halogen in lighting is usually a reference to the use of iodine and or bromine in combination with a tungsten filament, the iodine has the effect of reacting with the boil off tungsten to make a tungsten iodide which diffuses back to the hot filament where it decomposes to give tungsten again. the halogen recycles the tungsten back onto the filament preventing it from coating the inner surface of the bulb allowing the bulb to run hotter and longer.
