Tungsten halogen lamps

In a conventional incandescent lamp the filament loses material by evaporation. The inert
gas inside the bulb reduces the rate of evaporation but cannot prevent it completely. A
further improvement can be obtained by adding a halogen to the gas, which gives rise to a
reversible chemical reaction. Tungsten evaporates from the filament and diffuses towards
the bulb wall. Provided the temperature is favourable the tungsten combines with halogen
at the walls and the resulting tungsten halide diffuses back to the filament. At the
temperature prevailing at the filament the tungsten halide dissociates into tungsten and
halogen and the tungsten is deposited back on the filament.
Successful operation of such a lamp depends on the achievement of suitable
temperatures at both the filament and the bulb wall. This makes it necessary for the
filament to be at a higher temperature than in an ordinary tungsten lamp and for the bulb
to be smaller. The bulb has to be at a temperature of about 250°C, and in order to
withstand this it has to be made of fused silica or high melting-point glass.
For a given power a tungsten halogen lamp has a longer life and a higher light output
than an ordinary tungsten lamp. The high temperature makes it of limited use in domestic
or commercial lighting but it finds application in floodlighting and the lighting of film
and television studios. It is probably most widely known for its use in automobile
headlamps, but that is an application entirely outside the scope of this book.