Incandescent lamps operate simply by heating a metal filament inside a bulb filled with inert gas. Because they operate directly on variety of common power types including common household alternating current or direct current such as batteries or automobiles, they do not require a special power supply or ballast. They turn on up instantly, providing a warm light with excellent color rendition because the light is produced in much the same way as the light from the sun. They can also be easily dimmed using inexpensive controls and are available in a staggering variety of shapes and sizes. However, 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). Because their principle of operation is incandescence, they release most of their radiative energy as heat with only a small percentage given off as useful and visible light.
The three most common types of incandescent lamps are:
- Standard incandescent lamps
- Energy-Saving Incandescent (or Halogen)
- Reflector lamps