Fluorescent Tube and Circline Lamps

Fluorescent tube lamps—the second most popular type of lamps—are more energy efficient than the more popular A-type standard incandescent lamps.

The traditional tube-type fluorescent lamps are usually identified as T12 or T8 (12/8 or 8/8 of an inch tube diameter, respectively). They are installed in a dedicated fixture with a built-in ballast. The two most common types are 40-watt, 4-foot (1.2-meter) lamps, and 75-watt, 8-foot (2.4-meter) lamps.

Tubular fluorescent fixtures and lamps are preferred for ambient lighting in large indoor areas. In these areas, their low brightness creates less direct glare than incandescent bulbs.

Circular, tube-type fluorescent lamps are called circline lamps. They are commonly used for portable task lighting.

Illustration of a fluorescent tube lamp. The lamp is a long cylinder, and one side is cut away to show that mercury and inert gases are inside the lamp, and the glass has a phosphor coating on the inside of the tube. At one end inside the lamp is a ballast, which looks like two small wires projecting into the tube from the end, through a flat piece, and joined by another length of wire. At the other end, outside of the tube, is the base with a bi-pin plug.

In fluorescent tubes, a very small amount of mercury mixes with inert gases to conduct the electrical current. This allows the phosphor coating on the glass tube to emit light.

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