The graying of the inner surfaces of incandescent bulbs is the result of gradual evaporation of tungsten from the filament while the light is on. This evaporation eventually makes the filament so thin it burns out.
Various methods have been developed to reduce greying. Filaments of the first incandescent lamps burnt in a vacuum, but it was soon found that introducing inert gas to the bulb reduced the rate of greying.
A mixture of nitrogen and argon is used today. In addition, “getters” reactive metal such as tantalum and titanium can be placed near the filament to attract the tungsten so that it is not deposited on the glass.
Alternatively, a small amount of abrasive tungsten powder can be placed in the bulb. Shaking it occasionally will remove the grey coating from the surface of the glass.
Graying can almost eliminated by introducing a small amount of the halogens iodine and bromine. As tungsten evaporates from the filament, it reacts with the halogens which then redeposit the tungsten on the filament. This keeps the bulb wall clean.
To prevent the tungsten halides from condensing on the bulb and breaking the cycle, the temperature of the bulb wall must be at least 500 – C. this is too hot for glass bulbs, which normally operate at about 150 – C, so fused quartz (silicon dioxide) must be used instead
Compared with ordinary incandescent lamps, quartz-halogen lamps have longer lives and maintain their light output over time. For example, a quartz-halogen lamp with a 200-hour life will have dimmed by less than 5 percent by the time it burns out. When an incandescent lamp with 1000-hour life burns out, it will have dimmed by more than 15 percent.
