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A headlamp is a lamp, usually attached to the front of a vehicle such as a car or a motorcycle, with the purpose of illuminating the road ahead during periods of low visibility, such as darkness or precipitation. Headlamp performance has steadily improved throughout the automobile age, spurred by the great disparity between daytime and nighttime traffic fatalities: the U.S. National Highway Traffic Safety Administration states that nearly half of all traffic-related fatalities occur in the dark, despite only 25% of traffic travelling during darkness.

While it is common for the term headlight to be used interchangeably in informal discussion, headlamp is the term for the device itself, while headlight properly refers to the beam of light produced and distributed by the device.

A headlamp can also be mounted on a bicycle (with a battery or small electrical generator), and most other vehicles from airplanes to trains tend to have headlamps of their own.

Additionally automotive night vision systems work to supplement headlamps.

History

Mechanics

The earliest headlamps were fuelled by acetylene or oil and were introduced in the late 1880s. Acetylene lamps were popular because the flame was resistant to wind and rain. The first electric headlamps were introduced in 1898 on the Columbia Electric Car from the Electric Vehicle Company of Hartford, Connecticut, and were optional. Two factors limited the widespread use of electric headlamps: the short life of filaments in the harsh automotive environment, and the difficulty of producing dynamos small enough, yet powerful enough to produce sufficient current.

"Prest-O-Lite" acetylene lights were offered by a number of manufacturers as standard equipment for 1904, and Peerless made electric headlamps standard in 1908. A Birmingham firm called Pockley Automobile Electric Lighting Syndicate marketed the world's first electric car lights as a complete set in 1908, which consisted of headlights, sidelights and tail lights and were powered by an 8 volt battery.

In 1912, Cadillac integrated their vehicle's Delco electrical ignition and lighting system, creating the modern vehicle electrical system.

"Dipping" (low beam) headlamps were introduced in 1915 by the Guide Lamp Company, but the 1917 Cadillac system allowed the light to be dipped with a lever inside the car rather than requiring the driver to stop and get out. The 1924 Bilux bulb was the first modern unit, having the light for both low (dipped) and high (main) beams of a headlamp emitting from a single bulb. A similar design was introduced in 1925 by Guide Lamp called the "Duplo". In 1927, the foot-operated dimmer switch or dip switch was introduced and became standard for much of the century. The last vehicle with a foot-operated dimmer switch was the 1991 Ford F-Series. Fog lamps were new for 1938 Cadillacs, and their 1954 "Autronic Eye" system automated the selection of high and low beams.

In 1935 Tatra T77a introduced light with cornering function - the front had three headlamps of which the central unit was linked to the steering, making it possible to turn this lamp with the steering wheel.

The standardised 7-inch (178 mm) round sealed beam headlamp was introduced in 1940, and was soon required for all vehicles sold in the United States. Britain, Australia and other Commonwealth countries, as well as Japan, also made extensive use of 7-inch sealed beams. With some exceptions from Volvo and Saab, this headlamp size format was never widely accepted in continental Europe, leading to different front-end designs for each side of the Atlantic for decades.

The first halogen lamp for vehicle headlamp use, the H1 lamp, was introduced in 1962 by a European consortium of bulb and headlamp makers. Shortly thereafter, headlamps using the new light source were introduced. These were prohibited in the U.S., where sealed beam headlamps were required. In 1978, sealed beam headlamps with internal halogen burners became available for use in the United States. Halogen sealed beams now dominate the sealed beam market, though it is considerably smaller than it was before replaceable-bulb composite headlamps returned to the U.S. in 1983.

High-intensity discharge (HID) systems were introduced in 1991s BMW 7-series. European and Japanese markets began to prefer HID headlamps, with as much as 50% market share in those markets, but they found slow adoption in North America. 1996's Lincoln Mark VIII was an early American effort at HIDs, and was the only car with DC HIDs.

Regulations and requirements

Modern headlamps are electrically operated, positioned in pairs, one or two on each side of the front of a vehicle. A headlamp system is required to produce a low and a high beam, which may be achieved either by an individual lamp for each function or by a single multifunction lamp. High beams (called "main beams" or "full beams" or "driving beams" in some countries) cast most of their light straight ahead, maximizing seeing distance, but producing too much glare for safe use when other vehicles are present on the road. Because there is no special control of upward light, high beams also cause backdazzle from fog, rain and snow due to the retroreflection of the water droplets. Low beams (called "dipped beams" or "passing beams" in some countries) have stricter control of upward light, and direct most of their light downward and either rightward (in right-traffic countries) or leftward (in left-traffic countries), to provide safe forward visibility without excessive glare or backdazzle.

Low Beam

Low beam (dipped beam, passing beam, meeting beam) headlamps provide a distribution of light designed to provide adequate forward and lateral illumination with limits on light directed towards the eyes of other road users, to control glare. This beam is intended for use whenever other vehicles are present ahead. The international ECE Regulations for filament headlamps[ and for high-intensity discharge headlamps specify a beam with a sharp, asymmetric cutoff preventing significant amounts of light from being cast into the eyes of drivers of preceding or oncoming cars. Control of glare is less strict in the North American SAE beam standard contained in FMVSS / CMVSS 108.

High Beam

High beam (main beam, driving beam, full beam) headlamps provide a bright, centre-weighted distribution of light with no particular control of light directed towards other road users' eyes. As such, they are only suitable for use when alone on the road, as the glare they produce will dazzle other drivers. International ECE Regulations permit higher-intensity high-beam headlamps than are allowed under North American regulations.

Optical systems

Lens optics

A light source (filament or arc) is placed at or near the focus of a reflector, which may be parabolic or of non-parabolic complex shape. Fresnel and prism optics moulded into the headlamp lens refract (shift) parts of the light laterally and vertically to provide the required light distribution pattern. Most sealed-beam headlamps have lens optics.

Reflector optics

Starting in the 1980s, headlamp reflectors began to evolve beyond the simple stamped steel parabola. The 1983 Austin Maestro was the first vehicle equipped with Lucas-Carello's homofocal reflectors, which comprised parabolic sections of different focal length to improve the efficiency of light collection and distribution.CAD technology allowed the development of reflector headlamps with nonparabolic, complex-shape reflectors. First commercialised by Valeo under their Cibié brand, these headlamps would revolutionise automobile design.

The 1987 U.S.-market Dodge Monaco/Eagle Premier twins and European Citroën XM were the first cars with complex-reflector headlamps with faceted optic lenses. General Motors' Guide Lamp division in America had experimented with clear-lens complex-reflector lamps in the early 1970s and achieved promising results, but the U.S.-market 1990 Honda Accord was first with clear-lens multi-reflector headlamps; these were developed by Stanley in Japan. The optics to distribute the light in the desired pattern are designed into the reflector itself, rather than into the lens. Depending on the development tools and techniques in use, the reflector may be engineered from the start as a bespoke shape, or it may start as a parabola standing in for the size and shape of the completed package. In the latter case, the entire surface area is modified so as to produce individual segments of specifically calculated, complex contours. The shape of each segment is designed such that their cumulative effect produces the required light distribution pattern.

Modern reflectors are commonly made of compression-moulded or injection moulded plastic, though glass and metal optic reflectors also exist. The reflective surface is vapour deposited aluminium with a clear overcoating to prevent the extremely thin aluminium from oxidizing. Extremely tight tolerances must be maintained in the design and production of complex-reflector headlamps.