How Do Halo Headlights Work?
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Introduction
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Halogen lights operate on the same principle as other incandescent lamps; however, the different chemicals used to make halogen lamps create a brighter, longer-lasting and more efficient light source. The offset to this improvement is that halogen lamps require more expensive materials for their manufacture. Another issue with halogen headlights is the discomfort experienced by other motorists: because of the brightness of the lamps, the National Highway Traffic Safety Administration reported a record number of complaints when the agency sought comments on them in 2002.
How Incandescent Bulbs Work
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Generally, incandescent lights are composed of a tungsten filament and an inert gas contained within a glass bulb. Electricity heats the filament that incandesces, giving off light and heat. The inert gas in the bulb displaces oxygen and prevents the filament from erupting into flame. One of the side effects of this process is the gradual breakdown of the tungsten, which causes tungsten molecules to adhere to the glass bulb. This soot causes the bulb to become dimmer over time and eventually results in the filament's destruction.
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Halogen Bulb Construction
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In a halogen lamp assembly, a gas from the group of halogen (i.e., salt-forming) elements--usually iodine or bromine--is added to the inert gas and pressurized to more than 7 atmospheres. This high pressure reduces the degradation of the tungsten. The lighting assembly is encased in a quartz or hard, high-silica glass capable of withstanding the high pressure and extreme temperature the light gives off. The lamps are smaller than their traditional counterparts of similar wattage.
The Halogen Cycle
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Halogen lamps last longer and remain brighter than traditional bulbs because the chemical mixture inside the bulb regenerates itself through a process known as the Halogen Cycle. This process not only recycles the tungsten molecules that detach from the filament, it significantly slows the accumulation of tungsten soot.
The Halogen Cycle is a chemical reaction caused by the temperature variations within the bulb. When electricity flows through the filament, its temperature increases to more than 3,000-degrees Celsius, while the quartz or silica glass heats to around 1,000-degrees Celsius. The high temperature causes the free tungsten molecules to react with the halogen gas, forming a gaseous tungsten salt. This halide does not adhere to the bulb wall, but is instead circulated by convection currents to the filament. The high temperature near the filament breaks down the tungsten halide and the tungsten molecules are redeposited on the filament. The halogen gas is freed to repeat the Halogen Cycle with other tungsten molecules.
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