Quad lights were introduced in 1952. Rectangular lights were first used in 1961, but they were prohibited in the United States until 1975. The first halogen light was used in 1962, although they prohibited in the United States until 1978. But the use of H1 and H3 bulbs became legalized in United States in 1997.
In the late 1990s the round style of headlights again became popular on new cars. Except now they involve circular or oval optical elements within an architecturally shaped housing assembly.
Modern headlights are electrically operated and they are positioned in pairs. They are located on the front of the vehicle with one or two on each side. A headlamp system is required to produce a low and a high beam that can be achieved either by an individual lamp for each of the functions or by a single multifunction lamp.
High beams – also known as main beams, full beams or driving beams, cast their light straight ahead, which maximizes seeing distance. But this produces too much glare for safe use when there are other vehicles present on the road. As there is no control of upward light, they also cause backdazzle due to reflection from fog, rain and snow due to the refraction of water droplets.
Low beams – also known as dipped beams have a much stricter control of upward light. They direct most of their light downward and either rightward or leftward (depending on the country the vehicle is in). This provides safe forward visibility without excessive glare or backdazzle.
There are many different types of headlights:
Lens optics – here a light source, either a filament or arc is placed at or near the focus of the reflector. Fresnel and prism optics that are molded into the headlight lens then shift parts of the light laterally and vertically to provide the required light distribution pattern. Most sealed-beam have lens optics.
Reflector optics – these are required to give the proper light distribution pattern and are designed into the reflector itself. Optic reflectors are mostly made of compression-molded or injection molded plastic, although you can also get glass and metal optic reflectors.
Projector optics – in this system a filament is located at one focus of an elliptical and a condenser lens at the front of the lamp.
HID headlamps – known as high-intensity discharge, which is the technical term for the electric arc that produces light. These lamps are often called xenon headlights because of the xenon gas that is used in the lamps. The xenon gas allows the lamp to produce minimal amounts of light upon startup and speed the warm-up time. Vehicles that are equipped with HID headlights need to have a lens cleaning system and automatic beam leveling control. Both of these are to reduce the tendency for high-output headlights to cause high levels of glare to other vehicles on the road.
There are many different bulb types but they are not interchangeable, therefore the correct one for a specific vehicle must be purchased.
Headlights don’t require much care. When the filament burn out, the entire module is replaced. Since 1985 most vehicles in North America have used headlight lens-reflector assemblies that are part of the vehicle, and when it fails, just the bulb is replaced.
Headlight aim must be properly checked on a regular basis. As they can deteriorate over time and because of this they can become discolored, turning yellowish and reducing light output, making them unsafe.