| Halogen Lamp |
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Information AboutHalogen Lamp |
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PRINCIPLE OF OPERATION In an ordinary incandescent lamp, the thickness of the filament may vary slightly. The resistance of the filament is higher at the thinner portions which causes the thin areas to be hotter than the thicker parts of the filament. The rate of tungsten evaporation will be higher at these points due to the increased temperature, causing the thin areas to become even thinner, creating a runaway effect until the filament fails. A tungsten-halogen lamp creates an Equilibrium Reaction in which the tungsten that evaporates when giving off light is preferentially re-deposited at the hot-spots, preventing the early failure of the lamp. This Chemical Transport Reaction is based on WO2I2 or WO2Br2 formation at elevated temperatures and its decomposition at high temperatures at the tungsten filament. This also allows halogen lamps to be run at higher temperatures which would cause unacceptably short lamp lifetimes in ordinary incandescent lamps, allowing for higher luminous efficacy, apparent brightness, and whiter Color Temperature . Because the lamp must be very hot to prevent crystalistaion of WO2I2 or WO2Br2 at the glass surface, the halogen lamp's envelope must be made of hard glass or Fused Quartz , instead of ordinary soft glass which would soften and flow too much at these temperatures. The envelope material can be selected and modified (by means of Optical Coating ) to achieve whatever lamp characteristics are required. Halogen bulbs are widely used in automobile Headlamp s, for example, and because headlamps often contain plastic parts, halogen headlamp bulbs' envelopes are made out of hard glass, or out of quartz 'doped' with additives to block most of the UV output (hard glass blocks UV without need of dopants). Conversely, some applications ''require'' ultraviolet radiation, and in such cases, the lamp envelope is made out of undoped quartz. Thus, the lamp becomes a source of UV-B radiation. Undoped quartz halogen lamps are used in some scientific, medical and dental instruments as a UV-B source A typical halogen lamp is designed to run for about 2000 hours, twice as long as a typical incandescent lamp. HALOGEN INFRARED |
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