How does a halogen lamp work?
It works through the halogen cycle. As tungsten evaporates from the filament, it combines with halogen gas and becomes tungsten halide. This tungsten halide state is maintained at temperatures between 250°C and 1400°C. The temperature of the bulb is kept above 250°C in order to maintain this state and prevent the tungsten from sticking to the bulb wall and darkening it. When the tungsten halide nears the filament, the heat of the filament separates the tungsten from the halogen gas, redepositing the tungsten on the filament. The freed halogen gas then repeats the same process again thus the halogen cycle.