultant deposition of tungsten,occurs faster than the rate of tungsten evaporation so thatthe hot spot is healed. This regenerative transport cycle waslater confirmed by radio-chemical tracer experiments,17

these proved that tungsten does transfer from a cold region(e.g. tungsten on the envelope) to a high temperature region(e.g. filament hot spot) in the presence of a suitable fluoride.

From the results of such experiments it appeared that atrue steady-state condition, with no net transfer of filamentmaterial, could be achieved by a tungsten-fluorine transportcycle. However, in a practical lamp there is a considerabletemperature gradient from the centre of the filament to the

IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980 145

point where the lead-in wires (or filament tails) contact theenvelope. In this situation the essential envelope cleaningcycle is also accompanied by the transport of tungsten fromthe cold filament tails to the incandescent filament. Thisleads to gradual thinning and subsequent failure of the lampby so-called 'tail erosion'.

This problem, and the difficulty caused by the vigorousreaction of fluorine and fluorides with the fused quartzenvelope, has prevented the development of practicaltungsten-fluorine lamps. The early researchers recognisedthese problems, and through the late '60s and early '70s anumber of patents were filed21'22'23 suggesting a variety ofmeans of protection to avoid filament tail and envelopeattack. These included the use of lead-in wires fabricatedfrom, or coated with, fluorine resistant materials, speciallyshaped leadwires to provide excess tungsten at the criticaltemperatures for erosion, etc. It was also suggested that theinternal surfaces of the envelope could be protected bycoating them with stable fluorides such as MgF2 and CaF2 ;in some cases it was proposed that the coating should alsobe the source of fluorine. For instance, silver fluoride andcobalt fluoride were suggested21 since it was claimed thatthese materials were capable of taking up or releasing fluor-ine in a reversible manner and therefore would act as chemi-cal buffers against attack of the coldest parts of the filamentwhilst releasing fluorine at higher temperatures.

The most successful means to date of protecting thelamp envelope has been by the use of glassy films based onthe system of Al2 O3 —TiO2 —P2 O5. These are deposited insolution and after curing are converted to glassy pin-holefree film which are transparent, do not break down duringtemperature cycling between 300 and 900 K, and resistattack from fluorine and tungsten fluorides.24'25 It wasoriginally thought that these coatings were resistant inthemselves, but recent analytical evidence26 has led to theconclusion that the following reaction occurs between theglassy film and the fluorine in the lamp atmosphere:

2A12O3 + 6F2 ~> 4A1F3 + 3O2

Aluminium fluoride so formed acts as a passivation layerwhich protects the fused quartz envelope. Experimentalflat-grid filament projection lamps, internally coated withthis type of glassy film, gas filled with krypton and contain-ing fluorine, have been constructed in our laboratory. Theseoperate up to 150 h without observable envelope attack orblackening, whereas similar lamps without the protectivefilm blacken after only a few minutes running (Fig. 6). Therate of blackening of the unprotected lamps is much fasterthan may be predicted by normal evaporation and can beaccounted for by the following reaction between tungstenhexafluoride (WF6) and the fused quartz envelope (SiO2);

SiO2 + WF6 -» SiF4 + WO2 F2

The oxyfluorides then dissociate to give tungsten oxides,e.g.

2WO2 F2 -• W0F4 + WO3

which deposit on the lamp envelope. Also, the formation ofSiF4 effectively ties-up the fluorine so that it can no longertake part in a transport cycle. In consequence, a thickopaque layer is formed by the deposition of evaporatedtungsten over the tungsten oxide film.

Since fluorine is the most reactive halogen only a verysmall quantity is required to establish a regenerative trans-

Fig. 6 Tungsten-fluorine lamps

Those without protection of the fused quartz envelope blackenafter a few minutes running; lamps with the envelope internallycoated with a glassy layer of aluminium fluoride remain cleanthroughout their life.

port cycle (typically 25 jug). The amount must also be con-trolled within reasonable limits because an excess results inan overactive transport cycle, causing lamp failure by tailerosion, and insufficient fluorine allows the evaporationprocess to predominate, leading to envelope blackening.This was the reason for suggesting silver or cobalt fluorideas a protective coating in the early patents because theytake-up and release fluorine in a reversible manner, but inthe author's experience are not successful in providingadequate protection or control. The introduction of fluorineas a gaseous compound, such as NF3 and WF6, gave a varia-bility of ± 50% in halogen dose which proved to be anunacceptable variation. After much experimentation arepeatability of ± 15% has been achieved by introducing thefluorine as an inert involatile solid in solution in a volatilesolvent, this method has already been successfully employedfor bromine lamps12 and was adapted for this purpose byusing fluorinated polymer dissolved' in an appropriate sol-vent.27-28

As we have already discussed, the reactions that occur inany tungsten-halogen lamp are complex, involving a compli-cated system of chemical reactants and reaction products,including not only the tungsten and halogen, but gaseousand solid impurities (some deliberately added) such ashydrogen, oxygen, carbon and metallic impurities. Modelshave been proposed for the tungsten-fluorine system, basedupon thermodynamic equilibrium conditions and calcu-

146 IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

lations by Neumann29'30 using approximate data for manyof the tungsten fluoride species, which predict that asystem containing a 1:1 ratio of fluorine and oxygen will*obviate attack on the fused quartz envelope and prevent tailerosion. Unfortunately, our experiments do not confirmthese predictions and these data have been questioned byother authors who have carried out experimental measure-ments to obtain thernodynamic data for the tungsten-fluorine system based on mass transport31'32 and massspectrometry33 results.

However, the role of oxygen in a tungsten-fluorine lampis uncertain since it is extremely difficult to remove alltraces of oxygen during processing, or even to predict theamount that may be liberated during normal operation. Infact, one group of workers have deliberately omitted pro-cessing steps during lamp making in order to supply oxygento the system.34 They claim that tungsten-fluorine lampsmade in this way have a substantially improved life in com-parison with tungsten-bromine lamps, and show no attackon the unprotected fused quartz envelope, and very littletail erosion. In our opinion this experiment is inconclusivebecause the fluorine was added as gaseous CBrF3, whichthermally decomposes, producing BrF3 as the active fluori-nating agent. The stability of this compound, up to 2000 K,could account for the limited attack on the envelope andcold filament tails; also, it seems likely that the brominemay play a predominant role in preventing envelope black-ening rather than there being a genuine regenerative tungsten-fluorine cycle operating in the lamp.

Our own experiments using lamps having the envelopesprotected by a glassy aluminophosphate layer, and with thefluorine added as a fully fluorinated grease,27 showed anaverage increase in life of 45% in comparison with tungsten-bromine lamps. These also displayed a genuine hot spothealing mechanism but eventually failed by tail erosion.However, in this case the protective coating liberated someoxygen and the fluorinated grease contained 5% by weightof oxygen, plus carbon. Again, the role of the oxygen isuncertain but mass spectrometry of the gasfilling failed toidentify elemental oxygen, but definitely showed carbonmonoxide, thus suggesting that oxygen is gettered by thecarbon leaving tungsten and fluorine as the principal reac-tants.

These results are very encouraging; they confirm that aregenerative tungsten cycle can be made to operate byprotecting the envelope and controlling the fluorinequantity. The remaining obstacle to producing a practicallamp is tail erosion, which may eventually be solved bycontrolling the activity of the cycle by some form ofchemical buffering, as suggested in one of the first fluorinelamp patents.

To complete the review it should be mentioned thatexperiments have been carried out on carbon filamentincandescent lamps in which carbon-chlorine or carbon-fluorine transport cycle operates to return evaporatedcarbon to the filament.35 Since the melting point of carbonis some 200 K higher than tungsten this alternative seemsattractive, but suffers two severe disadvantages. Carbonis a grey body rather than a selective emitter like tungsten,so that, for a given temperature, a carbon filament radiatesless power in the visible region than a tungsten filament(about 25% lower luminous efficacy at 3000 K). Further-more, carbon has a much higher vapour pressure, or evapor-ation rate, than tungsten (e.g. 192 Pa and 0009 Pa, respect-ively, at 3000 K) and therefore has a shorter life at any

given temperature. The carbon-halogen transport cycleis scientifically interesting but unlikely to seriously challengetungsten-filament lamps.

5 Conclusion

Development in the technology and manufacturing processesof tungsten-iodine and tungsten bromine lamps has led tothe establishment of a wide range of sources. Although thefilament life of these is extended beyond that of a conven-tional incandescent lamp the ultimate failure mode isunchanged — burnout at a local hot spot. In recent timesprogress has been made towards producing a lamp havinga genuine self-healing filament, employing a tungsten-fluorine cycle, and experimental lamps have achieved a45% increase in life.

6 References

1 US Patent 254 780(1881)2 US Patent 1925 857 (1933)3 British Patent 807 137 (1959), (British version of U.S. Patent)4 COATON, J.R.: 'Modern tungsten-halogen lamp technology',

Proc. IEE, 1970, 117,(10), pp. 1953-19595 COATON, J.R., and REES, J.M.: 'Future of incandescent

and tungsten-halogen lamps', ibid., 1977, 124, (9), pp. 763-7676 COVINGTON, E.J.: The life-voltage exponents for tungsten

lamps',/, nium. Eng. Soc, 1973, 2 (2), pp. 83-917 HARVEY, F.J.: 'Failure of incandescent filaments by hot-

spot growth', J. Ilium. Eng. Soc, 1974, 3, pp. 295-3028 GEIJTENBEEK, J.J.F.: 'Calculations on the development of

hot-spots in straight wire tungsten filaments caused by axialmass transport'. Presented at 2nd International Symposium onIncoherent Light Sources, Enschede, Netherlands, 1979

9 GESZT1, T., and GAAL, I.: 'On the theory of the halogenlamp — Gas-controlled axial transport', Acta Tech. Acad. SciHung., 1974, 78, pp..479-488

10 GUPTA, S.K.: 'Thermodynamic and Kinetic aspects of brominelamp chemistry', J. Electrochem. Soc, Solid-State Scienceand Technology, 1978, 125, pp. 2064-2070

11GODDARD, V.W., and PETT, C: 'Kinetic investigation ofthe reaction between tungsten and bromine', /. Chem. Soc.Dalton Trans., 1975, pp. 767-771

12 REES, J.M.: 'Bromophosphonitrile lamps', Light. Res. & Technol.1970,2, pp. 257-260

13 US Patent 3 956 659 (1976)14 COATON, J.R., and PHILLIPS, N.J.: 'The influence of convec-

tion and thermal diffusion on halogen separation in verticalburning linear tungsten-halogen lamps', J. Phys. B, 1971, 4, pp.248-257

15 COATON, J.R., and PHILLIPS, N.J.: 'Universal burning lineartungsten-halogen lamps', Proc. IEE, 1971, 118, pp. 871-874

16 WOLFE, K.R.,: 'H4 Automobile lamp technology', Light. J.,1979, 20, pp. 5-7

17 HAIGH, I.: 'Thermochemical and related properties of halidesof refractory metals', Ph.D. thesis, University of Leicester, 1973

18 SCHRODER, J.: 'Chemical transport reactions at very hightemperatures using fluorine', Philips Tech. Rev., 1964, 25,pp. 359-364

19 British Patent 1019814 (1966)20 US Patent 3263113 (1966)21 British Patent 1047302 (1966)22 British Patent 1185663 (1970)23 British Patent 1185664 (1970)24 British Patent 1456242 (1973)25 ROTHON, R., and REES, J.M.: 'Fluorine resistant coatings for

use on silica envelopes of tungsten/fluorine lamps', Chem. &Industry, (1st July 1978), pp. 478-480

26 FITZPATRICK, J.R., and GODDARD, V.W.: 'Passivation layerof aluminium fluoride protects fused quartz lamp envelopesfrom attack by fluorine', 2nd International Symposium onIncoherent Sources, Enschede, Netherlands, (1979)

27 FITZPATRICK, J.R., and REES, J.M.: 'Progress towards apractical fluorine lamp', Lighting Research and Technology,1979 (to be published)

IEE PROC, Vol. 127, Pt. A, No. 3, APRIL 1980 147

28 British Patent Application (Thorn Lighting Ltd.) 1046/76 (1976)29 NEUMANN, G.M.: "Thermodynamics of heterogeneous gas

phase equilibria in the high temperature zone of the systemstungsten-fluorine and tungsten-fluorine - hydrogen', /. FluorineChem., 1971/72, 1, pp. 473-486

30 NEUMANN, G.M.: 'Thermodynamic analysis of the effect ofoxygen on the reaction equilibria in the high temperature zoneof the tungsten-fluorine system', ibid., 1973/74, 3, pp. 197-208

31 DITTMER, G., KLOPFER, A., ROSS, D. and SCHRODER, J.:'Transport reactions in the tungsten-fluorine system', /. Chem.Soc, D: Chemical Communications, 1973, pp. 846-847

32 DITTMER, G., KLOPFER, A., and SCHRODER, J.: 'Hetero-geneous reactions and chemical transport of tungsten withoxygen, fluorine, and fluorides of several metalloids', PhillipsRes. Rep., 1977, 32, pp. 341-364

33 HILDENBRAND, D.L.: 'Thermochemistry of the gaseoustungsten fluorides', /. Chem. Phys., 1975, 62, pp. 3074-3079

34 HILL, J.C. and DOLENGA, A.: 'Fluorine-cycle incandescentlamps',/. Appl. Phys., 1977, 48, pp. 3089-3092

35 VAN DER HOEK, W.J.: 'An evaluation of the carbon filamentincandescent lamp', Philips Res. Rep., 1976, 31, pp. 129-152

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