Headlight
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A headlight or headlamp 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.
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.
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Automotive headlights
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.
In the USA, many headlights are of the sealed beam type, meaning that the reflector, lens array on the front, as well as bulb, are all one unit that must be replaced together. In most European countries sealed beam headlights are not allowed on automobiles. 'H4' halogen incandescent light bulbs are used instead.
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 light 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) lamps, similar to the Mercury-vapor lamps 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.
Headlights usually have a distinctive shape with a light bulb positioned in the focus of a parabolic reflector cone. The glass lens usually has additional patterns to direct the lightbeam in a certain direction. These patterns are different (mirrored) for left- or right hand traffic.
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. Headlights are checked for alignment prior to leaving the production line, and may need realignment periodically throughout the car's lifetime, or after repairing body damage.
History of automotive headlights
The earliest headlights were fueled by acetyline or oil and were introduced by drivers in the late 1880s. Acetyline was popular because the flame was resistant to wind and rain. The first electric headlights were introduced in 1898 on the Columbia Electric Car from the Electric Vehicle Company of Hartford, Connecticut, but they were optional. "Prest-O-Lite" acetylene lights were offered by a number of manufacturers as standard equipment for 1904, and Peerless made electrical headlights standard in 1908. In 1912, Cadillac integrated their vehicle's Delco electrical ignition and lighting system, creating the modern vehicle electronics system.
"Dipping" (low beam) headlights were introduced in 1915 by the Guide Lamp Company, but the 1917 Cadillac system was much more useful as it 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 both low- and high-beams in a single bulb. A similar design was introduced the next year by Guide Lamp called the "Duplo". In 1927, the foot-operated dimmer was introduced and would become standard for much of the century. The last vehicle with a foot-operated dimmer was the 1991 Ford F-Series. Fog lights were new for 1938 Cadillacs, and that company's 1954 "Autronic Eye" system automated the switch between high and low beams.
The standardized 7 inch sealed beam headlight was introduced in 1940, and was soon required for all vehicles sold in the United States. Sealed beams were never accepted in other countries, however, leading to different front-end designs for each side of the Atlantic for decades. The U.S. DOT also outlawed transparent covered headlights (as famously used on the Jaguar E-Type and Ferrari Daytona) in 1968, further altering the look of European models sold in the United States. In 1984, these regulations were finally reversed. The first U.S.-market car with composite headlights was the Lincoln Mark VII of that year, but it would be another dozen years before sealed beam lights vanished from new cars altogether.
Directional headlights were a notable feature of the 1948 Tucker, but the technology went back to the 1937 Tatra T77A. The first real production car with steerable headlights was the 1967 Citron DS. The technology was slow to gain acceptance, but is today offered on a number of luxury vehicles, including those from Lexus, Infiniti, and the Porsche Cayenne.
In 1954, Cibie introduced an automatic headlight leveling system. First used in the Panhard Dyna, it would spread slowly. Although automatic headlight leveling never became a standard feature on all cars, it is still used on some today.
Cadillac introduced quad headlights (using 5¾ inch sealed lamps) in 1957. Chrysler soon followed suit, and by 1958, every new car would have four round headlights, instead of two. The vastly superior illumination afforded by this system makes it common in the next decade, changing the look of many cars for the 1960s.
Rectangular lamps were first used in 1961. Developed by Cibie for the Citron Ami 6, they would not be allowed in the United States until 1975, on the Cadillac Eldorado. By 1979, the majority of new cars now had the square headlights.
Pop-up headlights were introduced in 1963, on the Chevrolet Corvette. When the lights were turned on, they would appear from inside the car's front bumper. Many famous cars to use this feature include the Ferrari Testarossa, Mazda RX-7, RX-8 and Miata, Dodge Daytona, Pontiac Firebird, Nissan 300ZX, Toyota Celica and Supra, Honda Accord and Prelude, and Buick Reatta. The Corvette itself continued to use pop-up headlights until it was redesigned for 2004.
The first halogen bulbs for vehicle use were introduced in 1962 by Hella. First used only for auxiliary lights, they spread to headlight use in 1965. The modern dual-filament H4 bulb was patented in 1971. General Motors introduced halogen headlights in the U.S. in 1978.
Daytime running lights were mandatory in Finland for 1972. The technology spread around Scandanavia, with Sweden following in 1977, Norway in 1986, Iceland in 1988, and Denmark in 1990. Canada was the first large DRL market, requiring them as of 1990. General Motors, Saab, and Volkswagen introduce daytime running lights in the U.S. market in 1995, though they would not be mandated.
Projector beam lamps first appeared in 1983. Developed by Hella, the "DE" lights are much brighter than previous systems. The 1986 BMW 7 Series was the first to use projectors for low beams. Projector and CAD technology allowed the development of so-called "free form" headlights. First used by the 1989 Citron XM, free form headlights would revolutionize automobile design, and lead to the downfall of the United States' decades-long sealed beam standard. The 1990 Honda Accord was the first U.S.-market car with free form lights.
High-intensity discharge systems were introduced in 1991's BMW 7-series as an even-brighter system than projectors. The technology spread to the United States for 1997's Lincoln Mark VIII, which was also the first car with DC HIDs.
Directional headlights
These provide improved lighting for cornering. Some automobiles have their headlights connected to the steering mechanism so the lights will follow the movement of the front wheels. The Citron DS was one car equipped with such a system. Also, some automobiles have vertically adjustable lights, to compensate for dipping when carrying heavy loads.
Dual-beam headlights
Night driving has long been dangerous due to the glare of headlights that blind drivers approaching from the opposite direction. Therefore, headlights that satisfactorily illuminate the highway ahead of the automobile for night driving without temporarily blinding approaching drivers have long been sought. To correct this problem resistance-type dimming circuits, which decreased the brightness of the headlights when meeting another car, were first introduced. This gave way to mechanical tilting reflectors and later to double-filament bulbs with a high and a low beam, called sealed-beam units.
There was only one filament at the focal point of the reflector in the double-filament headlight unit of necessity. Greater illumination required for high-speed driving with the high beam, consequently, the lower beam filament was placed off center, with a resulting decrease in lighting effectiveness. From the 1950s, manufacturers equipped their models with four headlights to improve illumination.
In some cars, dimming is automatically achieved. This happens by means of a photocell-controlled switch in the lamp circuit that is triggered by the lights of an oncoming car. Larger double-filament lamps and halogen-filled lamp bulbs with improved photometrics permitted a return to two-headlight systems on some cars. At many places the law limits the total intensity of forward lighting systems to 75,000 candlepower (800,000 lux).
In most new automobiles, lowering front hood heights for improved aerodynamic drag and driver visibility reduces the vertical height available for headlights. Due to this, lower-profile rectangular sealed-beam units and higher-intensity bulbs, in conjunction with partial parabolic reflectors with reduced vertical axis, were adopted in the 1970s. In some cases, models featured full-size concealed headlights that were not visible until turned on. An electric motor linkage was used to rotate the lamp housing or a housing cover into proper position to supply lighting. Aerodynamic benefits were provided by this system only when the headlights were concealed.
In the 1960s, signal lamps and other special-purpose lights were increased in usage. Amber-colored front and red rear signal lights are flashed as a turn indication; all these lights are flashed simultaneously in the "flasher" system for use when a car is parked along a roadway or is traveling at a low speed on a high-speed highway. The law requires that marker lights that are visible from the front, side, and rear be also present. Red-colored rear signals are used to denote braking, and, on some models, cornering lamps to provide extra illumination in the direction of an intended turn are available. These are actuated in conjunction with the turn signals. To provide illumination to the rear when backing up, backup lights are required.ja:前照灯