Halogen lamp filament. Coloured scanning electron micrograph (SEM) of a tungsten filament from a halogen bulb. This filament of this incandescent bulb is enclosed in a small quartz envelope inside the bulb. The quartz envelope contains the vapour of a halogen gas, such as iodine or bromine, which allows the filament to operate at a much higher temperature than in a conventional tungsten bulb. This results in greater brightness and a longer life than standard incandescent bulbs. Magnification: x1600, when printed 10cm wide.