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Fluorescent Lighting Basics

October 17, 2013 - 5:39pm


Light from a fluorescent lamp is first created by an electric current conducted through an inert gas producing ultraviolet light that is invisible to the human eye. The ultraviolet light in turn interacts with special blends of phosphors coating the interior surface of the fluorescent lamp tube that efficiently converts the invisible light into useful white light. Fluorescent lamps require a special power supply called a ballast that is needed to regulate the lamp's operating current and provide a compatible start-up voltage.  Electronic ballasts must be used with all modern, high-efficiency and high-performance linear fluorescent lamps, especially those with small diameters like T8 or T5. (A few specialized fluorescent lamp types in North America still use magnetic ballasts.) Electronic ballasts enable fluorescent lamps to operate consistently and efficiently, to incorporate dimming functions, and to be networked for advanced control of lighting. 

The two general types of fluorescent lamps are:


Illustration of a linear fluorescent lamp.In linear fluorescent lamps (LFLs), a very small amount of mercury mixes with inert gases to conduct electrical current. This allows the phosphor coating on the glass tube to emit white, visible light.

LFLs are usually identified as T12, T8, or T5 (with the suffix digit designating the diameter of the lamp tube in inches divided by 0.125 or one-eighth of an inch). They must be used in a dedicated fixture or luminaire that is supplied with a suitable ballast. These lamps may be straight, bent (U-tube), or circular (e.g., CirclineTM) in shape. The most common LFL is the 40-watt, 4-foot (1.2-meter) lamp, followed by the popular 75-watt, 8-foot (2.4-meter) lamps found in most commercial and industrial buildings.

Visit Energy Saver for more information on using fluorescent lighting in your home.


Illustration of six types of compact fluorescent lamps.

Compact fluorescent lamps (CFLs) come in a variety of sizes and shapes, including (a) twin-tube integral, (b and c) triple-tube integral, (d) integral model with casing that reduces glare, (e) modular circline and ballast, and (f) modular quad-tube and ballast varieties.

CFLs work exactly like LFLs, only on a smaller scale. They consist of two parts: a gas-filled tube and a power supply called a ballast that is either magnetic or electronic. The gas in the tube glows with ultraviolet light when electricity from the ballast flows through it. This, in turn, excites a specially engineered phosphor coating on the inside of the tube, which emits white, visible light throughout the surface of the tube.

Electronic ballasts, although sometimes more expensive than the old magnetic ballasts, generally turn on more quickly and reliably, especially at very low temperatures. They are also more efficient than the old magnetic ballasts and produce much less electromagnetic interference.

Most CFL lamps will last about 10,000 hours and the ballast about 50,000 hours or more. Most currently available CFLs have electronic ballasts integrated into their bases that are used in residential applications. Most CFLs used in commercial applications have the electronic ballast permanently located in the fixture so only the lamp requires replacement.

CFLs are designed to operate within a specific temperature range. Temperatures below the range cause reduced luminous output. Most CFLs are designed for indoor use, but there are models available for outdoor operation. A CFL's temperature range is usually listed on its package.


CFLs may have two, four, or six tubes or circular or spiral-shaped tubes. The size or total surface area of the tube(s) determines how much light is produced.

In some CFLs, the tubes and ballast are permanently connected. These are commonly called integral CFLs. Other CFLs have separate lamp tubes and ballasts. This allows the tubes to be changed without changing the ballast, and are generally used in commercial environments where the lamp will be illuminated for long periods. There are also examples of both types where the tubes are enclosed in a glass globe. These look somewhat similar to conventional incandescent light bulbs, except they are often larger in diameter and length.

Most integral CFLs fit into light fixtures or luminaires originally designed for incandescent lamps that use common Edison bases such as "medium" E-26 or "candelabra" E-12. Although most CFLs fit into existing three-way E-26 lamp sockets, only some CFL lamps can be dimmed. This feature will be listed clearly on the lamp package. Learn more about the types of CFLs and their uses in homes on Energy Saver.