Halogen bulb

A halogen lamp, also known as a tungsten halogen, quartz-halogen or quartz iodine lamp, is an incandescent lamp consisting of a tungsten filament sealed into a compact transparent envelope that is filled with a mixture of 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 halogen cycle chemical reaction which redeposits evaporated tungsten to the filament, increasing its life and maintaining the clarity of the envelope. 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, producing light of a higher luminous efficacy and color temperature. The small size of halogen lamps permits their use in compact optical systems for projectors and illumination.

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

In ordinary incandescent lamps, evaporated tungsten mostly deposits onto the inner surface of the bulb, causing the bulb to blacken and the filament to grow increasingly weak until it eventually breaks. The presence of the halogen, however, sets up a reversible chemical reaction cycle with this evaporated tungsten. The halogen cycle keeps the bulb clean and causes the light output to remain almost constant throughout the bulb's life. At moderate temperatures the halogen reacts with the evaporating tungsten, the halide formed being moved around in the inert gas filling. At some point, however, it will reach higher temperature regions within the bulb where it then dissociates, releasing tungsten back onto the filament and freeing the halogen to repeat the process. The overall bulb envelope temperature must be significantly higher than in conventional incandescent lamps for this reaction to succeed, however: it is only at temperatures of above 250 °C (482 °F) on the inside of the glass envelope that the halogen vapor can combine with the tungsten and return it to the filament rather than the tungsten becoming deposited on the glass. A 300 watt tubular halogen bulb operated at full power quickly reaches a temperature of about 540 °C (1,004 °F), while a 500 watt regular incandescent bulb operates at only 180 °C (356 °F) and a 75 watt regular incandescent at only 130 °C (266 °F).

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