Does the intensity or color of the light emitted by the object after exposure to light depend on the intensity of the incoming light?
time of exposure to the incoming light?
energy of the incoming light? (Try UV and IR as well as various colors in the visible.)
temperature of the object? (If you can, try very low temperature (dry ice or liquid nitrogen.)
If you have a knowledge of energy bands, create a possible
energy band diagram for the phosphorescent materials. Then sketch
the electron transitions on your model.