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.alogen 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.