Luminescence

Other types of luminescence are defined by the source of the energy that causes the light emission. These include chemiluminescence, bioluminescence, electroluminescence, sonoluminescence, triboluminescence, and thermoluminescence.

Chemical reactions provide the energy to generate photons in chemiluminescence. These chemical reactions often involve oxygen.

Cyalume sticks are chemiluminescent: when you bend the flexible tube enough to break the barrier that separates two substances, the tube glows for several hours until the chemical reactions are completed. A method called enhanced chemiluminescent detection, developed by researchers in Paris, offers a non-radioactive way of keeping track of genes and is being used in the international Human Genome Project.

Bioluminescence is a subset of chemiluminescence in which the chemical change occurs in living things, such as fireflies. Such reactions are very efficient. They occur when a substance called a luciferin is oxidized with the aid of a catalyst called a luciferase.

Electroluminescent devices glow when a current is applied, although not because of chemical changes. In neon lamps, current causes electroluminescence of the gas in the tube. Another fun example of this is the pickle trick: if electrical contacts are connected to either end of a cucumber pickled in brine, then current passing through the ionized pickling salts glows. (It also smells very bad.) Many flat-panel displays, such as in laptop computers, are made of electroluminescent materials. (Although the most common type uses a fluorescent light to backlight a liquid-crystal mask.) Luminescence because of electron bombardment or an electrical field is related to electroluminescence. In fluorescent lamps, current ionizes the gas in the tube, and the ions activate a fluorescent coating on the inside of the lamps. The television example mentioned above is a case of cathodoluminescence, in which the phosphor is activated by a stream of electrons.

In sonoluminescence, the light is produced from energy provided by sound waves. This mechanism is fairly unusual. Recent research into sonoluminescence suggests that in some situations, sound may be concentrated to produce extremely high energies in small areas. This energy is dispelled as light, but it may be possible to harness that energy for other uses.

In triboluminescence, friction is responsible for the light. A famous example of this is the flash of light sometimes produced by crunching wintergreen flavor Life-Savers. (Do not confuse this with sparks given off by hitting flint and steel together. Those sparks, which can ignite a fire, are incandescent bits of metal.)