The periodic table of elements actually lists four halogens: fluorine, chlorine, bromine and iodine. Halogens are diatomic molecules – they have two of the same atoms – and they are all ions with a negative charge. They must be produced artificially, because none of them exist in nature in their elemental form. Halogens are highly reactive and generally considered dangerous, but they exhibited one characteristic that was valuable for making incandescent light bulbs.
Incandescent light bulbs use a tungsten filament inside a glass bulb with either a vacuum or an inert gas. A current is run through the filament, and the resistance causes the filament to glow. Over time, particles of tungsten break away and evaporate inside the bulb, causing the filament to weaken and break. It was discovered that introducing a small amount of halogen (either iodine or bromine) into the bulb caused the evaporating tungsten to re-deposit on the filament, prolonging the life of the light bulb -- and amplifying the light because the tungsten could now withstand greater heat. These bulbs are called halogen bulbs or halogen lamps.
The greatest single advantage of halogen lamps is energy conservation. While halogen bulbs cost more to buy, they more than compensate for the extra cost in a longer life for each bulb. Halogen lamps have applications that provide greater light intensity where needed, such as security lights, theater lights, search lights, headlamps and automobile headlights. Many of these applications also employ a sealed-beam design for the lights.
Sealed-beam is a term that applies to the whole construction of a lamp. This construction includes the bulb – which can be incandescent, halogen or fluorescent – but it also includes the encasement, the reflector, a protective glass cover and an attachment apparatus for the power source. Sealed-beam means that the entire assembly is sold as a single component – usually waterproof – that is not meant to be disassembled. The most common sealed-beam lamps are automobile headlights, stage lights and outdoor security lights.