Light micrograph of a halogen lamp filament. This filament of an 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: x100 when printed 10 centimetres wide.