Luminescence

Luminescence can be simply defined as any emission of visible electromagnetic radiation (light) not ascribable directly to incandescence. Incandescence emerges at high temperatures due to thermal radiation; in contrast, because luminescence is often independent of temperature, it is sometimes termed cold light. However, some kinds of luminescence (e.g., candoluminescence, pyroluminescence, thermoluminescence) are associated with or require elevated temperatures. Notably, luminescence is exhibited by primary light sources (which emit light) rather than by secondary light sources (which reflect or transmit light). Luminescence has also been defined to include all optical radiation, rather than being limited to visible radiation. In addition, it has been defined as a spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment.