Most incandescent lamps operate by heating a special filament inside a glass bulb filled that is usually filled with an inert gas. The descriptive name "incandescence" comes from the surface physics governed by the material used as a filament. Early lamps used steel and more modern lamps use Tungsten, but for most materials used today most of the energy released by the lamp is in the form of heat and not light. This is why incandescent lamps are so inefficient. Incandescent lamps have a low efficacy (10-17 lumens per watt) compared with other lighting options and a short average operating life (750–2,500 hours).
The three most common types of incandescent lamps are:
- Standard incandescent lamps, which are no longer manufactured but are still in use
- Energy-saving Incandescents (or halogen)
- Reflector lamps