Gas-Discharge Light Sources

devices in which an electric current passes through gases or other substances (for example, mercury) in a vaporous state and electrical energy is converted into visible radiation.

While doing research on arc discharge in 1802, the Russian scholar V. V. Petrov noted the light phenomena accompanying it. In 1876 the Russian engineer P. N. Iablochkov invented the AC carbon arc lamp, setting the basis for the practical application of electric discharge in lighting. The development of gas glow tubes took place between 1850 and 1910. In the 1930’s intensive research was undertaken on the application of luminophores in gas glow tubes. A group of scientists and engineers at the Physics Institute of the Academy of Sciences of the USSR, the Moscow Electrical Lamp Factory, and the All-Union Electrotechnical Institute carried out research, development, and production of gas-discharge light sources in the Soviet Union beginning in the 1930’s. The first models of mercury vapor lamps in the USSR were made in 1927; the first gas glow lamps, in 1928; and the first sodium vapor lamps, in 1935. Fluorescent lamps in the USSR were developed in 1938 by a group of scientists and engineers led by academician S. I. Vavilov.

Gas-discharge light sources consist of a glass or ceramic enclosure, or of a metallic enclosure with a transparent window. The enclosure may have a cylindrical, spherical, or some other shape. It contains a gas and sometimes a certain amount of metal or some other substance (for example, a halide salt) with a sufficiently high vapor pressure. The electrodes between which the discharge takes place are mounted on (for example, soldered to) the enclosure, sealing it hermetically. Some gas-discharge light sources, such as the carbon arc, have electrodes that operate in the open air or in a gas stream.

There are several types of light sources: gas glow lamps, in which the visible radiation is created by excitation of atoms, of molecules, and of recombining ions and electrons; fluorescent lamps, in which the source of radiation is luminophores that are excited by the radiation of a gas discharge; and incandescent lamps, in which the radiation is created by electrodes heated by a discharge.

The majority of gas-discharge light sources use the radiation of a positive column of an arc discharge (more rarely, of a glow discharge; for example, in gas glow tubes). Flash tubes use a spark discharge that is transformed into an arc discharge. Arc discharge lamps may be low pressure (from 0.133 newtons per sq m [10^-3 mm mercury]), for example, the sodium vapor lamp; high pressure (from 0.2 to 15 atmospheres [1 atmosphere = 98,066.5 newtons per sq m]); or superhigh pressure (20 to 100 atmospheres or more), for example, xenon discharge lamps.

Gas-discharge light sources may be used for general lighting, irradiation, signaling, and other purposes. High light output, proper color, and simplicity and reliability of operation are important when gas-discharge light sources are used for general lighting. Fluorescent lamps are the most popular gas-discharge light sources for general lighting. The output of fluorescent lamps ranges up to 80 lumens per watt, and the useful life can be 10,000 hours or more. Low-pressure sodium vapor lamps with an output of up to 140 lumens per watt are used to light suburban and rural highways. City streets are lighted by high-pressure color-corrected mercury vapor lamps. Various features may be required of gas-discharge light sources in specialized applications. Luminosity and color are important, for example, in superhigh-pressure xenon lamps for movie-making equipment. Spectral composition and power are important in immersible mercury-thallium lamps used in industrial photochemistry. Power and a spectral composition identical to the sun’s rays are important in metal-enclosed xenon lamps used to imitate solar radiation. Amplitude and duration of illumination are important features in flash tubes for high-speed photography, stroboscopy, and the like.

The prospects for further development and application of gas-discharge light sources are being broadened by work on new high-temperature and chemically resistant materials for the enclosure of lamps and by the discovery of technological processes for introducing highly volatile compounds into the lamp as radiant elements. For example, a mercury vapor lamp with thallium, sodium, and indium iodides added has a light output of up to 80-95 lumens per watt and good color emission. The light output of a high-pressure sodium vapor lamp may be as much as 100-120 lumens per watt; this lamp was created by using an enclosure of high-temperature ceramics composed basically of aluminum oxide.