Starting and Warm Up
A "cold" (below operating temperature) metal-halide lamp cannot immediately begin producing its full light capacity because the temperature and pressure in the inner arc chamber require time to reach full operating levels. Starting the initial argon arc sometimes takes a few seconds, and the warm up period can be as long as five minutes (depending upon lamp type). During this time the lamp exhibits different colors as the various metal halides vaporize in the arc chamber.
If power is interrupted, even briefly, the lamp's arc will extinguish, and the high pressure that exists in the hot arc tube will prevent restriking the arc; with a normal ignitor a cool-down period of 5–10 minutes will be required before the lamp can be restarted, but with special ignitors with specially designed lamps, the arc can be immediately re-established. On fixtures without instant restrike capability, a momentary loss of power can mean no light for several minutes. For safety reasons, many metal-halide fixtures have a backup tungsten-halogen incandescent lamp that operates during cool-down and restrike. Once the metal halide restrikes and warms up, the incandescent safety light is switched off. A warm lamp also tends to take more time to reach its full brightness than a lamp that is started completely cold.
Most hanging ceiling lamps tend to be passively cooled, with a combined ballast and lamp fixture; immediately restoring power to a hot lamp before it has re-struck can make it take even longer to relight, because of power consumption and heating of the passively cooled lamp ballast that is attempting to relight the lamp.
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