By definition photoluminescent objects emit visible light when light, visible or UV, is shined upon them. Two broad categories are phosphorescence (glow-in-the-dark) and fluorescence. The tutorial Modeling Light Emission by Fluorescent Lamps explores that type of emission. Here we will consider phosphorescence.
Photoluminescent materials such as the phosphor coating found on fluorescent tubes, fluorescent minerals, and phosphorescent toothbrushes contain many solid atoms. As a result, these materials have valence and conduction energy bands that are separated by an energy gap. As we have seen, photoluminescent solids also contain many impurity atoms which result in the formation of a band of energy levels found inside the energy gap of the solid material.
As a brief review, the animation represents an energy band
diagram for the phosphor coating found in a typical white
fluorescent lamp that emits visible light.