Measurements are reported on the time required for an infra-red phosphor to acquire maximum brightness when exposed to infra-red radiation. This time becomes smaller as the infra-red intensity is increased, but there seems to be no simple relationship between the two quantities. The data given are for phosphors composed of strontium sulfide or selenide activated with europium and samarium. The brightness of strontium sulfide activated with cerium and samarium reaches its maximum value in something less than 10−4 sec., a time too short to be measured by the present apparatus.
Results on some samples seem to indicate the existence of two simultaneous processes. An appreciable fraction, often about one-half, of the final brightness is reached in less than 10−4 sec., while the ultimate value is approached in a much more leisurely fashion.
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