JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 37, NUMBER 5 MAY, 1947

Letters to the Editor

Notes on the Stability of Phosphors under Ultraviolet Irradiation

GEORGE MEISTER AND RUDOLPH NAGY

Research Department, Westinghouse Electric Corporation, Lamp Division, Bloomfteld, New Jersey

March 12, 1947

THE main issues to be considered are: (a) Is the presence of mercury vapor necessary in the de­

terioration of a phosphor? (b) What ultraviolet radiations effect deterioration of a phosphor?

We maintain that phosphor deterioration can be effected in the absence of mercury vapor with radiation below 2000A, while Froelich contends in his first paper1 that the decline of lumen output of phosphors is caused by con­densed and adsorbed metallic mercury and mercury com­pounds but that the phosphor is unchanged by either excited mercury or ultraviolet radiation, and in his second paper2 he concludes that lamp depreciation is funda­mentally of photo-chemical nature involving excited mercury atoms caused by 2537A.

Our paper3 definitely shows that a well-baked 3500° white phosphor deteriorates when irradiated with a quartz mercury lamp in the absence of mercury, but Froelich maintains that an unbaked phosphor contains enough adsorbed gases to cause the reported losses in brightness. Therefore tests were made, and they revealed that on well-evacuated and well-baked phosphors, and in the absence of mercury vapor, deterioration only occurred when the phosphor was irradiated with a quartz mercury discharge lamp which is comparatively rich in 1849A but not when Vycor mercury discharge lamp was used which transmits practically no 1849A but an abundance of 2537A. The question of minute amounts of oxygen or other gaseous impurities has been anticipated and was investigated. A quartz bulb was half-coated with a normal zinc beryllium silicate. A charcoal trap was attached to the bulb and then both were well exhausted. The bulb was then exposed to a seasoned quartz lamp. During the entire test the charcoal trap was immersed in liquid nitrogen. The phosphor de­teriorated in the normal manner indicating that oxygen or other gas impurities were not involved in the loss of output. A similar test using a Vycor bulb did not show deterioration thus conclusively proving that radiations less than 2000A are the chief cause of phosphor deteriora­tion in fluorescent lamps and that the process is photo­chemical in nature.

I t will be recalled from the publication3 that the purpose of using Sterilamps was to demonstrate that the de­terioration of phosphors was proportional to the 1849A output. Since the germicidal lamps produce a great deal of 1849A radiation when they are new, loss of output of the phosphors shown by Froelich in his second paper clearly indicates he was dealing with the short radiations and not 2537A.

Froelich mentions that if short ultraviolet radiation

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produces photolysis then it should be observed to an even greater extent with argon and neon. The experiments in this field have been very limited. Also, the amount of fluorescence produced by these rare gases is exceedingly small so that experiments would have to be conducted over a very much longer time before definite conclusions can be drawn on this subject. In fact, preliminary experiments here indicated a deterioration with the rare gases.

We know that in the presence of oxygen 1849A produces ozone and also that 2537A destroys it, but since we have observed this photo-chemical effect under good vacuum conditions in the absence of mercury, we still maintain that radiations below 2000A are the chief cause of de­terioration and that secondary chemical effect, with or without mercury or oxygen, as expressed by Froelich plays only a minor role.

1 H. C. Froelich, Trans. Am. Electrochem. Soc. 87, 365 (1945). 2 H. C. Froelich, J. App. Phys. 17, 573 (1946). 3 G. Meister and R. Nagy, J. Opt. Soc. Am. 36, 696 (1946).