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a light source in which the conversion of electric energy into luminous energy occurs às a result of the heating of a refractory conductor by an electric current. Luminoús energy was first produced by this method in 1872 by the Russian scientist A. N. Lodygin, who passed an electric current through a carbon rod placed in a closed, evacuated vessel. In 1879 the American inventor T. A. Edison provided a sufficiently durable design of an incandescent lamp with a carbon filament that could be manufactured conveniently on an industrial scale. In 1898–1908 several metals (osmium, tantalum, and tungsten) were tested as incandescent bodies, and the use of incandescent lamps with a zigzag-shaped tungsten filament began in 1909. Incandescent lamps filled with nitrogen or inert gases (argon and krypton) appeared in 1912–13; the tungsten filament came to be manufactured in the shape of a helix (spiral). Further improvements in incandescent lamps were aimed at improving the luminous efficiency by increasing the temperature of the incandescent body, while preserving the service life of the lamp. The use of macromolecular inert gases with halogen additives to fill incandescent lamps made possible a reduction in contamination of the lamp bulb by particles of dispersed tungsten and a decrease in the rate of evaporation of the tungsten filament. The use of incandescent bodies shaped as a double helix (a spiral wound from a spiral) or triple helix reduced heat losses through the gas.
All the numerous varieties of incandescent lamps are made from standard parts, although the dimensions and the shape of the parts vary. The design of a typical incandescent lamp is shown in Figure 1. Inside the bulb the incandescent body (tungsten spiral) is attached to a glass or metal exhaust tube by holders made of molybdenum wire. The ends of the spiral are fastened to the ends of the leads. The middle part of the lead wires is made of platynite or molybdenum to produce a vacuum-tight seal with the glass stem. During vacuum processing the bulb of the lamp is filled with an inert gas; subsequently the exhaust tube is heat-sealed, forming a tip. To protect the tip and to facilitate the fastening of the bulb to a socket, the lamp is provided with a base fastened to the bulb with a sealing compound.
Incandescent lamps are classified according to area of use (lamps for general lighting, for headlights, and so on), according to their basic design and the lighting properties of the bulb (reflector lamps, ornamental lamps, and lamps with a diffusing finish), or according to the shape of the incandescent body (lamps with a flat spiral, a double helix, and so on). According to overall dimensions, incandescent lamps are classified as sub-miniature, miniature, small, standard, and large. For example, lamps less than 10 mm long and 6 mm in diameter are called subminiature, and large lamps are more than 175 mm long and 80 mm in diameter.
|Table 1. Luminous efficiency of certain lamps|
|Luminous efficiency (lumens per watt)||Notes|
|with carbon filament . . . .||2–3|
|with tantalum filament . . .||7||General lighting of buildings and transportation vehicles|
|with tungsten filament (vacuum) ........||8–9|
|with tungsten double helix (gas-filled, industrial krypton).........||12.5–13.5|
|with tungsten double helix (halogen lamps) .....||22–27||Special optical instruments|
|with flat tungsten helix (halogen lamps) ....||34.5||Small motion-picture projectors|
Incandescent lamps are manufactured for voltages from fractions of a volt to hundreds of volts and for power ratings up to dozens of kilowatts (kW). For example, a searchlight lamp rated at 10 kW is 475 mm long and 275 mm in diameter. An increase in the operating voltage of 1 percent relative to the rated voltage increases the light flux from an incandescent lamp by 4 percent but reduces its service life by 15 percent. Short-term connection of the lamp to a voltage exceeding the rated voltage by 15 percent causes breakdown of the lamp. The service life of an incandescent lamp ranges from 5 hr for aircraft lights to 1,000 hr for lamps used in the transportation industry; therefore, lamps should be installed in locations allowing easy replacement. The luminous efficiency of incandescent lamps depends on their design, voltage, and power and on the duration of service; it ranges from 10 to 35 lumens per watt. The values of luminous efficiency for several lamps of differing designs are given in Table 1 and 2.
|Table 2. Luminous efficiency of illumination lamps filled with krypton (for a service life of 1,000 hr)|
|Voltage||Power (W)||Light output (lumens per watt)|
The luminous efficiency of incandescent lamps is inferior to that of gas-discharge light sources. However, incandescent lamps are simpler to operate (they do not require starters and complicated fittings), and they have virtually no voltage and power limitations. The worldwide annual production of incandescent lamps is up to 10 billion; there are more than 2,000 types of lamps.
REFERENCESSkobelev, V. M. “Lampy nakalivaniia.” In Spravochnaia kniga po sveto- tekhnike. Moscow, 1956.
Ul’mishek, L. G. Proizvodstvo elektricheskikh lamp nakalivaniia, 5th ed. Moscow-Leningrad, 1966.
Gutorov, M. M. Osnovy svetotekhniki i istochniki sveta. Moscow, 1968.
V. M. SKOBELEV