The energy is thengradually released by the surrounding thermal energy as visibleemission.2The effect is also called phosphorescence, after-glow, or long lasting phosphorescence (LLP). However, sincethe long lifetime of persistent luminescence is due to trappingof energy in defects, the term to be used for the phenomenon isnot phosphorescence.3On the other hand, the afterglow can beconsidered an unwanted phenomenon in luminescent materialssuch as those used for TVs and other displays. Thus, persistent∗