Luminescence is light that usually occurs at low temperatures, and is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light generated by high temperatures.

The excitation is usually achieved with ultraviolet radiation, X rays, electrons, alpha particles, electric fields, or chemical energy. The colour, or wavelength, of the light emitted is determined by the material, while the intensity depends on both the material and the input energy.