Halogens work similar to regular incandescents. They have a tungsten filament which incandesce (glow) when charged with an electric current. This tungsten filament evaporates over time, getting progressively thinner to the point where it breaks (burns out). The critical difference between an incandescent bulb and a halogen bulb is the presence of halogen gas and how the filament burns. An incandescent filament leaves tungsten deposits on the inner shell of the bulb, resulting in a darkening or ‘blackening’ of the shell. Halogen gas recycles the tungsten back onto the filament, so you get a bulb that lasts longer and stays brighter.

the halogen cycle

A halogen has gas inside the bulb, which recycles the evaporated tungsten back onto the filament. This results in a longer life that emits brighter light than incandescent. The glass shell does not get blackened over time, resulting in a crisper, whiter light.