This technical page includes tips for fitting halogen headlights
If you really need to ask you have obviously never seen the difference between Tungsten bulbs and Halogen bulbs, but why are Halogen bulbs better?
Firstly let’s get things straight, all incandescent bulbs use a Tungsten filament, it gets so hot it glows WHITE-HOT and we can see by it. Halogen bulbs use a Halogen gas, often Iodine gas, whereas conventional bulbs use Argon or partial vacuum. Halogen bulbs go by several names, Quartz-Iodine, Quartz Halogen, Tungsten-Iodine so when Charlie Croaker asks, “are they Quartz Iodine” about half an hour into The Italian Job, the reply should be “yes Charlie, they are Halogen”.
In a conventional bulb, Tungsten from the filament evaporates and often leaves a black film on the inside of the glass, reducing the bulb’s brightness. As this gets worse the filament becomes thinner, burns hotter and evaporates more quickly.
Halogens are elements that compound to form salts and if these are added in precise quantities to the inert gas filling they have a significant effect on this evaporation process. Tungsten evaporates from the surface of the filament at about 2000 deg Celsius and migrates toward the glass envelope, cooling as it moves away from the filament. When it has cooled to 1450 Deg Celsius the Halogen compounds with the Tungsten to form Tungsten-Halides which do not deposit themselves on the glass but are attracted back to the filament. As the Tungsten-Halides move back to the filament their temperature rises again towards 2000 deg. Celsius and the Tungsten-Halides break up once more. The Halogen returns to the gaseous area whilst the Tungsten returns towards the filament, surrounding it and lowering further evaporation.
Simply because the Halogen eliminates evaporation of the Tungsten it is possible to have a hotter filament, thus the light produced is a brighter white.
Because Iodine is only gaseous above 250 deg Celsius it is necessary to have a very small glass envelope to keep the internal temperature up, this is not a problem because there is no blackening of the glass but the glass has to be Quartz due to the temperature. Because quartz is stronger the bulb can be pressurised, which in turn reduces evaporation further.
There are 2 disadvantages however.