The mechanism underlying this phenomenon is not fully understood. However, the phenomenon of persistent luminescence must not be mistaken for fluorescence and phosphorescence (see for definitions  and ). Indeed, in fluorescence, the lifetime of the excited state is in the order of a few nanoseconds and in phosphorescence, even if the lifetime of the emission can reach several seconds, the reason of the long emission is due to the deexcitation between two electronic states of different spin multiplicity. For persistent luminescence, it has been known for a long time that the phenomenon involved energy traps (such as electron or hole trap) in a material which are filled during the excitation. After the end of the excitation, the stored energy is gradually released to emitter centers which emit light usually by a fluorescence-like mechanism.