Halogen, Fluorescent and Specialty Bulbs

Halogen, fluorescent, HID or incandescent for your fixture

The fixture dictates the technology. Fluorescent tubes and pin-base compact fluorescents rely on a ballast in the fixture, and a tube's T-number gives its diameter in eighths of an inch, so a T5 will not fit a T8 fixture. High-intensity discharge lamps need their own ballast and a warm-up period before reaching full output. Halogen lamps are compact and bright but run hot, and many 12 V, 19 V and 120 V versions look alike. Pin codes such as GY9.5, GX5.3 and GX23 matter as much as watts, so copy the full code from the old lamp.

Color rendering, lifespan and special-purpose lamps

Color rendering index (CRI) runs to 100, and a value of 80 or more is a reasonable minimum for a home, while 90 and above suits art, makeup and photography. Lifespans vary enormously: studio and projector lamps may last only a few hundred hours in exchange for bright, steady output, whereas LED and some discharge lamps run for ten thousand hours or more. Germicidal lamps emit UV-C, which damages eyes and skin, so use them only in fixtures designed for them and never in an occupied room.

Questions about Halogen, Fluorescent and Specialty Bulbs

Can I put a T5 tube in a T8 fixture?

No. T5 tubes are thinner, use a smaller pin base and need a ballast made for them, whereas T8 tubes have their own base and ballast. Read the T-number and the length on the old tube and match both.

Why shouldn't I touch a halogen bulb with bare fingers?

Skin oils leave residue that heats unevenly on the hot quartz glass and can cause hot spots, shortening the lamp's life or even making it burst. Handle the bulb with a clean cloth or its packaging.

Why does my discharge lamp take minutes to reach full brightness?

Discharge lamps need to warm up so the metal salts vaporize, and most take a few minutes to reach full output. A lamp that cycles on and off, or cannot restart while hot, is often at end of life or needs a cool-down period.