Devoir de Philosophie

Electric Lighting.

Publié le 11/05/2013

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Electric Lighting. I INTRODUCTION Electric Lighting, illumination by means of any of a number of devices that convert electrical energy into light. The types of electric lighting devices most commonly used are the incandescent lamp, the fluorescent lamp, the various types of arc and electric-discharge vapor lamps (see Electric Arc), and light-emitting diodes. II 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 (see Conductor, Electrical). Inasmuch as any heated body will give off a certain amount of light at temperatures above 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 (see Tungsten), which has a melting point of 3422°C (6192°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 or krypton and a small amount of nitrogen. Radical changes in incandescent lamp design have resulted from substituting compact fused-quartz glass tubes for glass bulbs. These new, stronger-walled bulbs have made tungsten-halogen lamps, a variation of the incandescent lamp, possible. Tungsten-halogen lamps use the regenerative cycle of halogens to return evaporated tungsten particles to the filament, thus extending the life of the bulb. The high temperatures required to take advantage of halogen's regenerative cycle made this idea impossible until the walls of the bulb could be made stronger by the introduction of quartz. These bulbs are filled with a mixture of argon and halogen (usually bromine) gases along with a small amount of nitrogen. III TYPES OF LAMPS Electric-discharge lamps depend on the ionization and the resulting electric discharge in vapors 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-vapor 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 color and efficacy. Glasslike, translucent ceramic tubes have led to high-pressure sodium vapor 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 vapor lamp contained in a glass tube, which is coated on the inside with a fluorescent material known as phosphor. The radiation in the arc of the vapor 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. See Luminescence. 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 aluminum 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 aluminum foil or fine aluminum 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. See Stroboscope. IV LIGHT-EMITTING DIODES Light-emitting diodes (LEDs) are devices that emit visible light when an electric current passes through them (see Diode; Electromagnetic Radiation). LEDs are made of semiconductors, or electrical conductors, mixed with phosphors, substances that absorb electromagnetic radiation and reemit it as visible light (see Luminescence). When electrical current passes through the diode the semiconductor emits infrared radiation, which the phosphors in the diode absorb and reemit as visible light. The visible emission is useful for indicator lamps and alphanumeric displays in various electronic devices and appliances. Organic light-emitting diodes (OLEDs) have the potential to replace incandescent and fluorescent lamps because of their greater energy efficiency and longer lives. Currently, OLEDs are up to 75 percent more efficient than incandescent lamps at similar brightnesses. OLEDs are presently used in cellular telephone displays and MP3 players. However, in 2006, scientists reported a breakthrough in OLED technology that could enable these light-emitting diodes to replace lamps and other types of lighting in homes and offices. Because they are made of wafer-thin layers of plastics and give off little or no heat, walls, ceilings, or even furniture could be used to light a room in place of the traditional lamp. The technology could achieve 100 percent efficiency in converting electricity to light. V HISTORY The earliest experiments in electric lighting were conducted by 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. None were commercially successful, however, both because the vacuum pumps of the time could not create a vacuum strong enough to protect the wire filaments and because electricity was expensive to obtain. In 1878 and 1879, British inventor Joseph Swan and American inventor Thomas Edison simultaneously developed the carbon-filament lamp. Improved vacuum pumps and the increased availability of electricity made these lamps a success. 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. See also Lamp. For most of the 20th century the incandescent light bulb was widely used for lighting in homes. More energy-efficient and longer-lasting fluorescent lamps were adopted for industrial and office use. In 1979 a compact fluorescent bulb that screwed into ordinary light fixtures was introduced. The compact bulbs used 75 percent less electricity and lasted 10 times longer than regular incandescent light bulbs. However, the compact fluorescent bulbs were also much more expensive and had only modest success with consumers. In 2007 the United States Congress passed the Energy Independence and Security Act, which included provisions that phase out the use of incandescent light bulbs because of their energy inefficiency. By 2014 incandescent bulbs will no longer be sold for home lighting or other uses. American consumers will have a choice of compact fluorescent bulbs or LED lighting fixtures. The use of compact fluorescent bulbs is seen as an interim solution because the bulbs contain mercury and so present a potential pollution hazard. Researchers are developing LED lighting fixtures that are brighter and more energy efficient. Improved LED lighting could be available by the time of the final phase-out of incandescent bulbs. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« successful arc lamp in 1878.

Tungsten filaments were substituted for carbon filaments in incandescent lamps in 1907, and gas-filled incandescent lamps were developedin 1913.

The fluorescent lamp was introduced in 1938.

See also Lamp. For most of the 20th century the incandescent light bulb was widely used for lighting in homes.

More energy-efficient and longer-lasting fluorescent lamps were adoptedfor industrial and office use.

In 1979 a compact fluorescent bulb that screwed into ordinary light fixtures was introduced.

The compact bulbs used 75 percent lesselectricity and lasted 10 times longer than regular incandescent light bulbs.

However, the compact fluorescent bulbs were also much more expensive and had onlymodest success with consumers. In 2007 the United States Congress passed the Energy Independence and Security Act, which included provisions that phase out the use of incandescent light bulbsbecause of their energy inefficiency.

By 2014 incandescent bulbs will no longer be sold for home lighting or other uses.

American consumers will have a choice ofcompact fluorescent bulbs or LED lighting fixtures.

The use of compact fluorescent bulbs is seen as an interim solution because the bulbs contain mercury and sopresent a potential pollution hazard.

Researchers are developing LED lighting fixtures that are brighter and more energy efficient.

Improved LED lighting could beavailable by the time of the final phase-out of incandescent bulbs. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

All rights reserved.. »

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