Incandescent Filament Lamps
In conventional filament lamps or light bulbs, electric current is passed through a coiled tungsten filament, contained in a glass envelope that is filled with an inert gas. When heated by an electrical current, the filament emits electromagnetic radiation. At lower temperatures, radiation is mainly emitted in the infrared part of the spectrum as heat. At higher temperatures, the proportion of radiation at wavelengths ranging from 380 to 780 nm increases and visible light is produced. In a conventional lamp, the filament temperatures are limited to about 2700 Kelvin. The tungsten filament starts to evaporate and as a result it leads to the blackening of the inside of the lamp envelope.