Headlamp
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"Headlight" redirects here. For other uses, see Headlight (disambiguation).
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 night or precipitation. 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 [1]
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. Single small headlamps may also be mounted on a helmet or attached to a strap around the head 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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[edit] History of automotive headlamps
[edit] Mechanics
The earliest headlamps were fueled by acetylene or oil and were introduced by drivers in the late 1880s. Acetylene was 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, 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 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 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 the light for both low (dipped) and high (main) beams of a headlamp emitting from 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. Foglamps 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 (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 British Commonwealth countries, as well as Japan, also made extensive use of sealed beams, but they were 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 is considered a technological advance because it makes incandescent filaments much more efficient and can produce more light than was available from nonhalogen filaments at the same power consumption. These were prohibited in the United States where non-halogen sealed beam lamps were required until 1978.
High-intensity discharge systems were introduced in 1991's BMW 7-series. European and Japanese markets rapidly came to prefer HID headlamps, which have as much as 50% marketshare in those markets, but the technology was slow to be adopted in North America. 1996's Lincoln Mark VIII was an early American effort at HIDs; it was also the first and only car with DC HIDs.
[edit] 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. Early headlamps were always round, because that is the easiest shape in which to manufacture a parabolic reflector.
[edit] Headlight Styling outside of the United States
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 idea involved conventional round lamps covered with aerodynamic glass or polycarbonate covers, such as those on the 1961 Jaguar E-Type.
[edit] Headlamp Styling in the United States
In 1940, the US government mandated a 7 in. (178 mm) round sealed beam headlamp on all automobiles. Headlamp styling in the United States virtually ceased for many decades after this event.
Quad headlamps—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 centerline 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 models of Oldsmobile 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. Pontiac, American Motors, and Cadillac made extensive use of this design. 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.
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 outboard low-beam units. The 1965 Gordon-Keeble, Triumph Vitesse and Bentley S3 Continental used such an arrangement. (source: World Car Catalog)
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 U.S. permitted only two standardized sizes of rectangular sealed-beam lamp: A system of two 200 mm x 142 mm (7½ in. x 5½ in.) high/low beam units corresponding to the existing 7" round format, or a system of four 165 mm x 100 mm (6½ in. x 4 in.) units, two high/low and two high-beam, corresponding to the existing 5¾ inch (146 mm) round format.
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, the pre-1968 VW Beetle, the Porsche 356, the Citroën DS and Ferrari Daytona) therefore had to be equipped with uncovered headlamps for the US market, further altering the look of European models sold in the United States. This change meant that vehicles designed for solid aerodynamic performance could not achieve it for the US market.
[edit] International headlamp styling
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, when new to the U.S. market, were commonly referred to as "Euro" headlamps, since aerodynamic headlamps were already common in Europe. Though conceptually similar to European headlamps with nonstandardized shape and replaceable-bulb construction, these headlamps conform to the SAE headlamp design standards contained in U.S. Federal Motor Vehicle Safety Standard 108, and not to the internationalized European safety standards used worldwide outside North America. Nevertheless, this significant change to US regulations largely united the formerly disparate paths of headlamp styling within and outside the North American market.
In the late 1990s, headlamps with round styling themes 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.
Pop-up headlamps were introduced in 1937, on the Cord 812. They were mounted in the front fenders, which were smooth until the lights were activated, aiding aerodynamics when the headlamps were not in use. They also provided a means of fitting a large 7 inch round headlamp into an otherwise pointed front end.
Many (more or less) famous cars used this feature, but no current volume-produced car models use pop-up headlamps, largely because they are expensive to construct. The system requires one or more vacuum-operated servos and reservoirs, with associated plumbing and linkage, or electric lift motors, geartrains and linkages of sufficient robustness and precision to raise the lamps to an exact position each time to assure correct beam aim despite ice, snow and age. Some early pop-up headlamps used a lever-operated mechanical linkage (e.g. Saab Sonett III). Fashions have also changed, and greater priority is given to aerodynamics with lamps off and on, further reducing the attractiveness of pop-up headlamps. In addition, recent ECE Regulations contain stringent standards regarding protuberances on car bodies, in an effort to minimize injury to pedestrians struck by cars.
Hidden headlamps are a variant of a similar concept. In cars with hidden headlamps, panels designed to blend in with the front styling of the car (e.g. adjacent fender and/or grille) cover the headlamps when they are switched off. When the lamps are switched on, the cover doors are swung out of the way, usually downward or upward and into the space within the fender above or below the headlamps. Actuation of the cover door mechanism may be by means of vacuum pots (as in the 1967-1969 Mercury Cougar) or an electric motor.
[edit] Regulations and requirements
[edit] Functions and fitment
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 retroflection 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.
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 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 Canada, Iceland, Hungary and most Scandinavian countries.
[edit] Construction, performance, and aim
There are two different beam pattern and headlamp construction standards in use in the world: The UNECE ("United Nations Economic Commission for Europe") standard, which is allowed or required in virtually all industrialized countries except the United States, and the SAE standard that is mandatory only in the US. The differences between the two 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 characterized 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 several different forms. Proponents of each 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, England 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, and from 1936 until 1993 this was required on all vehicles registered in France.
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 all aimed alike, regardless of 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 declination is linked to headlamp mounting height. This gives all vehicles roughly equal seeing distance and all drivers roughly equal glare.
[edit] Optical systems
[edit] Reflector lamps
[edit] Lens optics
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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.
[edit] Reflector optics
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Starting in the 1980s, CAD technology allowed the development of reflector headlamps with nonparabolic, complex-shape reflectors. First made by Valeo under their Cibie brand, these headlamps would revolutionize 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, with such a unit being known as 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 precise 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 adhered to in the design, tooling and production of complex-reflector headlamps.
[edit] Dual-beam reflector headlamps
Night driving has long been dangerous due to the glare of headlights from oncoming traffic which temporarily blinds drivers approaching from the opposite direction. Therefore, headlamps that satisfactorily illuminate the road ahead of the automobile without causing this effect have long been sought. The first attempts to address this problem involved resistance-type dimming circuits, which decreased the brightness of the headlamps when meeting another car. This gave way to mechanical tilting reflectors and later to double-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.
[edit] 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 energized, 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 optimize 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 maximize 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.
[edit] 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 1cm 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.
This system was first used with the Bilux/Duplo 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-style bulbs. Named HB2 or 9003, for the U.S. market, and with very slightly different production tolerances stipulated, these bulbs are physically and electrically interchangeable with H4 bulbs. Similar optical techniques are used, but with different reflector and/or lens optics to create a U.S. 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 U.S. In addition, the American system makes it difficult to create markedly different low and high beam light distributions; the high beam is usually simply 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 optimization of both low and high beams.
[edit] 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.
[edit] Projector (polyellipsoidal) lamps
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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, or it may be stationary in which case separate high-beam lamps are required. The condenser lens may have slight fresnels 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.
The first modern polyellipsoidal "projector beam" automotive lamp was the Super-Lite, an auxiliary low beam 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 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 1983. Developed more or less simultaneously in Germany by Hella and in France by Cibie, the projector low beam permitted accurate beam focus and a much smaller-diameter (though much deeper) optical package for any given beam output. The 1986 BMW 7 Series was the first to use projectors for low beams.
[edit] Light sources
[edit] 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.
[edit] 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 emphasized 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 candela (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 favor long bulb life and low power consumption, while European designs continued to prioritize 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 13.2v. H2 (55w @ 12.0v, 1820 lm @ 13.2v) followed in 1964, and the transverse-filament H3 (55w @ 12.0v, 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 withdrawn 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 legalized in the United States in 1997. More recent single filament bulb designs include the H7 (55w @ 12.0v, 1500 lm ±10% @ 13.2v), H8 (35w @ 12.0v, 800 lm ±15% @ 13.2v), H9 (65w @ 12.0v, 2100 lm ±10% @ 13.2v), and H11 (55w @ 12.0v, 1350 lm ±10% @ 13.2v). 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 watt, 12.8 V).
[edit] 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.
[edit] HID light sources (Xenon and Bi-Xenon)
HID stands for high-intensity discharge, the technical term for the electric arc that produces the light. Automotive HID lamps are commonly called 'xenon headlamps', because of the xenon gas used in the lamps. The xenon gas allows the lamps to produce minimally adequate amounts of light immediately upon startup and speed the warmup time. If argon were used instead, as is commonly done in street and other stationary HID lamps, it would take several minutes for the lamps to reach their full output. HID headlamps use a small, purpose-designed metal halide lamp which produces more light than ordinary tungsten and tungsten-halogen bulbs. The light from HID headlamps has a distinct bluish tint when compared with tungsten-filament headlamps. The high intensity of the arc comes from metallic salts that are vapourised within the arc chamber.
HID headlamp bulbs produce between 2,800 and 3,000 lumens from 42 watts of electrical power, while halogen filament headlamp bulbs produce between 700 and 2,100 lumens from between 40 and 65 watts. Because of the increased amounts of light available from HID 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.
HID headlamp bulbs do not run on low-voltage DC current, so they require a ballast with either an internal or external ignitor. The ballast controls the current to the bulb. When the headlamps are switched on, the ignitor provides rapidly pulsed current at several thousand volts to initiate the arc between the electrodes within the bulb. Once the arc is started, its heat begins to vapourise the metallic salts within the arc chamber, and the ballast gradually transitions from startup operation to arc-maintenance operation. Once the arc is completely stabilised, the ballast provides 85v in conventional D1 and D2 systems, or 42v with Mercury-free D3 and D4 systems.
Despite marketing claims to the contrary, 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.
The arc within an HID headlamp bulb generates considerable short-wave ultraviolet (UV) light, but none of it escapes the bulb. 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. The lamps do emit considerable near-UV light.
European 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.
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 US EPA regulations. Newer HID bulb designs D3R, D3S, D4R, and D4S contain no mercury, but are not electrically or physically compatible with headlamps designed for previous bulb types.
The arc light source in an HID headlamp is fundamentally different from the filament light source used in tungsten/halogen headlamps. For that reason, HID-specific optics are used to collect and distribute the light. Installing HID bulbs in headlamps designed to take filament bulbs results in improperly-focused beam patterns and excessive glare, and is therefore illegal in almost all countries.
[edit] LED light sources
Automotive headlamp applications using LEDs are not yet in volume production, but have been undergoing very active development, and present prototypes give performance roughly equal to existing halogen headlamps. These prototype designs 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. LED headlamps, foglamps and other forward illumination devices have so far generally been featured only on manufacturers' concept cars, but the first series-production LED headlamps will be factory-installed on the 2007 Lexus LS 600h / LS 600h L. They will also appear on the version of the 2007 Audi R8 sports car sold outside North America.
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). These needs, to exhaust heat from the headlamps and to keep LED emitter junction temperatures under control, require expensive powered ventilation systems.
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 LEDs' 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 the front lenses of the lamps—which are not heated by the cold light beam produced by LEDs—to provide rapid and complete thawing of snow and ice accumulation.
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.
[edit] Dynamic Headlight Beam Control
[edit] Headlamp Levelling Control
In 1954, Cibie 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. Internationalized 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.
[edit] 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 was an 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 deleted from those models when sold in the USA.
[edit] 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 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 as Audi and Lexus have released vehicles equipped with AFS since 2002. 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. Development is underway of AFS systems that use GPS signals to anticipate changes in road curvature.
[edit] Care
Headlamps require very little care. Sealed beam headlamps are modular. When the filament burns out, the entire module is replaced. Most 1985 and later-model vehicles in North America use headlamp lens-reflector assemblies that are considered a part of the car, and just the bulb is replaced if it fails. There are many different bulb types, and they are not interchangeable, so the correct bulb for the specific vehicle year, make and model must be purchased. Manufacturers vary the means by which the bulb is accessed and replaced.
Headlamp aim must be properly checked and adjusted on a regular, periodic basis. Misaimed lamps are dangerous and ineffective.
Over time, the front lens can deteriorate. It can become pitted due to abrasion of road sand and pebbles. It can become cracked, admitting water into the headlamp. "Plastic" (polycarbonate), can become cloudy and discolored, turning yellowish. 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 extremely thin vaporized aluminum deposited on a metal, glass or plastic base, can become oxidized or burnt and lose its specular reflective properties. This can happen if water enters the headlamp, if bulbs of higher wattage than specified is 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.
[edit] Headlamp washers
Headlamp washers come in two main varieties: a small wiper blade and motor as seen on the windshield of the car, or a pop-up sprayer which cleans the lenses.
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[edit] See also
[edit] External links
- Headlamp Optical Systems Illustrated, Explained & Compared
- HID Retrofitting — optical physics
- Headlamp aiming page
- "Where Does The Glare Come From?" (white paper on headlamp performance, glare, and regulation)
- Sidemarkers, flashing and non-flashing
- New-generation LED headlamp prototype with performance equal to HID
- VW Golf V LED headlamp study and prototype
- HID discussion board
