Headlight

A headlight is a light, usually attached to the front of a vehicle such as a car, with the purpose of illuminating the road ahead during periods of low visibility, such as night or precipitation.

Modern headlights are electric floodlights, positioned in pairs, one on each side of a moving vehicle. Headlights are usually combinations of multiple lamps and reflectors and can be switched between two modes or configurations: low beams and high beams. High beams cast more light at a higher angle, allowing the driver to see further away, but at the cost of potentially blinding drivers of oncoming vehicles. High beams also increase reflection from fog, due to the refraction of the water droplets.

Several countries, including Canada and the Scandinavian countries, require daytime running lights (DRLs) or the use of low-beam headlights during daytime driving as well. Most often, this is the high beams running at half power. Many models of automobiles sold elsewhere are also equipped with daytime running lights, which are automatic when the car is running. This is entirely for the visibility of the car itself by other drivers, especially when there are low-light conditions which a driver may forget to turn on his or her regular headlights. A slight disadvantage of the DRL is that it wastes a small amount of gasoline or petrol, especially with incandescent bulbs.

Most headlights use incandescent light bulbs (usually halogen-type), either with separate high and low beam bulbs, or a single bulb on each side with dual filaments. More and more are using high-intensity discharge (HID) lighting, such as the mercury vapour ones used in white street lights. These have a distinct purplish or bluish cast to them, which also causes fluorescence in certain materials. HID bulbs are also extremely energy-efficient, using over 80% less power, and drawing less electrical current from the alternator and improving fuel efficiency.

A headlight can also be mounted on a bicycle (with a battery or small generator, and most other moving vehicles from airplanes to trains tend to have headlights of their own. Single small headlights may also be mounted on a helmet designed to be worn in situations where light is required but both hands are needed, for example in subterranean mines or for spelunking in caves.

Headlights usually have a distinctive shape with a light bulb positioned in the middle of a reflector cone.

Headlights must be adjusted, aimed to an ISO-regulated inclination and center. This applies most of all to low beams (US name) or dipped beams (UK name), where the aiming results in a specific position of the cut-off line in the beam when projected on a wall at a minimum 10-metre (33-foot) distance from the car. (The cut-off line is the transition between the illuminated and dark section of the beam.) Vehicles are aimed at the end of the production line (often combined with the alignment of the wheels) or in the garage workshop after body repair.