- Tungsten-halogen incandescent lights use the same illumination technology and produce the same amount of light as conventional bulbs. Because filament temperatures are so high, however, halogen bulbs utilize a different material for the bulb's shell to resist the heat, and a modified gas filling to maintain bulb clarity and increase its life. As a result, a halogen light stays bright and burns longer than comparable conventional bulbs. For example, manufacturers use halogen bulbs as the light source in LCD televisions and computer monitors they expect to last for years.
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A typical light bulb uses electrical energy to heat a tungsten filament to a glowing, red-hot state. In fact, the filament gets so hot that some of its material "evaporates," migrating to the cooler inner walls of the bulb where it "blackens" the bulb and diminishes its light output.
Eventually, the filament loses enough of itself that it breaks, cutting the electrical circuit, and the bulb "burns out." The black you see inside the dead bulb comes from the tungsten in the filament, not from any actual burning inside the bulb. -
Advancing technology created a need for smaller light bulbs. Unfortunately, as the inner wall of a standard light bulb moves closer to the filament, tungsten evaporation accelerates, making the resulting bulbs commercially unacceptable. Thus, the halogen bulb.
In a halogen bulb, fused quartz (silica, Si02), high-silica glass or aluminosilicate replace the glass and an inert fill gas, with a trace of halogen vapor (iodine or bromine), replaces the argon or nitrogen.
The shell of a halogen bulb (called an envelope) doesn't "blacken" (keeping the light brighter throughout its life), and the filament lasts significantly longer. Here's why. -
High heat causes the filament to shed tungsten atoms. In a gaseous state, the free atoms float toward the bulb's inner wall.
The free tungsten atoms combine with oxygen and halogen atoms near or on the bulb's inner wall to form tungsten oxyhalides. Because the tungsten oxyhalides remain vaporous at the bulb's wall temperatures, the cooler vapor floats back toward the hot filament.
Back at the filament, the high temperature breaks the vapor molecules apart. The oxygen and halogen atoms float back toward the wall, leaving the tungsten atoms behind to replenish the filament.
