The cycle works like this:
1: Tungsten atoms evaporate from the hot filament and diffuse toward the cooler bulb wall. The filament temperature is about 3030º Celsius (or about 5480º Fahrenheit). The temperature at the bulb wall is about 730º C (or about 1340º F).
2: Tungsten, oxygen and halogen atoms combine on or near the bulb-wall to form tungsten oxyhalide molecules. Bromine is now the most common halogen. Chlorine is used in some special photocopying lamps that operate only for brief intervals.
3: Tungsten oxyhalides remain in a vapor phase at the bulb-wall temperatures and this vapor moves toward the hot filament. A combination of diffusion and convection currents are responsible for the movement.
4: High temperatures near the filament break the tungsten oxyhalide molecules apart. The oxygen and halogen atoms move back toward the bulb wall and the tungsten atoms are re-deposited on the filament. The cycle then repeats. GoodMart sells thousands of halogen light bulbs.