Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation.
This activity is reminiscent of the bacteria which provide light for the well-known luminescent organs of fish living in the greatest depths of the oceans, . . . an environment inimical to the multiplication of the more familiar bacterial species. Presumably animal tissues are able to provide a substrate which fuels the bioluminescent reaction at low temperatures.
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