Luminescence, the emission of light from causes other than high temperature. (The emission of light due to heat is called incandescence.) Luminescence may occur in a solid, liquid, or gas. A solid object that will become luminescent is called a phosphor.
One kind of luminescence is caused by a chemical reaction. An example is bioluminescence (luminescence in a living organism), such as that of a firefly. Another kind of luminescence is caused by an object being subjected to an outside source of energy. Thus the lining of a fluorescent tube produces light when it receives energy from ultraviolet radiation; barium sulfide glows when struck by X rays; the picture tube of a television set glows when struck by electrons; and an electroluminescent light panel gives off light when subjected to an alternating electric field.
Luminescent substances may be either fluorescent or phosphorescent. Fluorescent substances glow only while receiving energy. Phosphorescent substances continue to glow for a time after the energy has been cut off.
