Halogen lamps can give off more light than standard incandescent lamps because they operate at higher temperatures. The higher temperature also means that the light is whiter. The molecules of the gas perform a service called the halogen cycle which permits the higher temperature.
The halogen gas does it job by catching atoms from the filament which are driven from surface by the high temperature. In standard incandescent lamps those atoms collect on the glass and can be seen as the black deposit in an old lamp.
If a halogen gas is present in the lamp with a tungsten filament then the atoms which are driven off combine with halogen molecules. Instead of collecting on the lamp wall, the tungsten atoms stay attached to the halogen gas until they are returned to the filament where the tungsten and halogen are separated. The tungsten is deposited on the filament and the halogen gas is free to circulate, again available to intercept a tungsten atom.
Because the atoms return to to the filament, halogen lamps can be used at higher temperatures in spite of the fact that the heat causes atoms to leave the surface at a faster rate.
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