Halogen lamp

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A halogen lamp is an incandescent lamp with a tungsten filament contained within an inert gas and a small amount of a halogen such as iodine or bromine. The combination of the halogen gas and the tungsten filament produces a chemical reaction known as a halogen cycle (see below) which increases the lifetime of the filament and prevents darkening of the bulb by redepositing tungsten from the inside of the bulb back onto the filament. Because of this, a halogen lamp can be operated at a higher temperature than a standard gas-filled lamp of similar power and operating life. The higher operating temperature results in light of a higher color temperature. This, in turn, gives it a higher luminous efficacy (10–30 lm/W). Because of their smaller size, halogen lamps can advantageously be used with optical systems that are more efficient in how they cast emitted light.

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History

A carbon filament lamp using chlorine to prevent darkening of the envelope was patented[1] in 1882, and chlorine-filled "NoVak" lamps were marketed in 1892.[2] The use of iodine was proposed in a 1933 patent,[3] which also described the cyclic redeposition of tungsten back on the filament. In 1959 General Electric patented[3] a practical lamp using iodine.[4]

Halogen cycle

The function of the halogen is to set up a reversible chemical reaction with the tungsten evaporating from the filament. In ordinary incandescent lamps, this tungsten is mostly deposited on the bulb. The halogen cycle keeps the bulb clean and the light output remains almost constant throughout life. At moderate temperatures the halogen reacts with the evaporating tungsten, the halide formed being moved around in the inert gas filling. At some time it will reach higher temperature regions, where it dissociates, releasing tungsten and freeing the halogen to repeat the process. In order for the reaction to operate, the overall bulb temperature must be higher than in conventional incandescent lamps. The bulb must be made of fused silica (quartz) or a high melting point glass (such as aluminosilicate glass). Quartz being very strong, the gas pressure can be higher,[5] which reduces the rate of evaporation of the filament, permitting it to run a higher temperature (and so efficacy) for the same average life. The tungsten released in hotter regions does not generally redeposit where it came from, so the hotter parts of the filament eventually thin out and fail. Regeneration of the filament is also possible with fluorine, but its chemical activity is so great that other parts of the lamp are attacked.[6][7]

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