How Does a Halogen Light Bulb Work?

Unlike LED lights and HID (High Intensity Discharge), or Xenon lights, Halogen light bulbs are not a "new" technology, but rather a refinement of an existing technology. Halogen bulbs are a variant of the traditional incandescent bulbs that have been used since the early days of the electric light.

An incandescent bulb works by applying electrical current to heat a filament in a glass bulb until it glows. The bulb is either evacuated (has a vacuum) or filled with an inert gas to keep the filament from oxidizing. Early light bulbs like those developed by Thomas Edison used a carbon filament, but in 1906 the tungsten filament, which produced a brighter light and lasted longer, was introduced. At around the same time, electric lighting using incandescent bulbs began replacing acetylene lights on automobiles.

Halogen Bulb Types
How Halogen Bulb Works

The tungsten filament bulb was the mainstay of automotive lighting for most of the 20th century, but it was not without its problems. During operation, the tungsten filament evaporates, and the hotter it operates the faster it evaporates. This means that there had to be a tradeoff between the amount of light produced and the bulb's longevity. It was found that using an inert gas like argon, instead of a vacuum, would prolong filament life, but the gas would also cool the filament and reduce its efficiency. Another problem with the tungsten filament bulb was bulb blackening. As the tungsten filament evaporated, it would condense on the inner surface of the bulb. This darkened the bulb and reduced the light output.

A halogen bulb contains a halogen element like iodine or bromine instead of inert gas like a conventional bulb. The halogen gas and the filament combine to produce a chemical reaction called the halogen cycle, which redeposits evaporated tungsten on the filament. This increases the bulb's useful life by prolonging the life of the filament, and prevents bulb blackening. However, the bulb must be operated at a higher temperature for the reaction to take place. This is why the halogen bulb is made of a high melting point glass like fused quartz, and also why such lights are also known as quartz-halogen lights. Since quartz is so strong, the gas pressure in the bulb can be increased, which reduces the rate of filament evaporation and allows the bulb to be operated at a higher temperature without a reduction in lifespan. The higher operating temperature and bulb clarity of halogen bulbs results in increased light output. They're also more efficient than conventional incandescent bulbs, producing more light from the same amount of power. The first halogen bulbs for automobiles were used in Europe in 1962, but they did not become legal for use in the United States until 1978. Today, halogen bulbs are most often used for headlights and fog lights, and their increased light output increases driver visibility in darkness and inclement weather. Halogen bulbs are available in several degrees of brightness and color variation ranging from bright white to blue to yellow.

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