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, 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 and 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. End of life behaviour Metal halide lamps, usually lose their output or change color due to the loss of halides and arctube blackening. They stop working at the end of a life that is similar to mercury lamps. In rare cases, they can also cycle on/off. Some can exhibit major color shift, and in rare cases, explode. Risk of lamp explosion All metal halide arc tubes deteriorate in strength over their lifetime due to chemical attack, thermal stress and mechanical vibration. As the lamp ages the arc tube becomes discoloured (often obtaining a dark grey shade), absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the breakup of the tube. Early failure of the arc tube may occur due to manufacturing defects. Manufacturers may "season" new lamps to check for such defects before sale. Since a metal-halide lamp contains gases at a significant high pressure (up to 3.4 atmospheres), failure of the arc tube is inevitably a violent event. Fragments of arc tube will break the outer bulb, and hot glass fragments may fall on people or objects below. Hot fragments may present a fire hazard. Fixtures are designed to contain hot fragments with a hard glass cover, or may be designed for lamps with a quartz tube surrounding the arc tube to