Optics 101

What is a Halogen Lamp?

A halogen lamp is an incandescent light source. It consists of a tungsten filament, enclosed in an environment of an inert gas and a small amount of a halogen (bromine or iodine). The combination of the tungsten filament and the halogen results in a chemical reaction called the halogen cycle, which increases the lifetime of the filament.

Halogen lamps are widely used in optical systems, mainly due to their compact size and efficient casting of light. Enhancements in the design of the halogen lamps, such as the provision of integral reflectors, enable redirection of light into the illumination system in an orderly manner. This datasheet will look into the working, construction, and application areas of these lamps.

Working Principle and Spectral Output

Tungsten-halogen lamps work as a thermal radiator, which means that light is generated by heating a solid to very high temperatures. The brightness of the emitted light is proportional to the heating temperature.

The spectral output of halogen lamps is continuous, and is similar to that of a blackbody radiator. A major portion (up to 85%) of the emitted light lies in the infrared and near infrared regions; the rest (15-20%) lies in the visible region, and less than 1% of the light falls in the ultraviolet region.

Construction

Tungsten-halogen lamps consist of a filament, made of tungsten, housed in fused silica quartz glass. The glass envelope is filled with an inert fill gas, which may be xenon, krypton, argon, or nitrogen. There are two leads emerging out from the lamp for heating the filament.

Applications

Typical areas of application of tungsten-halogen lamps are listed below:

  • Automotive – as headlamps in vehicles
  • Architectural – house lighting, dichroic and plain reflector spots
  • Stage lighting – for theatrical and studio lighting
  • Projection lamps - in motion pictures and slide projectors
  • Inspection lights and microscope illuminators

Sources and Further Reading

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