of multifaceted reflector "MR" lamps of 20–50 watts were originally conceived for the projection of 8 mm film, but are now widely used for display lighting and in the home.

The combination of the halogen gas and the tungsten filament produces a halogen-cycle chemical reaction, which redeposits evaporated tungsten on the filament, increasing its life and maintaining the clarity of the envelope.For a fixed power and life, the luminous efficacy of all incandescent lamps is greatest at a particular design voltage. This allows the filament to operate at a higher temperature than a standard incandescent lamp of similar power and operating life; this also produces light with higher luminous efficacy and color temperature.

A carbon filament lamp using chlorine to prevent darkening of the envelope was patented [2] in 1882, and chlorine-filled "NoVak" lamps were marketed in 1892. However the light output is reported as proportional to V 3 {\displaystyle V Incandescent lamp variety A halogen lamp operating in its fitting with the protecting glass removed A halogen lamp behind a round UV filter. The first lamps used only tungsten for filament supports, but some designs use molybdenum – an example being the molybdenum shield in the H4 twin filament headlight for the European Asymmetric Passing Beam.

Tungsten halogen lamps behave in a similar manner to other incandescent lamps when run on a different voltage. At some point, however, it will reach higher temperature regions within the bulb where it then dissociates, releasing tungsten back onto the filament and freeing the halogen to repeat the process. A 300 watt tubular halogen bulb operated at full power quickly reaches a temperature of about 540°C (1,004°F), while a 500 watt regular incandescent bulb operates at only 180°C (356°F) and a 75 watt regular incandescent at only 130°C (266°F).

The small size of halogen lamps permits their use in compact optical systems for projectors and illumination. The halogen cycle keeps the bulb clean and causes the light output to remain almost constant throughout the bulb's life.

Halogen lamps made for 12 to 24 volt operation have good light outputs, and the very compact filaments are particularly beneficial for optical control (see picture). The presence of the halogen, however, sets up a reversible chemical reaction cycle with this evaporated tungsten.