Lighting Glossary

Shedding Light on Lighting Terminology

T-12 lamp – A fluorescent lamp that is 12/8ths of an inch (1.5") in diameter. These were the most common lamps until the mid-1990s.

T-8 lamp – A fluorescent lamp that is 8/8ths of an inch (1") in diameter. These lamps offer improved efficiency and light output, but require a new ballast to replace T-12 bulbs.

T-5 lamp – A fluorescent lamp that is 5/8ths of an inch (0.625") in diameter. In most retrofits T-5 lamps require an entire fixture replacement due to differences in length and sockets, but in appropriate applications can provide significant energy savings.

T5HO lamp – A high output (HO) version of a T-5 lamp. These lamps deliver more light than standard T-5 lamps and are available in higher wattages.

Ballast – A fluorescent fixture device that regulates the current flow to the lamp.

Magnetic ballast – Older, less efficient ballast that produces a 60Hz hum.

Electronic ballast – Newer, more compact and more efficient.

Ballast factor – Fraction of lamp light output. A ballast with a standard ballast factor of 0.88 will produce 10 percent more light than one with a ballast factor of 0.78.

Lamp lumens – The light output of a lamp.

Initial lumens – The light output of a new lamp.

Mean Lumens – Lamps produce more light when they are new and less as lamps age. Mean lumens output is the light level at 40 percent of the lamp life and is a better term for comparing light output.

Fluorescent lamp designations – The letter and number designations describe the wattage, the lamp type and the light color of the lamp. The letters CW (cool-white) or WW (warm-white) describe lamp color. For example, a 34-Watt T12 lamp with warm-white light is labeled F34T12WW.