Halogen headlights are an improvement over traditional incandescent headlights for automobiles, introduced first into Europe in 1962 and later into the United States in 1978. Early halogen headlamps had to be encased in quartz bulb fixtures on automobiles, as the halogen gas environment for the filament created a temperature that could melt conventional glass. In the 1980s, halogen technology improved to the level of self-contained bulbs that could now be mounted in clear plastic headlight fixtures, and were much more affordable and easier to change. This led to a proliferation in diverse shapes and designs for halogen headlights as automakers customized their look to make their car models distinctive from one another.
The basic design of a halogen headlight is not all that different from its predecessor. While an incandescent headlight uses a tungsten filament surrounded by an inert gas, such as a combination of nitrogen, argon, and krypton, a halogen headlight uses the same tungsten filament, but instead has a small amount of halogen gas added in with the standard gas mixture. This is what gives halogen headlights advantages over previous versions of headlights. The halogen undergoes a chemical reaction with the tungsten filament creating halides, which deposit oxidized compounds of tungsten back onto the filament, and prevent tungsten particles from adhering to the surface of the bulb. This process prevents the bulb from dimming as it ages, and also allows it to generate more light with the same amount of electrical power.
In order for halogen headlights to work, however, they must burn at a high temperature, usually around 482° Fahrenheit (250° Celsius). This temperature, and a necessity for a higher gas pressure inside the bulb, has led to halogen headlights being designed as smaller bulb units that are mounted in the center of a traditional headlight enclosure. The enclosure lens and light reflector can, therefore, be unpressurized, and made from plastics that don't have to withstand the high temperatures or pressures for which incandescent headlights were built.
These design specifications for halogen headlights that took root in the 1980s in auto manufacturing have led to bulbs that burn brighter and last about twice as long as conventional incandescent versions. These features were taken advantage of in different ways both in Europe and the US. European laws utilized the ability of halogen headlights to burn brighter with the same amount of electrical consumption of a standard headlight to increase light output levels to 225,000 candela, a measure of luminosity. US automakers, by contrast, chose to utilize the power savings of halogen headlights by restricting their candela output to a rating of 150,000, which consumed less electricity than a standard incandescent headlight that usually generated 75,000 candela of light. Cars in European cities typically have brighter lights that reach farther down the road at night than their US counterparts, therefore, but US cars have a slight increase in fuel efficiency due to their need to produce less electricity to power halogen headlights.