Halogen lamps - What is so special about them?

A halogen lamp contains a gas known as halogen gas. Halogens are gases such as chlorine, bromine, iodine, etc. In a halogen lamp, generally there is a small amount of iodine. Iodine displays a property known as sublimation - when heated, it goes directly from solid to gas, without going through the intermediate liquid state. Halogen lamps are brighter, whiter and long lasting. They are actually similar to ordinary incandescent bulb. There is a tungsten filament, which is heated to about 4000°C by passing an electric current. At this temperature, some tungsten from the filament will start to evaporate. This will make the filament weak and eventually the lamp will blow out. What the iodine atoms do is they react with the evaporated tungsten atoms before they condense as a black spot on the glass of the lamp. Tungsten iodide is a gaseous substance. These molecules float around inside the bulb - sometimes they hit the hot tungsten filament. The high temperature breaks the molecules (dissociation reaction) into tungsten and iodine. The evaporated tungsten atom is returned to the filament and the iodine atom returns back as vapor. The freed iodine atom can now catch other tungsten atoms floating around in the lamp. This recycling process apparently doubles the life of the filament and hence the lamp.

Halogen lamps can therefore operate at much higher temperatures than ordinary incandescent bulbs. They also give out whiter, brighter light.

If bromine is placed instead of iodine, the halogen lamp may have a reddish - orange light.