Light Sources

radiators of electromagnetic energy in the visible region of the spectrum (or the optical region—that is, not only the visible but also the ultraviolet and infrared regions). The sun, moon, and stars, as well as atmospheric electrical discharges, are natural light sources; devices that convert energy of any type into the energy of visible or optical radiation are artificial light sources.

A distinction is made between thermal light sources, in which light is produced when bodies are heated to high temperatures, and luminescent light sources, in which light is produced as a result of the conversion of various types of energy directly into optical radiation, regardless of the thermal state of the radiating body. Artificial light sources may be subdivided into chemical, electrical, and radioactive types (according to the nature of the energy used) and into illumination, signaling, and other types (according to function). Each of these types may in turn be classified on the basis of various additional features, such as structural, industrial, and operational characteristics.

The first artificial light sources (campfires, splinters, and torches) appeared in remote antiquity. Until the end of the 19th century, thermal light sources based on the burning of combustibles (candles, oil and kerosine lamps, or incandescent mantles) were used. The radiation in these devices is created by extremely fine particles of solid carbon that have been heated in a flame or by incandescent mantles. They provide a continuous spectrum of radiation. Their luminous efficiency is very low; it does not exceed one lumen per watt (the theoretical limit for white light is about 250 lumens per watt).

The first practical electric light sources appeared at the end of the 19th century. The Russian scientists P.N. Iablochkov, V.N. Chikolev, and A.N. Lodygin made a major contribution to the development of these sources. In the early 20th century the incandescent electric lamp, by virtue of its economy, cleanliness, and convenience of operation, rapidly began to replace combustion light sources in all areas. A modern incandescent electric lamp is a thermal light source in which radiation is generated in a spiral from a tungsten filament heated to a high temperature (about 3000°K) by an electric current passing through it. Incandescent lamps are the most widely used light sources. Their luminous efficiency is 10–30 lumens per watt.

Gas-discharge light sources, which use the radiation of an electrical discharge in inert gases or in vapors of various metals, especially mercury, came into use in the 1930’s. According to their principle of operation they are luminescent light sources or mixed-radiation light sources—that is, luminescent and thermal. As a result of their higher efficiency of radiation and greater breadth of spectrum than incandescent lamps, they are used in illumination, signaling, and advertising. Luminescent lamps, in which the ultraviolet radiation of the mercury discharge is converted into visible radiation by means of phosphors, are used on a particularly wide scale for illumination. The luminous efficiency of modern luminescent lamps producing white light may be as high as 80–85 lumens per watt. In so-called electroluminescent panels the luminescence of powdered phosphors in a dielectric medium appears upon exposure to an alternating electric field. They are close in efficiency to incandescent lamps and are used primarily as light indicators, signal panels, and decorative elements.

In semiconductor light sources the luminescence appears during passage of a current. For example, gallium arsenide produces infrared radiation, and gallium phosphide and silicon carbide produce visible radiation. These light sources are used for special purposes; as yet their efficiency is low. In cathode-luminescent light sources the phosphor is excited by fast electrons (indicator tubes, electro-optical image converters, and cathode ray tubes).

In radioisotopic light sources the phosphor is excited by the radioactive decay products of certain isotopes, such as tritium. Such light sources do not require an external energy source and have a long life, but they produce small light fluxes of low brightness. In principle, chemoluminescent light sources, in which luminescence results from the conversion of the energy of chemical reactions into radiation (as is the case in the luminosity observed in the animal and plant world—deep-water fish and fireflies, for example), are possible.

Lasers, which produce coherent light beams of high intensity, exceptional frequency uniformity, and pinpoint directionality, are a totally new type of light source.