Halogen light bulbs are hybrid incandescent bulbs also known as tungsten-halogen light bulbs. Similar to the incandescent light bulb, halogen bulbs have a tungsten filament; however, in a halogen light bulb, the tungsten filament is sealed into a small envelope filled with a halogen gas, such as iodine or bromine.
In an ordinary incandescent light bulb, the thickness of the filament may vary slightly. The resistance of the filament is higher at the thinner portions, which causes the thin areas to be hotter than the thicker parts of the filament. The rate of tungsten evaporation will be higher at these points due to the increased temperature, causing the thin areas to become even thinner and creating a runaway effect until the filament fails. As the tungsten burns off the filament, it deposits itself on the inside of the bulb wall. This reduces lumens in two ways. First, as the filament gets thinner, there is less material to generate light. Second, as the tungsten builds up on the inside of the bulb, it blocks the light from shining through the glass.
A tungsten-halogen light bulb creates a balanced, cyclical reaction in which the tungsten that evaporates when giving off light is absorbed by the halogen gas and preferentially re-deposited at the hot-spots on the filament, preventing the early failure of the light bulb. Also known as the halogen cycle, this equilibrium reaction allows for the halogen light bulb to maintain its lumen rating over the life of the light bulb because there is no loss of tungsten from the filament and no dirty buildup inside the bulb wall.
Halogen light bulbs must be very hot in order to create the equilibrium reaction; therefore, the halogen bulb’s envelope must be made of hard glass or fused quartz instead of ordinary soft glass, which would soften and flow too much at these high temperatures. The high operating temperature allows the halogen light bulb to have a higher luminous efficacy, apparent brightness, and whiter color temperature.
Halogen light bulbs can be placed on a dimming circuit; however, this does not allow the light bulb to get hot enough to activate the halogen cycle. Without the cycle, the tungsten depletion will continue. If you wish to place a halogen light bulb on a dimmer, you should turn the light bulb to full voltage for at least 30 minutes each day to allow the cycle to clean the light bulb. If you do not do this, your halogen light bulbs will experience short life.
Even though the halogen cycle prevents the filament from becoming thinner, the halogen light bulb will still burn out eventually. This is because the filament – which is a series of coils – acts as a mini electromagnet, and every time the light bulb is turned on, the initial voltage makes the coil jump very slightly as the magnetic fields build up. Moreover, every time electrical current flows through the filament, the filament becomes a little stiffer. This continues throughout the life of the light bulb, and the filament becomes very brittle. Eventually, at the end of life, the halogen coil is so brittle that the slight movement at the start-up causes the filament to fracture and fail. That is why most incandescent and halogen light bulbs fail when the light switch is first turned on.