Fluorescent Lamp

Fluorescent lamp, pronounced floo uh REHS uhnt, is a tube-shaped electric light that has wide use in factories, offices, and schools. In homes, incandescent lamps are more widely used than fluorescent lamps. A fluorescent lamp uses only about a fifth as much electricity as an incandescent lamp uses to produce the same amount of light. It also produces only a fifth as much heat for the same amount of light. For this reason, fluorescent lamps are sometimes called "cool" lights. In addition, fluorescent lamps last much longer than incandescent lamps.

A fluorescent lamp consists of a glass tube containing a small amount of mercury and another chemically inactive gas at low pressure. The gas in most fluorescent tubes is argon. The inside surface of the tube has a coating of chemicals called phosphors. At each end of the tube is an electrode, a coil of tungsten wire coated with chemicals called rare earth oxides. A fluorescent circuit includes a device called a ballast, which provides voltage to start the lamp. The ballast also regulates the flow of current in the lamp circuit.

There are three main kinds of fluorescent lamp circuits: (1) preheat, (2) rapid-start, and (3) instant-start. Fixtures using a preheat circuit cost the least and are found in some homes. Lamps last longer in rapid-start fixtures, and so these fixtures are cheaper to operate and maintain. They are widely used commercially.

When a preheat or rapid-heat lamp is turned on, electricity flows through the tungsten wire. The wire becomes heated, and the earth oxides on it give off electrons. Some of the electrons strike the argon atoms and ionize them--that is, the electrons give the atoms a positive or negative electric charge. When ionized, the argon can conduct electricity. A current flows through the gas from electrode to electrode, forming an arc (stream of electrons). Instant-start lamps start at such high voltage that the arc forms immediately. When an electron in the arc strikes a mercury atom, it raises the energy level of an electron in the atom. As this electron returns to its normal state, it emits invisible ultraviolet rays. The phosphors on the inside walls of the tube absorb the rays, which cause the phosphors to fluoresce (glow), producing visible light. The color of the light produced depends on the phosphors used.

The fluorescent lamp was introduced at the New York World's Fair of 1938-1939. Since 1952, fluorescent lamps have ranked ahead of incandescent lamps in sales in the United States.

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

Fluorescent Lamp

Master Index