Phosphorescence is a kind of photoluminescence (i.e., a kind of light emission of a medium) which lasts relatively long after excitation of the medium. The excitation energy is stored in metastable electronic states (often triplet states), exhibiting only forbidden transitions to lower states. The stored energy can be released only through relatively slow processes, which are often thermally activated.
Important phosphorescent materials are phosphorus, europium-doped strontium aluminate (Eu:SrAl2O4) and zinc sulfide (ZnS). They are used e.g. in safety products such as exit signs, which are visible even under conditions of power failure.
Fluorescent lamps also exhibit a low level of phosphorescence for some time after being switched off.
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