Halogen Cycle

Halogen lamp is filled with the same inert gases as incandescent lamps mixed with small amounts of a halogen gas (generally less than 1% bromine). The halogen chemically reacts with the tungsten deposit to produce tungsten halides. When the tungsten halide reaches the filament, the intense heat of the filament causes the halide to break down, releasing tungsten back to the filament. This process is known as the tungsten-halogen cycle and it maintains a constant light output over the life of the lamp. To make this halogen cycle to work following are the basic requirements:

• The bulb surface must be very hot otherwise the halogen may not effectively vaporize or fail to adequately react with condensed tungsten.
• The bulb needs to be smaller and made of either quartz or a high-strength, heat-resistant grade of glass known as aluminosilicate.

Since the bulb is small and reasonably strong due to its thicker walls, it can be filled with gas to a higher than normal pressure. This slows down the evaporation of the tungsten from the filament, increasing the life of the lamp. The higher pressure and better fill gases can extend the life of the bulb and permit a higher filament temperature which results in better efficiency.