Electric Light

The word lamp can refer either to a source of electric light, such as a bulb, or to the appliance that holds the source. This article discusses sources of electric light, and it uses the word lamp to refer to a light source. There are two main kinds of lamps: (1) incandescent and (2) gaseous-discharge. For information on the uses of electric light, see the article on Lighting.

The amount of light given off by an incandescent lamp depends on how much electricity it uses. Most home lamps operate on 120 volts and use from 40 to 150 watts of power. Lighting engineers measure the amount of light given off by a lamp in units called lumens. An ordinary 100-watt lamp produces about 1,750 lumens. The number of watts a lamp uses is printed on the bulb.

Every incandescent lamp has three basic parts: (1) the filament, (2) the bulb, and (3) the base. The filament produces the light. The bulb and the base help it do its job.

The filament is a thin, coiled wire. Electricity flows through the wire when the lamp is on. But the electricity must overcome resistance in the filament. In doing so, the electricity heats the filament to more than 4500 ºF (2482 ºC). The high temperature makes the filament give off light.

Lamp manufacturers make filaments from tungsten, a strong metal that can withstand high temperatures without melting. The light from a tungsten filament is a mixture of all the colors of light found in sunlight.

Some lamps have more than one filament. These filaments may be turned on individually so that the lamp produces different amounts of light. For example, a lamp may have a 50-watt filament and a 100-watt filament. Depending on whether the filaments are lit separately or together, the lamp can produce light from 50, 100, or 150 watts of electricity.

The bulb keeps air away from the filament to prevent it from burning up. Most bulbs contain a mixture of gases, commonly argon and nitrogen, instead of air. The gases help lengthen the life of the filament and prevent electricity from jumping inside the lamp.

The bulb also is coated to diffuse (scatter) light from the filament and reduce glare. Manufacturers coat the inside of the bulb with silica or etch (cut) it with acid. Colored lamps have a coating that filters out all but the desired color of light. Manufacturers produce bulbs in many shapes, including flame-shaped, pear-shaped, round, and tubular bulbs.

When most incandescent lamps burn out, the cause is usually the slow evaporation and eventual breakage of the filament. Before the filament breaks, currents of gas within the bulb spread the evaporated tungsten over the bulb's inside surface. The evaporated tungsten forms a dark deposit on the surface called bulb-wall blackening. This deposit blocks some of the light.

One kind of lamp, the tungsten-halogen lamp, eliminates bulb-wall blackening. A tungsten-halogen lamp has a quartz bulb that contains a small amount of a chemical element from the halogen family, such as bromine or iodine. Inside the bulb, the halogen combines with the evaporated tungsten and forms a gas. This gas moves through the bulb until it touches the filament. The higher temperature of the filament breaks down the gas. The tungsten is redeposited on the filament, and the halogen combines with more tungsten evaporated from the filament.

The base holds a lamp in its fixture. It also connects the lamp to an electric circuit.

Fluorescent lamps have limited use in homes, but they are widely used in offices, schools, and stores. Lighting engineers install other types of gaseous-discharge lamps in large indoor and outdoor areas. Such areas include factories, highways, parking lots, shopping centers, and sports stadiums. Most neon lamps are used in advertising signs.

Except for fluorescent lamps, most gaseous-discharge lamps are not used for home lighting. The colors of objects appear different in the light from these lamps. Gaseous-discharge lamps also cost more than incandescent lamps. But they produce more light per watt and have a longer life, so that their total cost may be less.

Low-pressure gaseous-discharge lamps use argon, neon, and other gases under low pressure to produce light. They include fluorescent lamps, neon lamps, and low-pressure sodium lamps.

Fluorescent lamps. A fluorescent lamp is a glass tube that contains mercury vapor and argon gas under low pressure. Electricity flowing through the glass tube causes the vaporized mercury to give off ultraviolet energy. The eye cannot see the ultraviolet energy as light. But the inside of the lamp is coated with substances called phosphors that give off visible light when struck by the ultraviolet energy. See Fluorescent Lamp.

Neon lamps are gas-filled glass tubes that glow when an electric discharge takes place in them. Pure neon gas in a clear tube gives off red light. Other colors result from mixtures of neon and other gases, from colored tubes, or from a combination of these two factors.

Low-pressure sodium lamps consist of two glass tubes, one inside the other. The inner tube contains solid sodium and a mixture of neon and argon gas. When the lamp starts, it emits the reddish-orange light that is characteristic of neon. But as the sodium heats up, it vaporizes, and the light becomes yellow.

High-pressure gaseous-discharge lamps use mercury, metal compounds, or other chemical compounds under high pressure to produce light. These lamps are also called high-intensity discharge (HID) lamps. They include mercury vapor lamps, metal halide lamps, and high-pressure sodium lamps.

Mercury vapor lamps have two bulbs, one inside the other. The inner bulb, made of quartz, is called the arc tube. The outer bulb protects the arc tube. The arc tube contains mercury vapor at a higher pressure than that of a fluorescent lamp and thus allows the vapor lamp to produce visible light without using phosphor coating.

The mercury vapor gives off greenish-blue light as well as ultraviolet rays. A mercury vapor lamp made with clear glass produces no red light, and so red objects appear brown, gray, or black. Mercury vapor lamps with a phosphor coating inside the outer bulb produce light with more colors. The phosphor gives off red light when struck by ultraviolet rays.

Mercury vapor lamps have a longer life than other electric lights of similar wattage. However, the lamps take from five to seven minutes to build up the mercury vapor pressure and reach full brightness.

Metal halide lamps contain chemical compounds of a metal and a halogen. These compounds produce a more natural color balance than mercury vapor lamps without using phosphors. Metal halide lamps also have a long life and a high light output. These lamps make an excellent light source for outdoor--and some indoor--uses.

High-pressure sodium lamps resemble mercury vapor lamps. But the arc tube is made of aluminum oxide instead of glass or quartz, and it contains a solid mixture of sodium and mercury, as well as a rare gas. The lamp produces orange-white light, which darkens blues and greens, and turns red to orange. This lamp has a long life and a very high light output.

Light-emitting diodes (LED's) are small chips of gallium arsenide or some other solid semiconductor material. LED's give off red, yellow, or green light when electric energy excites their atoms (see Light). LED's use little power and last almost indefinitely. Groups of LED's are used in computers, pocket calculators, and digital watches to form letters or numbers. A typical LED display has many small LED's that are individually controlled by the computer circuits. The circuits light a pattern of LED's to form each letter or number.

Electroluminescent panels consist of a layer of phosphors between a metal plate and a transparent coating that conducts electricity. The phosphors produce a bluish-green glow when electricity flows through the plate and the coating. Electroluminescent panels use little power. But the brightest panel produces less light than the smallest ordinary lamp. Electroluminescent panels are used as night lights and on the instrument panels of some aircraft and automobiles.

The widespread use of electric light required not only a lamp, but also a method of distributing cheap electricity to users of the lamp. The American inventor Thomas A. Edison developed such a method and became famous as the inventor of the electric light. In 1879, Edison invented his incandescent lamp. It had a filament of carbonized thread. During the early 1880's, he developed one of the first central stations (power plants) to generate and distribute electricity. Edison's first station, on Pearl Street in New York City, began operating in 1882.

During the early 1900's, engineers began to experiment with gaseous-discharge lamps containing mercury. Their work resulted in fluorescent and mercury vapor lamps in the 1930's.

Electroluminescence was discovered in 1936. LED's resulted from research on semiconductor devices during the 1960's. In the 1970's, researchers developed energy-conserving light sources, such as metal halide lamps and high-pressure sodium discharge lamps.

Contributor: Ronald N. Helms, Ph.D., Chairperson of Architectural Engineering, North Carolina Agricultural and Technical State Univ.; Lighting Consultant; Expert Witness.

Related articles include:

Edison, Thomas Alva; Electric Circuit; Electric Current; Flashlight; Fluorescent Lamp.

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