JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Causes of Early Loss of Light Output of Fluorescent LampsGEORGE MEISTER AND RUDOLPH NAGY

Research Department, Westinghouse Electric Corporation, Bloomfield, New Jersey

To determine the cause of loss of light output of fluorescent lamps the photo-decompositionof zinc silicate, zinc beryllium silicate, calcium tungstate, magnesium tungstate, and calciumcerium phosphate phosphors has been studied in a vacuum, air, and inert gases. The loss oflight output has been shown to be caused by the photolysis of the commercial phosphors whichis in contrast to the general theory in which the loss has been attributed to a film formation onthe fluorescent particles. The photolysis of the silicate and tungstate phosphors is mainlycaused by radiations less than 2000A. The calcium cerium phosphate is most sensitive tothese radiations. The photochemically active center in the silicate phosphors appears to bethe manganese atom. The tungsten and cerium atoms may be involved in the tungstate andphosphate phosphors, respectively.

INTRODUCTION

THE loss of light output for the first hundredhours of operation of a fluorescent lamp

runs 10 to 15 percent. The depreciation of lightoutput for the next 2000 hours of operation isonly of the same order of magnitude as the firsthundred hours. The cause for this loss of light,especially during. the first hundred hours, hasnever been fully explained. Davis, Ruff, andScott' attributed the loss to a film formation onthe surface of the fluorescent particles whichprevented their full excitation by the discharge;others believed the film was finely divided mer-cury or deposits from the electrodes.2

Since mercury is used in fluorescent lamps,these deductions were reasonable. However, itwas also known that the ultraviolet radiationsfrom the mercury discharge, especially at 1850A

FIG. 1. Exposure chamber with phosphor disks beingirradiated. In the foreground an evacuated quartz tubepacked with various fluorescent compounds before ex-posing to ultraviolet radiations.

1 L. J. Davis, H. R. Ruff, and W. J. Scott, "Fluorescentlamps," Part II, J.I.E.E. 89, 447 (1942).

2 G. E. Inman, "Characteristics of fluorescent lamps,"Trans. I.E.S. 34, 65 (1939). H. C. Froelich, "Chemical andphysical stability of silicate phosphors," Trans. E.C.S. 87,365 (1945).

and 2537A, were present. Therefore, it wasdecided to investigate this radiation effect ofthe mercury discharge exclusive of any mercuryvapor. Since the completion of this work, Froelich2

has published that these short radiations haveno photochemical effect but that the loss offluorescence is mainly caused by a film of mer-cury or mercury compounds on the phosphor.All of his tests were conducted in the presenceof mercury vapor.

Since most chemical reactions which proceedwith measurable velocities require approximately20,000-60,000 calories of energy per mol, andthe energy equivalent for radiations at 2000A is142,000 calories per mol, it would seem thatthese short ultraviolet radiations have sufficientenergy to initiate almost any chemical reactions.3

Photochemical reactions are numerous but

FIG. 2. An experimental lamp for the exposing ofphosphors in vacuum to radiations from a quartz lamp.The quartz window in the center of the lamp is for themeasurement of the intensity of ultraviolet radiations.

3 A. B. O'Day and R. F. Cissell, "Fluorescent lamps andtheir applications," Trans. I.E.S. 34, 1165 (1939). A. D. S.Atkinson, Fluorescent Lighting (George Newnese, Ltd.,England, 1944). Farrington Daniels, Chemical Kinetics(Cornell University Press, Ithaca, New York, 1938).

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DECEMBER, 1946VOLUME 36, NUMBER 12

LOSS OF LIGHT OUTPUT

FIG. 3. Light outputof fluorescent-jacketedquartz lamp.

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most of them have been done in air, mercuryvapor, in gases, liquids, and solutions but not ininert gases or vacuum, and comparatively littleis found in the literature on photolysis in thesolid state. 4

One of the compounds that had been thor-oughly investigated was zinc sulphide which to-gether with cadmium sulphide was used as acolor corrective for the high pressure mercurydischarge lamps.5 The zinc sulphide phosphorsdeteriorate more rapidly than most of the presentphosphors. 6 O'Brien7 as early as 1915 has sum-marized the research on the loss of fluorescenceof zinc sulphide and has attributed it to a photo-decomposition. Gloor 5 studied the photolysis ofzinc sulphide in the absence of oxygen and at-tributed the blackening of the compound to theliberation of zinc atoms. Baur9 stated that

I G. K. Rollefson and M. Burton, Photochemistry (Pren-tice-Hall, Inc., New York, 1942). W. A. Noyes and P. A.Leighton, The Photochemistry of Gases (Reinhold PublishingCorporation, New York, 1941).

5 H. Krefft and K. Larche, "High pressure mercurylamps with phosphors," Das Licht 8, 133 (1938).

6 Protective and Decorative Coatings, Vol. III, J. Mattiello,editor (John Wiley & Son, Inc., New York, 1943), Chapter22A, p. 68.

7 W. J. O'Brien, "A study of lithopone," J. Phys. Chem.19, 113 (1915).

8 Karl Gloor, "Photolysis with zinc sulfide," Helv.Chim. Acta. 20, 853 (1937).

9 Emil Baur, "The chemistry of phosphorescence of zincsulfide," Helv. Chim. Acta. 20, 878 (1937).

weighable amounts of zinc were produced in thephotolysis reaction. Recently Gordon, Seitz, andQuinlan0 have studied the mechanism of theblackening of zinc sulphide, and also reported thatzinc atoms were produced. According to Eibner"and Seitz12 the blackening occurs only in thepresence of moisture. 3

Since there is no literature on the photolysisof silicate, tungstate, and phosphate phosphorsin the absence of mercury and in view of itsimportance in the fluorescent lamp industry itwas decided to investigate the' compounds forphoto-decomposition.

EXPERIMENTAL

The phosphors that were used in the followingexperiments were prepared by the conventionalmethod as described in the literature.'4 The

10 N. T. Gordon, F. Seitz, and F. Quinlan, "The blacken-ing of zinc sulphide phosphors," J. Chem. Phys. 7, 4 (1939).

11 A. Eibner, "Lithopone," Farben Zeit. 30, 2600 (1925).12 F. Seitz, "The darkening of materials by light,"

J. App. Phys. 13, 639 (1942).13 C. Ellis and A. Wells, The Chemical Action of Ultra-

violet Rays (Reinhold Publishing Corporation, New York,1941).

14 Preparation of Phosphors: zinc silicate-Brit. Pat.457,126 Nov. 23, 1936; magnesium tungstate-Brit. Pat.469,732 July 27, 1937; zinc beryllium silicate-U. S.Pat. 2,103,085; calcium tungstate-Brit. Pat. 494,299Oct. 20 (1938); calcium cerium phosphate-U. S. Pat.2,306,567.

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purified ingredients were mixed in an agatemortar and fired for the proper length of timein platinum dishes in an electrically heatedfurnace. The magnesium tungstate and the zincberyllium silicate phosphors were generally ob-tained from the phosphor manufacturing de-partment.

The phosphors to be tested for photo-decom-position were pressed with a spatula into aholder made from a 1-inch diameter copperring cemented onto glass. A number of theseholders could be placed horizontally into a tubeat one time (Fig. 1) and exposed to ultravioletradiations.

The ends of the exposure chamber were closedso that the photo-decomposition could be con-ducted in 99.7 percent argon, pure nitrogen, orin air.

Quartz lamps of the "Sterilamp"* ultraviolettype were employed when both 1850A and 2537Aradiations were desired, and standard Sterilampwhen only the latter radiation was wanted.A platinum photo-cell was used to measureradiations of less than 2000A and a tantalumphoto-cell for radiations between 2900A and2200A. The photo-cells were connected to a

* Trade mark Reg. U.S. Pat. Off.

Rentschler clickmeter15 which gave the relativeintegrated energy for each of the two spectralregions.

The measurement of the fluorescent output ofthe phosphors was made with a barrier-layer cellof the selenium class. The sample was placedunder a circular quartz mercury lamp and thefluorescent as well as the reflective output ofthe sample was measured by alternately exposingthe phosphor to direct ultraviolet and glass-shielded ultraviolet. This method is similar tothe one described by Marden and Meister.6

The difference between these two readings wasthen taken as the ultraviolet response. Allmeasurements were made on a sensitive micro-ammeter.

EXPOSURE OF PHOSPHOR IN VACUUM

The preliminary radiation effects were madeon 3500'K white phosphor which is a mixtureof zinc beryllium silicate and magnesium tung-state. This phosphor was placed in a vacuum onthe inner wall of a glass jacket surrounding aquartz ultraviolet lamp (Fig. 2). In order toprevent even a trace of mercury from entering

15 H. C. Rentschler, "An ultraviolet light meter," J.A.I.E.E. 49, 113 (1930).

1G J. W. Marden and G. Meister, "Effects of impuritieson fluorescent compounds," Trans. I.E.S. 34, 503 (1939).

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the outer chamber the evacuation was performedon a mercury free system. The lamp was designedso that the intensity of ultraviolet radiationscould be measured periodically through a quartzwindow, with a tantalum photo-cell. This wasto determine whether the loss of output couldbe caused by a decrease in the amount ofactivating radiations. One-half of the phosphoron the jacket was exposed directly to the ultra-violet radiations while the other half was shieldedby placing a Pyrex glass sleeve over that portionof the lamp so that the latter received onlyvisible radiations. To determine the intensity offluorescence of the control portion, the lamp wastilted so that the sleeve moved to the irradiatedend. The fluorescent intensity was measured witha footcandle meter. The results in Fig. 3 definitelyshow that the ultraviolet radiation decomposesthe phosphor and that mercury or electrodedeposits play only a very minor role. The smallloss of output of the control phosphor protectedby the Pyrex glass sleeve was probably caused byscattered radiations from the other half of thelamp during operation.

Since this test did not differentiate betweenthe effects from 1850A and 2537A radiation, andalso did not show which radiation contributedmost to the photolysis, further experiments were

FIG. 5. Exposure ofphosphors to radia-tions from a standardSterilamp.

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conducted on the individual phosphors withquartz, and standard Sterilamps.

PHOTO-DECOMPOSITION IN VARIOUSGAS ATMOSPHERES

By exposing the phosphors in air to the ultra-violet radiation and measuring the intensity offluorescence periodically, the rate of photo-de-composition was obtained. In Figs. 4 and 5 are afew typical curves obtained by exposing phos-phors to radiations from a quartz ultravioletlamp and a standard Sterilamp. Curves for therate of photolysis in argon or nitrogen are notgiven since they are nearly identical to thoseobtained by exposing in air. The occluded airon the phosphors cannot be readily removed bythe inert gases, and therefore, the rate of pho-tolysis is only slightly less than in air.

The results of these tests can be summarizedas follows:

1. The silicate, tungstate, and phosphate phosphors arephotodecomposed by ultraviolet radiations.

2. Only the short radiations, less than 2000A, are photo-chemically active. The total amount of 2537A radiation inboth experiments was approximately the same whereasthe short radiations for the standard Sterilamp were only10 percent of those in the quartz ultraviolet lamps.

3. The rate of decomposition of silicate phosphors isslightly more in air than in nitrogen or argon indicatingthat oxygen may be involved in the reaction.

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4. The manganese atom is involved in the photochemicalreaction. It can be shown that by titrating with hydriodicacid that the manganese is oxidized from a valence of twoto some higher valence. No zinc or appreciable quantitiesof free zinc oxide could be found by titrating withdilute acid.

PHOTO-DECOMPOSITION IN A VACUUM

Not many phosphors were tested by themethod utilizing an evacuated jacket because ofthe difficulty of making such a lamp. To test alarge number of phosphors at one time a secondmethod was used. A thin-walled quartz tube,packed with phosphors, was evacuated, bakedfor one-half hour at 4750 C and then sealed fromthe exhaust system. The tube containing thephosphors was exposed to radiations by placingit alongside a quartz type of Sterilamp. The in-tensity of fluorescence was measured with thesame apparatus as previously described.

The results shown in Fig. 6 confirm the lampexperiment in that the phosphors are photo-chemically decomposed in a vacuum. The man-ganese atom is also involved under these con-ditions just as was found when the reaction wasconducted in air. In this instance, the manganesewas apparently reduced instead of oxidized asmeasured by the hydriodic acid titration. No zincor zinc oxide was found that could be attributedto the photochemical reaction.

FIG. 6. Exposure ofphosphors in an evac-uated quartz tubeto radiations from aquartz lamp.

Phosphors in the evacuated tube exposed tostandard Sterilamp decomposed only to a verysmall extent. For a given amount of decomposi-tion under a quartz type Sterilamp and a stand-ard Sterilamp the ratio of the time required toproduce the same amount of photolysis was ofthe same order as the ratio of their platinumphoto-cell readings, which cell responds to radia-tions shorter than 2000A.

DISCUSSION

The photochemically active center of zincsilicate and zinc beryllium silicate phosphorsappears to be the manganese atom. The color ofthe phosphor exposed to short radiations in avacuum gradually becomes darker, suggesting themetallic manganese may be liberated. Upon ad-mitting air into such a tube or a fluorescent lampthe phosphor becomes whiter and the intensity offluorescence increases. The manganese appar-ently is oxidized by the oxygen of the air anddoes not appreciably absorb visible radiation.When the photochemical reaction is conductedin air, the liberated manganese is immediatelyoxidized by the oxygen to some higher form ofoxide presumably Mn2O3. The darkening of thesilicates does not appear to depend upon thepresence of water as reported for the zinc sulphidephosphors.", 12

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RP3DUCING COATINGS

The photochemically active centers in thetungstates and calcium cerium phosphate cannotbe determined at this time but from the litera-ture 3 it would appear that the tungsten andcerium atoms play the major role in the decom-position of these phosphors.

The penetration of the short ultraviolet radia-tion 1850A, into a silicate phosphor has beenfound to be very small and, therefore, the photo-chemical action only occurs on the surface.Heating the darkened phosphor to 500'C forone-half hour will almost entirely restore thefluorescence, while at 10000 C the phosphor willbe completely reactivated. The photochemicalchange is not only a rearrangement of electronsor atoms of the phosphor, but also a decom-position similar to that found in silver salts andsolarization of glasses. Heating causes a recom-

bination of the elements, a reaction similar tothe formation of the original phosphor.

CONCLUSIONS

The loss of light output of commercial phos-phors is mainly caused by photolysis of thefluorescent particles.

The photolysis of the phosphors is largelycaused by very short wave radiations of less than2000A.

In the zinc silicate and zinc beryllium silicatephosphors the manganese atom is involved inthe photolysis.

The photolysis and blackening of the silicateand tungstate phosphors can occur in vacuumand apparently does not depend upon moistureas does the zinc sulphide phosphor.

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA \TOLUME 36, NUMBER 12 DECEMBER, 1946

German Reflection Reducing Coatings for Glass

HOWARD A. TANNER* AND LUTHER B. LOCKHART, JR.Naval Research Laboratory, Anacostia Station, Washington 20, D. C.

(Received October 3, 1946)

INTRODUCTION

IN the course of a postwar technical investi-gation of the German optical industry, infor-

mation and samples were obtained relative to theproduction of reflection reducing coatings onglass. The samples have been compared withcurrent domestic coatings for abrasion resistanceand reflection characteristics.

A. COATING METHODS

The Germans employed three general methodsof coating: a. vapor deposition in high vacuum,b. deposition from solution using a centrifuge,and c. deposition by reaction of gases at theglass surface. Both mono-layer and multi-layercoatings were produced, the latter on an experi-mental scale only.

* Formerly on temporary duty with the 'U. S. NavalTechnical Mission in Europe.

1. Vapor Deposition Process

A vapor deposition process similar to thoseused in this country was demonstrated by Dr. A.Smakula at the Carl Zeiss plant in Jena. The glasselements to be coated were placed in holes ofthe proper size in a parabolic metal dome whichwas supported on three glass legs above a heavyglass base plate. A small tungsten boat forvaporizing cryolite was mounted between twoelectrodes at the center of the base plate.A horizontal metal disk, with a hole in thecenter for passage of the cryolite vapor, wasmounted slightly above the vaporizer. Thedistance from the cryolite vaporizer to the workwas 35 cm at the center of the dome and about3 cm less at the edge of the dome. The dome was35 cm in diameter. The necessary electrical andvacuum line connections were made through thebase plate.' A carefully weighed amount ofcryolite was placed in the vaporizer, an open

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