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