Technology of Electric Lighting
If an electric current is passed through any conductor other than a perfect one, a certain amount of energy is expended that appears as heat in the conductor. Inasmuch as any heated body will give off a certain amount of light at temperatures over 525° C (977° F), a conductor heated above that temperature by an electric current will act as a light source. The incandescent lamp consists of a filament of a material with a high melting point sealed inside a glass bulb from which the air has been evacuated, or which is filled with an inert gas. Filaments with high melting points must be used, because the proportion of light energy to heat energy radiated by the filament rises as the temperature increases, and the most efficient light source is obtained at the highest filament temperature. Carbon filaments were employed in the first practical incandescent lamps, but modern lamps are universally made with filaments of fine tungsten wire, which has a melting point of 3410° C (6170° F). The filament must be enclosed in either a vacuum or an inert atmosphere, otherwise the heated filament would react chemically with the surrounding atmosphere. Using an inert gas instead of a vacuum in incandescent lamps has the advantage of slowing evaporation of the filament, thus prolonging the life of the lamp. Most modern incandescent lamps are filled with a mixture of argon and halogen gases or a small amount of nitrogen or krypton. Radical changes in incandescent lamp design have resulted from substituting compact fused-quartz glass tubes for glass bulbs.
Types of Lamps
Electric-discharge lamps depend on the ionization and the resulting electric discharge in vapours or gases at low pressures, if an electric current is passed through them (see Ion). Representative examples of these types of devices are the mercury-vapour arc lamp, which gives an intense blue-green light and is used for photographic and roadway illumination; and the neon lamp, which is employed for decorative sign and display lighting. In newer electric-discharge lamps, other metals are added to mercury and phosphor on the enclosing bulbs to improve colour and efficacy. Glass-like, translucent ceramic tubes have led to high-pressure sodium vapour lamps of unprecedented lighting power.
The fluorescent lamp is another type of electric-discharge device used for general-purpose illumination. It is a low-pressure mercury vapour lamp contained in a glass tube, which is coated on the inside with the fluorescent material, phosphor. The radiation in the arc of the vapour lamp causes the phosphor to become fluorescent. Much of the radiation from the arc is invisible ultraviolet light, but this radiation is changed to visible light if it excites the phosphor. Fluorescent lamps have several important advantages. By choosing the proper type of phosphor, the light from such lamps can be made to approximate the quality of daylight. In addition, the efficiency of the fluorescent lamp is high. A fluorescent tube taking 40 watts of energy produces as much light as a 150-watt incandescent bulb. Because of this illuminating power, fluorescent lamps produce less heat than incandescent bulbs for comparable light production.
One advance in the field of electric lighting is the use of electroluminescence, known commonly as panel lighting. In panel lighting, particles of phosphor are suspended in a thin layer of nonconducting material such as plastic. This layer is sandwiched between two plate conductors, one of which is a translucent substance, such as glass, coated on the inside with a thin film of tin oxide. With the two conductors acting as electrodes, an alternating current is passed through the phosphor, causing it to luminesce. Luminescent panels may serve a variety of purposes, for example, to illuminate clock and radio dials, to outline the risers in staircases, and to provide luminous walls. The use of panel lighting is restricted, however, because the current requirements for large installations are excessive.
A number of different kinds of electric lamps have been developed for such special purposes as photography and floodlighting. These bulbs are generally shaped to act as reflectors when coated with an aluminium mirror (see Optics). One such lamp is the photoflood bulb, an incandescent lamp that is operated at a temperature higher than normal to obtain greater light output. The life of these bulbs is limited to 2 or 3 hours, as opposed to that of the ordinary incandescent bulb, which lasts from 750 to 1,000 hours. Photoflash bulbs used for high-speed photography produce a single high-intensity flash of light, lasting a few hundredths of a second by the ignition of a charge of crumpled aluminium foil or fine aluminium wire inside an oxygen-filled glass bulb. The foil is ignited by the heat of a small filament in the bulb. Increasingly popular among photographers is the high-speed gas discharge stroboscopic lamp known as an electronic flash.
The earliest experiments in electric lighting were conducted by the British chemist Sir Humphry Davy, who produced electric arcs and who also made a fine platinum wire incandescent in air by passing a current through it. Beginning about 1840 a number of incandescent lamps were patented, but none were commercially successful until the American inventor Thomas Alva Edison produced his carbon-filament lamp in 1879. During the same period various arc lamps were introduced. The first practical arc lamp was installed in a lighthouse at Dungeness, England, in 1862. The American pioneer in electrical engineering, Charles Francis Brush, produced the first commercially successful arc lamp in 1878. Tungsten filaments were substituted for carbon filaments in incandescent lamps in 1907, and gas-filled incandescent lamps were developed in 1913. The fluorescent lamp was introduced in 1938.
Contributed by: Illuminating Engineering Society