Shedding Light on Lighting Terminology .

T-12 lamp — A fluorescent lamp that is twelve-eighths 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 eight-eighths of an inch (1") in diameter. These lamps offer improved efficiency and light output, but require a new ballast.

Ballast — A device in the fluorescent fixture that regulates the current flow to the lamp, and produces the voltage necessary to operate the lamp.

Magnetic ballast — The old way - less efficient, produces a 60Hz hum.

Electronic ballast — Newer, more compact, more efficient, operates at a higher frequency.

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 reduced ballast factor of 0.78.

Lamp lumens — A measure of the light output of a lamp.

Initial lumens — The light output of a new lamp.

Mean Lumens — All lamps produce more light when they are new. As the lamps age, the light output decreases. The mean lumens output is determined at 40 percent of the lamp life and is a better term for comparing light output.

Fluorescent lamp designations — Until recently, every 4' long fluorescent lamp was labeled F40, regardless of wattage. The letters CW or WW referred to lamp color (CW = cool-white, WW = warm-white) and WM meant Watt Miser (34-Watt). A F40T12WW/WM was a 4' long, T-12, warm-white, Watt Miser 34-Watt lamp. A new designation is now used. The same lamp will now be labeled F34T12WW.