Light generation by a process other than by heating is luminescence. For example, an incandescent light bulb, in which the filament is heated until it is literally white-hot, is not luminescent; a fluorescent light tube (which is cool to the touch) is luminescent. Luminescence is generated as part of a process in which atoms or molecules with electrons excited into higher energy states shed energy by emitting visible light.

People have observed luminescence in nature for centuries. In the early twentieth century, Marie Curie, in her doctoral thesis, mentioned that calcium fluoride glows when exposed to the radioactive material, radium. In the past 50 years, the use of luminescent devices, such as fluorescent lights and television screens, have become widespread.

Luminescence can be divided into categories by duration (fluorescence or phosphorescence) or by the mechanism that creates the light. By definition, fluorescent things stop emitting light very soon (about...