Incandescents are traditional lightbulbs that use a filament (a thin thread of metal, usually tungsten wire) inside a glass bulb that glows white hot as electricity passes through it. The filament is prevented from burning either by creating a vacuum inside the bulb or filling it with inert gas. They are far less energy efficient than fluorescents/CFLs or LEDs because most of their radiation is given off as heat rather than visible light.