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A headlamp is a lamp, 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 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 technically correct 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.
Mechanics
The earliest headlamps were fueled by acetylene or oil and were introduced in the late 1880s.
Discussion
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Encyclopedia
A headlamp is a lamp, 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 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 technically correct 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.
History of automotive headlamps
Mechanics
The earliest headlamps were fueled 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 electrical headlamps standard in 1908. 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 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. Foglamps were new for 1938 Cadillacs, and their 1954 "Autronic Eye" system automated the switch between high and low beams.
The standardised 7 in (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 in. sealed beams. With some exceptions from Volvo and Saab, this headlamp size format was never widely accepted in Europe, leading to different front-end designs for each side of the Atlantic for decades.
The first halogen headlamp for vehicle use was introduced in 1962 by a consortium of European bulb and headlamp makers. Halogen technology makes incandescent filaments more efficient and can produce more light than from non-halogen filaments at the same power consumption. These were prohibited in the US, where non-halogen sealed beam lamps were required until 1978. From 1978 to 1983, all halogen headlamps in the U.S. were sealed beams with halogen bulbs inside. These halogen sealed beams remain available, 25 years after replaceable-bulb headlamps returned to the US in 1983.
High-intensity discharge systems were introduced in 1991's 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.
Design & Style
Beyond the engineering, performance and regulatory-compliance aspects of headlamps, there is the consideration of the various ways they are designed and arranged on a motor vehicle. Headlamps were round for many years, because that is the easiest shape for parabolic reflector manufacture.
Headlamp styling outside of the United States, pre-1983
There was no requirement in Europe for headlamps of standardised size or shape. Automakers were free to design their lamps to whatever shapes and sizes they wished, as long as the lamps met the engineering and performance requirements contained in the applicable European safety standards. That design freedom permitted the development of rectangular headlamps, first used in 1961. Developed by Cibié for the Citroën Ami 6 and by Hella for the German Ford Taunus, they were prohibited in the United States where round lamps were required until 1975. Another early headlamp styling concept involved conventional round lamps faired into the car's bodywork with aerodynamic glass covers, such as those on the 1961 Jaguar E-Type.
Headlamp styling in the United States, 1940-1983
In 1940, the US government mandated a system of two 7 in. (178 mm) round sealed beam headlamps on all vehicles. Headlamp styling in the United States virtually ceased for many decades after this event.
A system of four round lamps, rather than two—one high/low and one high-beam 5¾ in. (146 mm) sealed beam on each side—were introduced in 1952 when the Prevost Car company included them in its Citaden bus model. Cadillac, Chrysler and Nash placed them in some of their car models in states that permitted the new system for the 1957 model year, and other American marques followed suit when all states permitted quad lamps in 1958. These lamps had some photometric advantages, but the primary advantage was the styling novelty permitted by the use of two small rather than one large lamp per side of the vehicle. The freedom was not absolute, however. Auto stylists such as Virgil Exner carried out design studies with the low beams in their conventional outboard location, and the high beams vertically stacked at the centreline of the car. No such designs reached volume production. Most cars had their headlights in pairs side by side on each side of the car. Some Oldsmobiles had a parking light in the middle of each pair.
Also popular was an arrangement in which the two headlamps on each side were stacked, low beams above high beams. Nash used this arrangement in the 1957 model year. Pontiac used this design starting in the 1963 model year; American Motors, Ford, Cadillac and Chrysler followed two years later. Also in the 1965 model year, the Buick Riviera had concealable stacked headlamps. The Mercedes-Benz W100, W108, W111, and W112 models sold in America used this arrangement because their home-market composite lamps were illegal in the US. The British firm Alvis and the French firm FACEL also used this setup for some of their cars, as did Nissan in Japan.
In the late 1950s and early 1960s, Lincoln, Buick, and Chrysler arranged the headlamps diagonally by placing the low-beam lamps outboard and above the high-beam lamps. Certain British cars used a less extreme diagonal arrangement, with the inboard high-beam lamps placed only slightly lower than the outboard low-beam units. The 1965 Gordon-Keeble, Triumph Vitesse and Bentley S3 Continental used such an arrangement. (source: World Car Catalog)
In 1968 the U.S. DOT prohibited any decorative or protective element in front of the headlamps whenever the headlamps are switched on. Glass-covered headlamps, used on e.g. the Jaguar E-Type, pre-1968 VW Beetle, 1965 Chrysler and Imperial models, Porsche 356, Citroën DS and Ferrari Daytona were no longer permitted and vehicles had to be imported with uncovered headlamps for the US market. This change meant that vehicles designed for good aerodynamic performance could not achieve it for the US market.
When Federal Motor Vehicle Safety Standard 108 was amended in the early 1970s to permit rectangular headlamps, these were placed in horizontally-arrayed or vertically-stacked pairs. By 1979, the majority of new cars in the US market were equipped with rectangular lamps. Again, the US permitted only two standardised sizes of rectangular sealed-beam lamp: A system of two 200 mm × 142 mm (7½ in. × 5½ in.) high/low beam units corresponding to the existing 7-inch round format, or a system of four 165 mm x 100 mm (6½ in. × 4 in.) units, two high/low and two high-beam, corresponding to the existing 5¾-inch (146 mm) round format.
International headlamp styling, 1983 to present
In 1983, the 44-year-old US headlamp regulations were amended to allow replaceable-bulb, nonstandard-shape, architectural headlamps with aerodynamic lenses. The first U.S.-market car since 1939 with composite headlamps was the 1984 Lincoln Mark VII. These composite headlamps were commonly referred to as "Euro" headlamps, since aerodynamic headlamps were common in Europe. Though conceptually similar to European headlamps with nonstandardised shape and replaceable-bulb construction, these headlamps conform to the SAE headlamp standards of US Federal Motor Vehicle Safety Standard 108, and not the internationalised European safety standards used outside North America. Nevertheless, this change to US regulations largely united headlamp styling within and outside the North American market.
In the late 1990s, round headlamps returned to popularity on new cars. These are generally not the discrete self-contained round lamps as found on older cars (certain Jaguars excepted), but rather involve circular or oval optical elements within an architecturally-shaped housing assembly.
Hidden headlamps
Hidden headlamps were introduced in 1936, on the Cord 810. They were mounted in the front fenders, which were smooth until the lights were cranked out, each with its own small dash-mounted crank. They aided aerodynamics when the headlamps were not in use, and were among the Cord's signature design features.
Many notable cars used this feature, but no current volume-produced car models use hidden headlamps, because they present difficulties in complying with pedestrian-protection provisions recently added to international auto safety regulations, and because the mechanisms are costly and heavy. Hidden headlamps require one or more vacuum-operated servos and reservoirs, with associated plumbing and linkage, or electric motors, geartrains and linkages to raise the lamps to an exact position to assure correct aiming despite ice, snow and age. Some early hidden headlamps, such as those on the Saab Sonett III, used a lever-operated mechanical linkage to raise the headlamps into position. Current market demands place a premium on vehicles' aerodynamic performance with lamps off and on, further reducing the attractiveness of pop-up headlamps. In addition, recent ECE regulations contain standards regarding protuberances on car bodies to minimise injury to pedestrians struck by cars.
Some hidden headlamps themselves do not move, but rather are covered when not in use by panels designed to blend in with the car's styling. When the lamps are switched on, the covers are swung out of the way, usually downward or upward, for example on the 1992 Jaguar XJ220. The door mechanism may be actuated by vacuum pots, as on some Ford vehicles of the late 1960s through early 1980s such as the 1967-1969 Mercury Cougar, or by an electric motor as on various Chrysler products of the middle 1960s through late 1970s such as the 1966-1967 Dodge Charger.
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 especial 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" 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.
Compatibility with traffic directionality
Most low-beam headlamps are specifically designed for use on only one side of the road. Headlamps for use in left-traffic countries have low-beam headlamps that "dip to the left"; the light is distributed with a downward/leftward bias to show the driver the road and signs ahead without blinding oncoming traffic. Headlamps for right-traffic countries have low beams that "dip to the right", with most of their light directed downward/rightward. Within Europe, when driving a vehicle with RH-traffic headlamps in a LH-traffic country or vice versa for a limited time (as for example on vacation or in transit), it is a legal requirement to adjust the headlamps temporarily so that the wrong-side hot spot of the beam does not dazzle oncoming drivers. This may be achieved by adhering blackout strips or plastic prismatic lenses to a designated part of the lens, but some varieties of the projector-type headlamp can be made to produce a proper left- or right-traffic beam by shifting a lever or other movable element in or on the lamp assembly.
Because wrong-side-of-road headlamps blind oncoming drivers and do not adequately light the driver's way, and blackout strips and adhesive prismatic lenses reduce the safety performance of the headlamps, most countries require all vehicles registered or used on a permanent or semipermanent basis within the country to be equipped with headlamps designed for the correct traffic-handedness. North American vehicle owners sometimes privately import and install Japanese-market (JDM) headlamps on their car in the mistaken belief that the beam performance will be better, when in fact such misapplication is quite hazardous and illegal.
Use in daytime
Some countries require automobiles to be equipped with automatic daytime running lamps (DRL), which are intended to increase the conspicuity of vehicles in motion during the daytime. DRL may consist of the manual or automatic illumination of the low beams at full or reduced intensity, or the high beams at reduced intensity, or may not involve the headlamps at all. Countries requiring DRL include Bosnia and Herzegovinia, Canada, Colombia, Estonia, Iceland, Hungary, Latvia, Lithuania, Macedonia, Poland, Slovenia, Sweden, Finland and Norway.
Construction, performance, and aim
There are two different beam pattern and headlamp construction standards in use in the world: The ECE standard, which is allowed or required in virtually all industrialised countries except the United States, and the SAE standard that is mandatory only in the US. Japan formerly had bespoke lighting regulations similar to the US standards, but for the left side of the road. However, Japan now adheres to the ECE standard. The differences between the SAE and ECE headlamp standards are primarily in the amount of glare permitted towards other drivers on low beam (SAE permits much more glare), the minimum amount of light required to be thrown straight down the road (SAE requires more), and the specific locations within the beam at which minimum and maximum light levels are specified.
ECE low beams are characterised by a distinct horizontal "cutoff" line at the top of the beam. Below the line is bright, and above is dark. On the side of the beam facing away from oncoming traffic (right in right-traffic countries, left in left-traffic countries), this cutoff sweeps or steps upward to direct light to road signs and pedestrians. SAE low beams may or may not have a cutoff, and if a cutoff is present, it may be of two different general types: VOL, which is conceptually similar to the ECE beam in that the cutoff is located at the top of the left side of the beam and aimed slightly below horizontal, or VOR, which has the cutoff at the top of the right side of the beam and aimed at the horizon.
Proponents of each headlamp system decry the other as inadequate and unsafe: U.S. proponents of the SAE system claim that the ECE low beam cutoff gives short seeing distances and inadequate illumination for overhead road signs, while international proponents of the ECE system claim that the SAE system produces too much glare. Comparative studies have repeatedly shown that there is little or no overall safety benefit to either SAE or ECE beams; the two systems' acceptance and rejection by various countries is based primarily on inertial and philosophical grounds.,
In North America, the design, performance and installation of all motor vehicle lighting devices are regulated by Federal and Canada Motor Vehicle Safety Standard 108, which incorporates SAE technical standards. Elsewhere in the world, ECE internationalised regulations are in force either by reference or by incorporation in individual countries' vehicular codes.
US laws required sealed beam headlamps on all vehicles between 1940 and 1983, and other countries such as Japan, United Kingdom and Australia also made extensive use of sealed beams. In most other countries, and in the US since 1984, replaceable-bulb headlamps predominate.
Headlamps on new vehicles must produce white light, according to both ECE and SAE standards. Previous ECE regulations also permitted selective yellow light, which from 1936 until 1993 was required on all vehicles registered in France. Yellow headlamps are no longer required anywhere, but remain permitted in France, Belgium, The Netherlands, Switzerland, Japan, New Zealand, and some other countries.
Headlamps must be kept in proper alignment (or "aim"). Regulations for aim vary from country to country and from beam specification to beam specification. US SAE headlamps are aimed without regard to headlamp mounting height. This gives vehicles with high-mounted headlamps a seeing distance advantage, at the cost of increased glare to drivers in lower vehicles. ECE headlamps' aim angle is linked to headlamp mounting height. This gives all vehicles roughly equal seeing distance and all drivers roughly equal glare.
Optical systems
Reflector lamps
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 then shift parts of the light laterally and vertically to provide the required light distribution pattern. The lens may use both refraction and TIR to achieve the desired results. Most sealed-beam headlamps have lens optics.
Reflector optics
Starting in the 1980s, CAD technology allowed the development of reflector headlamps with nonparabolic, complex-shape reflectors. First made by Valeo under their Cibié brand, these headlamps would revolutionise automobile design. The 1987 Dodge Monaco/Eagle Premier was the first U.S.-market car with complex-reflector headlamps, while the 1990 Honda Accord was the first U.S.-market car with such headlamps employing a completely clear, nonfaceted front lens.
The optics to distribute the light in the desired pattern are designed into the reflector itself, called an "optic reflector". 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.
Optic reflectors are commonly made of compression-moulded or injection molded plastic, though glass and metal optic reflectors also exist. The reflective surface is vapor deposited aluminum with a clear overcoating to prevent the extremely thin aluminum from oxidizing. Extremely tight tolerances must be maintained in the design and production of complex-reflector headlamps.
Dual-beam reflector headlamps
Night driving has long been dangerous due to the glare of headlights from oncoming traffic which temporarily blinds approaching drivers. Headlamps that satisfactorily illuminate the road ahead without causing glare have long been sought. The first solutions involved resistance-type dimming circuits, which decreased the brightness of the headlamps. This yielded to tilting reflectors, and later to dual-filament bulbs with a high and a low beam. Automatic headlamp dimmers were also introduced.
In a two-filament headlamp, there can only be one filament exactly at the focal point of the reflector. There are two primary means of producing two different beams from a two-filament bulb in a single reflector.
American system
One filament is located at the focal point of the reflector. The other filament is shifted axially and radially away from the focal point. In most 2-filament sealed beams and in 2-filament replaceable bulbs type 9004, 9007 and H13, the high beam filament is at the focal point and the low beam filament is off focus. For use in right-traffic countries, the low beam filament is positioned slightly upward, forward and leftward of the focal point, so that when it is energised, the light beam is widened and shifted slightly downward and rightward of the headlamp's axis. Transverse-filament bulbs such as 9004 can only be used with the filaments horizontal, but axial-filament bulbs can be rotated or "clocked" by the headlamp designer so as to optimise the beam pattern or to effect the traffic-handedness of the low beam. The latter is accomplished by clocking the low-beam filament in an upward-forward-leftward position to produce a right-traffic low beam, or in an upward-forward-rightward position to produce a left-traffic low beam.
The opposite tactic has also been employed in certain 2-filament sealed beams. Placing the low beam filament at the focal point to maximise light collection by the reflector, and positioning the high beam filament slightly rearward-rightward-downward of the focal point. The relative directional shift between the two beams is the same with either technique—in a right-traffic country, the low beam is slightly downward-rightward and the high beam is slightly upward-leftward, relative to one another—but the lens optics must be matched to the filament placements selected.
European system
The traditional European method of achieving low and high beam from a single bulb involves two filaments along the axis of the reflector. The high beam filament is on the focal point, while the low beam filament is approximately 1 cm forward of the focal point and 3 mm above the axis. Below the low beam filament is a cup-shaped shield (called a "Graves Shield") spanning an arc of 165°. When the low beam filament is illuminated, this shield casts a shadow on the corresponding lower area of the reflector, blocking downward light rays that would otherwise strike the reflector and be cast above the horizon. The bulb is rotated (or "clocked") within the headlamp to position the Graves Shield so as to allow light to strike a 15° wedge of the lower half of the reflector. This is used to create the upsweep or upstep characteristic of ECE low beam light distributions. The bulb's rotative position within the reflector depends on the type of beam pattern to be produced and the traffic directionality of the market for which the headlamp is intended.
This system was first used with the Bilux/Duplo R2 bulb of 1954, and later with the halogen H4 bulb of 1971. In 1992, U.S. regulations were amended to permit the use of H4 bulbs redesignated HB2 and 9003, and with slightly different production tolerances stipulated. These are physically and electrically interchangeable with H4 bulbs. Similar optical techniques are used, but with different reflector and/or lens optics to create a US beam pattern rather than a European one.
Each system has its advantages and disadvantages. The American system historically permitted a greater overall amount of light within the low beam, since the entire reflector and lens area is used, but at the same time, the American system has traditionally offered much less control over upward light that causes glare, and for that reason has been largely rejected outside the US. In addition, the American system makes it difficult to create markedly different low and high beam light distributions. The high beam is usually a rough copy of the low beam, shifted slightly upward and leftward. The European system traditionally produced low beams containing less overall light, because only 60% of the reflector's surface area is used to create the low beam. However, low beam focus and glare control are easier to achieve. In addition, the lower 40% of the reflector and lens are reserved for high beam formation, which facilitates the optimisation of both low and high beams.
Recent developments
Complex-reflector technology in combination with new bulb designs such as H13 is enabling the creation of European-type low and high beam patterns without the use of a Graves Shield, while the 1992 US approval of the H4 bulb has made traditionally European 60% / 40% optical area divisions for low and high beam common in the US. Therefore, the difference in active optical area and overall beam light content no longer necessarily exists between US and ECE beams. Dual-beam HID headlamps employing reflector technology have been made using adaptations of both techniques.
Projector (polyellipsoidal) lamps
In this system a filament is located at one focus of an ellipsoidal reflector and has a condenser lens at the front of the lamp. A shade is located at the image plane, between the reflector and lens, and the projection of the top edge of this shade provides the low-beam cutoff. The shape of the shade edge, and its exact position in the optical system, determines the shape and sharpness of the cutoff. The shade may have a solenoid actuated pivot to provide both low and high beam — the shade is removed from the light path to create high beam, and placed in the light path to create low beam, and such optics are known as BiXenon or BiHalogen projectors, depending on the light source used. If there is no such arrangement, the cutoff shade is fixed in the light path, in which case separate high-beam lamps are required. The condenser lens may have slight fresnel rings or other surface treatments to reduce cutoff sharpness. Recent condenser lenses incorporate optical features specifically designed to direct some light upward towards the locations of retroreflective overhead road signs.
Hella introduced its "projector beam" optics for acetylene headlamps in 1911, but following the electrification of vehicle lighting, this optical technology wasn't used for many decades. The first modern polyellipsoidal automotive lamp was the Super-Lite, an auxiliary headlamp produced in a joint venture between Chrysler Corporation and Sylvania and optionally installed in 1969 and 1970 full-size Dodge automobiles. It used an 85 watt transverse-filament tungsten-halogen bulb and was intended as a mid-beam, to extend the reach of the low beams during turnpike travel when low beams alone were inadequate but high beams would produce excessive glare.
Projector main headlamps first appeared in 1981 on the Audi Quartz, the Audi Quattro-based concept car designed by Pininfarina for Geneva Auto Salon. Developed more or less simultaneously in Germany by Hella and in France by Cibié, the projector low beam permitted accurate beam focus and a much smaller-diameter optical package, though a much deeper one, for any given beam output. The version of the 1986 BMW 7 Series sold outside North America was the first volume-production auto to use polyellipsoidal low beam headlamps.
Light sources
Tungsten light sources
The first electric headlamp light source was the tungsten filament, operating in a vacuum or inert-gas atmosphere inside the headlamp bulb or sealed beam. Compared to newer-technology light sources, tungsten filaments give off small amounts of light relative to the power they consume. Also, during normal operation of such lamps, tungsten boils off the surface of the filament and condenses on the bulb glass, blackening it. This reduces the light output of the filament and blocks some of the light that would pass through an unblackened bulb glass. For these reasons, plain tungsten filaments are all but obsolete in automotive headlamp service.
Tungsten-halogen light sources
Halogen technology (also "quartz-halogen", "quartz-iodine", "iodine", "iode") makes tungsten filaments more efficacious producers of light—more lumens out per watt in—and Europeans chose to use this extra efficacy to provide drivers with more light than was available from nonhalogen filaments at the same power consumption. Unlike the European approach which emphasised increased light output, most U.S. low beam halogens were low current versions of their nonhalogen counterparts, producing the same amount of light with less power. A slight theoretical fuel economy benefit and reduced vehicle construction cost through reduced wire and switch ratings were the claimed benefits. There was an improvement in seeing distance with U.S. halogen high beams, which were permitted for the first time to produce 150,000 candelas (cd) per vehicle, double the nonhalogen limit of 75,000 cd but still well shy of the international European limit of 225,000 cd. After replaceable halogen bulbs were permitted in U.S. headlamps in 1983, development of U.S. bulbs continued to favour long bulb life and low power consumption, while European designs continued to prioritise optical precision and maximum output.
The first halogen bulb for vehicle use, the H1, was introduced in 1962 by a consortium of European bulb and headlamp makers. This bulb has a single axial filament that consumes 55 watts at 12.0 volts, and produces 1550 lumens ±15% when operated at 13.2 V. H2 (55 W @ 12.0 V, 1820 lm @ 13.2 V) followed in 1964, and the transverse-filament H3 (55 W @ 12.0 V, 1450 lm ±15%) in 1966. H1 still sees wide use in low beams, high beams and auxiliary foglamp and driving lamps, as does H3. The H2 does not see wide use any more because it requires an intricate bulb holder interface to the lamp, has a short life and is difficult to handle. For those reasons, H2 was from ECE Regulation 37 for use in new lamp designs (though H2 bulbs are still manufactured for replacement purposes in existing lamps). The use of H1 and H3 bulbs was legalised in the United States in 1997. More recent single filament bulb designs include the H7 (55 W @ 12.0 V, 1500 lm ±10% @ 13.2 V), H8 (35 W @ 12.0 V, 800 lm ±15% @ 13.2 V), H9 (65 W @ 12.0 V, 2100 lm ±10% @ 13.2 V), and H11 (55 W @ 12.0 V, 1350 lm ±10% @ 13.2 V). 24-volt versions of many bulb types are available for use in trucks, buses, and other commercial and military vehicles.
The first dual-filament halogen bulb (to produce a low and a high beam with only one bulb), the H4, was released in 1971. The U.S. prohibited halogen headlamps until 1978, when halogen sealed beams were released. To this day, the H4 is still not legal for automotive use in the United States. Instead, the Americans created their own very similar standard (HB2/9003). The primary differences are that the HB2 sets more strict requirements on filament positioning, and that the HB2 are required to meet the lower maximum output standards set forth by the United States government.
The first U.S. halogen headlamp bulb, introduced in 1983, was the 9004/HB1. It is a 12.8-volt, transverse dual-filament design that produces 700 lumens on low beam and 1200 lumens on high beam. The 9004 is rated for 65 watts (high beam) and 45 watts (low beam) at 12.8 volts. Other U.S. approved halogen bulbs include the 9005/HB3 (65 W, 12.8 V), 9006/HB4 (55 W, 12.8 V), and 9007/HB5 (65/55 watts, 12.8 V).
Halogen infrared reflective light sources
A further development of the tungsten-halogen bulb has a dichroic coating that passes visible light and reflects infrared radiation. The glass in such a bulb is spherical, rather than tubular. The reflected infrared radiation strikes the filament located at the centre of the sphere, heating the filament to a degree greater than occurs by passing an electric current through the filament. The filament thus superheated emits more light, without an increase in power consumption or a decrease in lifespan.
HID (xenon) light sources
HID stands for high-intensity discharge, a technical term for the electric arc that produces the light. The high intensity of the arc comes from metallic salts that are vapourised within the arc chamber. These lamps are formally known as gas-discharge burners, and produce more light for a given level of power consumption than ordinary tungsten and tungsten-halogen bulbs. Because of the increased amounts of light available from HID burners relative to halogen bulbs, HID headlamps producing a given beam pattern can be made smaller than halogen headlamps producing a comparable beam pattern. Alternatively, the larger size can be retained, in which case the xenon headlamp can produce a more robust beam pattern.
Automotive HID lamps are commonly called 'xenon headlamps', though they are actually metal halide lamps that contain xenon gas. The xenon gas allows the lamps to produce minimally adequate light immediately upon powerup, and accelerates the lamps' run-up time. If argon were used instead, as is commonly done in street lights and other stationary metal halide lamp applications, it would take several minutes for the lamps to reach their full output. The light from HID headlamps has a distinct bluish tint when compared with tungsten-filament headlamps.
History
Xenon headlamps were introduced in 1991 as an option on the BMW 7-series. This first system used an unshielded, non-replaceable burner designated D1 — a designation that would be recycled years later for a wholly different type of burner. The AC ballast was about the size of a building brick. The first American-made effort at HID headlamps was on the 1996-98 Lincoln Mark VIII, which used reflector headlamps with an unmasked, integral-ignitor burner made by Sylvania and designated Type 9500. This was the only system to operate on DC; reliability proved inferior to the AC systems. The Type 9500 system was not used on any other models, and was discontinued after Osram's takeover of Sylvania. All HID headlamps worldwide presently use the standardised AC-operated bulbs and ballasts.
Burner and ballast operation
HID headlamp bulbs do not run on low-voltage DC current, so they require a ballast with either an internal or external ignitor. The ignitor is integrated into the bulb in D1 and D3 systems, and is either a separate unit or integral with the electronic ballast in D2 and D4 systems. The ballast controls the current to the bulb. The ignition and ballast operation proceeds in three stages:
- Ignition: a high voltage pulse is used to produce a spark — in a manner similar to a spark plug – which ionises the Xenon gas, creating a conducting tunnel between the tungsten electrodes. In this tunnel, the electrical resistance is reduced and current flows between the electrodes.
- Initial phase: the bulb is driven with controlled overload. Because the arc is operated at high power, the temperature in the capsule rises quickly. The metallic salts vapourise, and the arc is intensified and made spectrally more complete. The resistance between the electrodes also falls; the electronic ballast control gear registers this and automatically switches to continuous operation.
- Continuous operation: all metal salts are in the vapour phase, the arc has attained its stable shape, and the luminous efficacy has attained its nominal value. The ballast now supplies stable electrical power so the arc will not flicker.
Stable operating voltage is 85 volts AC in D1 and D2 systems, 42 volts AC in D3 and D4 systems. The frequency of the square-wave alternating current is typically 400 hertz or higher.
Burner types
HID headlamp burners produce between 2,800 and 3,500 lumens from between 35 and 38 watts of electrical power, while halogen filament headlamp bulbs produce between 700 and 2,100 lumens from between 40 and 72 watts at 12.8 V .
Current-production burner categories are D1S, D1R, D2S, D2R, D3S, D3R, D4S, and D4R. The D stands for discharge, and the number is the type designator. The final letter describes the outer shield. The arc within an HID headlamp bulb generates considerable short-wave ultraviolet (UV) light, but none of it escapes the bulb, for a UV-absorbing hard glass shield is incorporated around the bulb's arc tube. This is important to prevent degradation of UV-sensitive components and materials in headlamps, such as polycarbonate lenses and reflector hardcoats. "S" burners — D1S, D2S, D3S, and D4S — have a plain glass shield and are primarily used in projector-type optics. "R" burners — D1R, D2R, D3R, and D4R — are designed for use in reflector-type headlamp optics. They have an opaque mask covering specific portions of the shield, which facilitates the optical creation of the light/dark boundary (cutoff) near the top of a low-beam light distribution. Automotive HID burners do emit considerable near-UV light, despite the shield.
Color
The correlated color temperature of HID headlamp bulbs, at between 4100 K and 4400 K, is often described in marketing literature as being closer to the 6500 K of sunlight compared with tungsten-halogen bulbs at 3000 K to 3550 K. Nevertheless, HID headlamps' light output is not similar to daylight. The spectral power distribution (SPD) of an automotive HID headlamp is discontinuous, while the SPD of a filament lamp, like that of the sun, is a continuous curve. Moreover, the color rendering index (CRI) of tungsten-halogen headlamps (=0.98) is much closer than that of HID headlamps (~0.75) to standardised sunlight (1.00). Studies have shown no significant safety effect of this degree of CRI variation in headlighting.
Advantages
Increased safety
The HID headlamp light sources (bulbs) offer substantially greater luminance and luminous flux than halogen bulbs — about 3000 lumens and 90 mcd/m2 versus 1400 lumens and 30 mcd/m2. If the higher-output HID light source is used in a well-engineered headlamp optic, the driver gets more usable light. Studies have demonstrated drivers react faster and more accurately to roadway obstacles with good HID headlamps rather than halogen ones. Hence, good HID headlamps contribute to driving safety.
Efficacy and output
HID burners produce more light from less power than halogen bulbs. The highest-intensity halogen headlamp bulbs, H9 and HIR1, produce 2100 to 2530 lumens from approximately 70 watts at 13.2 volts. A D2S HID burner produces 3200 lumens from approximately 42 watts during stable operation.. The reduced power consumption means less fuel consumption, with resultant less CO2 emission per vehicle fitted with HID lighting (1.3 g/km assuming that 30% of engine running time is with the lights on).
Longevity
The average service life of an HID lamp is 2000 hours, compared to between 450 and 1000 hours for a halogen lamp.
Disadvantages
Glare
Vehicles equipped with HID headlamps are required by ECE regulation 48 also to be equipped with headlamp lens cleaning systems and automatic beam levelling control. Both of these measures are intended to reduce the tendency for high-output headlamps to cause high levels of glare to other road users. In North America, ECE R48 does not apply and while lens cleaners and beam levellers are permitted, they are not required; HID headlamps are markedly less prevalent in the US, where they have produced significant glare complaints. Scientific study of headlamp glare has shown that for any given intensity level, the light from HID headlamps is 40% more glaring than the light from tungsten-halogen headlamps.
Mercury content
HID headlamp bulb types D1R, D1S, D2R, D2S and 9500 contain the toxic heavy metal mercury. The disposal of mercury-containing vehicle parts is increasingly regulated throughout the world, for example under . Newer HID bulb designs D3R, D3S, D4R, and D4S which are in production since 2004 contain no mercury , but are not electrically or physically compatible with headlamps designed for previous bulb types.
Lack of backward-compatibility
The arc light source in an HID headlamp is fundamentally different in size, shape, orientation, and luminosity distribution compared to the filament light source used in tungsten-halogen headlamps. For that reason, HID-specific optics are used to collect and distribute the light. HID burners cannot effectively or safely be installed in optics designed to take filament bulbs; doing so results in improperly-focused beam patterns and excessive glare, and is therefore illegal in almost all countries.
Cost
HID headlamps are significantly more costly to produce, install, purchase, and repair. The extra cost of the HID lights may exceed the fuel cost savings through their reduced power consumption, though some of this cost disadvantage is offset by the longer lifespan of the HID burner relative to halogen bulbs.
LED light sources
Automotive headlamp applications using LEDs have been undergoing very active development since 2004. The first series-production LED headlamps are factory-installed on the 2008 Lexus LS 600h / LS 600h L (low beam, front position light and sidemarker only; high beam and turnsignal are filament based. The headlamp is supplied by Koito), and on the version of the 2008 Audi R8 sports car sold outside North America supplied by Automotive Lighting. The LED headlamp supplied by Hella for the 2009 Escalade Platinum is the first U.S. market headlamp with both a LED low and high beam. Present designs give performance between halogen and HID headlamps, with system power consumption slightly higher than halogen headlamps. These lamps currently require large packaging and a large number of the most powerful LED emitters available. As LED technology continues to evolve, the performance of LED headlamps is predicted to improve to approach, meet, and perhaps one day surpass that of HID headlamps.
The limiting factors with LED headlamps presently include high system expense, regulatory delays and uncertainty, glare concerns related to the output spectrum of white LEDs, and logistical issues created by LED operating characteristics. LEDs are commonly considered to be low-heat devices due to the public's familiarity with small, low-output LEDs used for electronic control panels and other applications requiring only modest amounts of light. However, LEDs actually produce a significant amount of heat per unit of light output. Rather than being emitted together with the light as is the case with conventional light sources, an LED's heat is produced at the rear of the emitters. The cumulative heat of numerous high-output LED emitters operating for prolonged periods poses thermal-management challenges for plastic headlamp housings. In addition, this heat buildup materially reduces the light output of the emitters themselves. LEDs are quite temperature sensitive, with many types producing at 30 °C (85 °F) only 60% of the rated light output they produce at an emitter junction temperature 16 °C (60 °F). Prolonged operation above the maximum junction temperature will permanently degrade the LED emitter and ultimately shorten the device's life. The need to keep LED junction temperatures low at high power levels always requires additional thermal management measures such as heatsinks and exhaust fans which are typically quite expensive.
Additional facets of the thermal issues with LED headlamps reveal themselves in cold ambient temperatures. Many types of LEDs produce at -12 °C (10 °F) up to 160% of their 16 °C (60 °F) rated output. The temperature-dependency of LED's light output creates serious challenges for the engineering and regulation of automotive lighting devices, which are in some cases required to produce intensities within a range smaller than the variation in LED output with temperatures normally experienced in automotive service.
Cold weather also brings another thermal-management conundrum: Not only must heat be removed from the rear of the headlamp so that the housing does not deform or melt and the emitters' output does not drop excessively, but heat must in addition be effectively applied to thaw snow and ice from the front lenses, which are not heated by the comparatively small amount of infared radiation emitted forward with the light from LEDs.
LEDs are increasingly being adopted for signalling functions such as parking lamps, brake lamps and turn signals as well as daytime running lamps, as in those applications they offer significant advantages over filament bulbs with fewer engineering challenges than headlamps pose.
Dynamic headlight beam control
Headlamp levelling control
In 1954, Cibié introduced an automatic headlamp leveling system linked to the vehicle's suspension system to keep the headlamps correctly aimed regardless of vehicle load. The first vehicle to be so equipped was the Panhard Dyna Z. Beginning in the 1970s, Germany and some other European countries began requiring remote-control headlamp levelling systems that permit the driver to lower the lamps' aim by means of a dashboard control lever or knob if the rear of the vehicle is weighted down with passengers or cargo, which would tend to raise the lamps' aim angle and create glare. Such systems typically use stepper motors at the headlamp and a rotary switch on the dash marked "0", "1", "2", "3" for different beam heights, "0" being the "normal" (and highest) position for when the car is lightly loaded. Internationalised ECE Regulation 48, in force in most of the world outside North America, currently requires such systems on all vehicles. The regulation stipulates a more stringent version of this antiglare measure for vehicles equipped with headlamp bulbs producing more than 2,000 lumens, such as xenon headlamps; such vehicles must be equipped with headlamp self-levelling systems that sense the vehicle's degree of squat due to cargo load and road inclination, and automatically adjust the headlamps' vertical aim to keep the beam correctly oriented without any action required by the driver.
Directional headlamps
These provide improved lighting for cornering. Some automobiles have their headlamps connected to the steering mechanism so the lights will follow the movement of the front wheels. Czech Tatra and 1920s Cadillacs were early implementer of such a technique, producing in the 1930s a vehicle with a central directional headlamp. The American 1948 Tucker Sedan was likewise equipped with a third central headlamp connected mechanically to the steering system. The 1967 French Citroën DS and 1970 Citroën SM were equipped with an elaborate dynamic headlamp positioning system that adjusted the headlamps' horizontal and vertical positioning in response to inputs from the vehicle's steering and suspension systems, though US regulations required this system to be removed from those models when sold in the USA.
Advanced front-lighting system (AFS)
There has been a recent resurgence in interest in the idea of moving or optimizing the headlight beam in response not only to vehicular steering and suspension dynamics, but also to ambient weather and visibility conditions, vehicle speed, and road curvature and contour. A task force under the EUREKA organisation, composed primarily of European automakers, lighting companies and regulators began working to develop design and performance specifications for what is known as advanced front-lighting systems, commonly AFS.
Manufacturers such Toyota, Skoda, and Vauxhall/Opel have released vehicles equipped with AFS since 2003.
Rather than the mechanical linkages employed in earlier directional-headlamp systems, AFS relies on electronic sensors, transducers and actuators. Other AFS techniques include special auxiliary optical systems within a vehicle's headlamp housings. These auxiliary systems may be switched on and off as the vehicle and operating conditions call for light or darkness at the angles covered by the beam the auxiliary optics produce. A typical system measures steering angle and vehicle speed to swivel the headlamps. Development is underway of AFS systems that use GPS signals to anticipate changes in road curvature, rather than simply reacting to them.
Automatic beam switching
Even when the high beam is warranted by prevailing conditions, drivers generally don't use them. There have long been efforts, particularly in America, to devise an effective automatic beam selection system to relieve the driver of the need to select and activate the correct beam as traffic, weather, and road conditions change. Early systems like Cadillac's Autronic Eye appeared in 1952 with an electric eye atop the dashboard (later behind the radiator grill) which was supposed to switch between low and high beam in response to oncoming traffic. These systems could not accurately discern headlamps from non-vehicular light sources such as streetlights, they did not switch to low beam when the driver approached a vehicle from behind, and they spuriously switched to low beam in response to road sign reflections of the vehicle's own headlamps. Present systems based on imaging CMOS cameras can detect and respond appropriately to leading and oncoming vehicles while disregarding streetlights, road signs, and other spurious signals. Camera-based beam selection was first released in 2005 on the Jeep Grand Cherokee, and has since then been incorporated into comprehensive driver assistance systems by automakers worldwide.
Care
Headlamp systems require periodic maintenance. Sealed beam headlamps are modular; when the filament burns out, the entire sealed beam is replaced. Most vehicles in North America made since the late 1980s use headlamp lens-reflector assemblies that are considered a part of the car, and just the bulb is replaced when it fails. Manufacturers vary the means by which the bulb is accessed and replaced. Headlamp aim must be properly checked and adjusted frequently, for misaimed lamps are dangerous and ineffective
Over time, the headlamp lens can deteriorate. It can become pitted due to abrasion of road sand and pebbles, and can crack, admitting water into the headlamp. "Plastic" (polycarbonate) lenses can become cloudy and discolored. This is due to oxidation of the painted-on lens hardcoat by ultraviolet light from the sun and the headlamp bulbs. If it is minor, it can be polished out using a reputable brand of a car polish that is intended for restoring the shine to chalked paint. In more advanced stages, the deterioration extends through the actual plastic material, rendering the headlamp useless and necessitating complete replacement. Sanding or aggressively polishing the lenses can buy a small amount of time, but doing so removes the protective coating from the lens, which when so stripped will deteriorate faster and more severely.
The reflector, made out of vaporised aluminum deposited in an extremely thin on a metal, glass or plastic substrate, can become oxidised or burnt and lose its specular reflective properties. This can happen if water enters the headlamp, if bulbs of higher wattage than specified are used, or simply with age and use. If the reflector when viewed by itself is not mirror-perfect, the headlamp should be replaced, for reflectors cannot effectively be restored.
Lens cleaners
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