Headlight
From Exampleproblems
A headlight or headlamp is a light, usually attached to the front of a vehicle such as a car, with the purpose of illuminating the road ahead during periods of low visibility, such as night or precipitation.
A headlight can also be mounted on a bicycle (with a battery or small generator), and most other moving vehicles from airplanes to trains tend to have headlights of their own. Single small headlights may also be mounted on a helmet designed to be worn in situations where light is required but both hands are needed, for example in subterranean mines or for spelunking in caves.
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History of automotive headlights
The earliest headlights 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 headlights were introduced in 1898 on the Columbia Electric Car from the Electric Vehicle Company of Hartford, Connecticut, but they were optional. "Prest-O-Lite" acetylene lights were offered by a number of manufacturers as standard equipment for 1904, and Peerless made electrical headlights standard in 1908. In 1912, Cadillac integrated their vehicle's Delco electrical ignition and lighting system, creating the modern vehicle electronics system.
"Dipping" (low beam) headlights were introduced in 1915 by the Guide Lamp Company, but the 1917 Cadillac system was much more useful as it allowed the light to be dipped with a lever inside the car rather than requiring the driver to stop and get out. The 1924 Bilux bulb was the first modern unit, having both low- and high-beams in a single bulb. A similar design was introduced the next year by Guide Lamp called the "Duplo". In 1927, the foot-operated dimmer was introduced and would become standard for much of the century. The last vehicle with a foot-operated dimmer was the 1991 Ford F-Series. Fog lights were new for 1938 Cadillacs, and that company's 1954 "Autronic Eye" system automated the switch between high and low beams.
The standardized 7 inch (178 mm) sealed beam headlight 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 U.S. DOT also in 1968 outlawed covered headlights (as famously used on the Jaguar E-Type, VW Beetle, Porsche 356 and Ferrari Daytona), further altering the look of European models sold in the United States. In 1984, US regulations once again allowed replaceable-bulb, nonstandard-shape headlamps. The first U.S.-market car since 1939 with composite headlights was the Lincoln Mark VII of that year. These U.S.-market composite headlamps were frequently, though incorrectly, referred to as "Euro" headlamps. Despite their aerodynamic, nonstandardized shape and replaceable-bulb construction, these headlamps conform to the SAE standard, and not the international European standard.
Directional (steering) headlights were a notable feature of the 1948 Tucker, but the technology went back to the 1937 Tatra T77A. The first real production car with steerable headlights was the 1967 Citroën DS. The technology was slow to gain acceptance, but is today offered on a number of luxury vehicles, including those from Lexus, Infiniti, and the Porsche Cayenne. There is a pan-European-and-Japanese working group steering development, implementation and standardization of steerable headlamps, which are now known as "AFS" (Advanced Frontlighting System") or "bending light".
In 1954, Cibie introduced an automatic headlight 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. Beginning in the 1970s, Germany and some other European countries began requiring remote headlamp leveling systems by means of which the driver could lower the aim of his lamps from the driver's seat, to avoid glaring other drivers in the event he was carrying a heavy load that would tend to cause the front of the car to point upward. This requirement spread slowly at first, but picked up speed to the point where most European countries required this equipment by the early 1990s. Xenon HID headlamps and very high output halogen bulbs are required in Europe to have automatic leveling systems that keep the beams aimed correctly regardless of vehicle attitude, without any effort on the part of the driver. These leveling systems are permitted but not required outside Europe.
Cadillac and Chrysler introduced quad headlights (using one high/low and one high-beam 5¾ inch (146 mm) sealed beam on each side of the car) in 1957. 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.
Rectangular lamps were first used in 1961. Developed by Cibie 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, on the Cadillac Eldorado. By 1979, the majority of new cars now had the square headlights. Again, the U.S. permitted only two standardized sizes of rectangular sealed-beam lamp: A system of two 200 mm x 142 mm high/low beam units corresponding to the existing 7" round format, or a system of four 165 mm x 100 mm units (two high/low and two high-beam) corresponding to the existing 5¾" (146 mm) round format.
Pop-up headlights were introduced in 1963, on the Chevrolet Corvette. When the lights were turned on, they would appear from inside the car's front bumper. Many famous cars to use this feature include the Dodge Charger, Ferrari Testarossa, Mazda RX-7, RX-8 and Miata, Dodge Daytona, Pontiac Firebird, Nissan 300ZX, Toyota Celica and Supra, Honda Accord and Prelude, Acura NSX and Buick Reatta. The Corvette itself continued to use pop-up headlights until it was redesigned for 2004.
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 produces 1500 lumens when operated at 13.2 volts. Halogen technology makes incandescent filaments much more efficient, and Europeans chose to use this extra efficiency to produce much more light than was available from nonhalogen filaments at the same power consumption. H2 followed in 1964, and the transverse-filament H3 in 1966. H1 still sees wide use in low beams, high beams and auxiliary fog and driving lamps, as does H3. The first halogen dual-filament 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. 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 candelas per vehicle, double the nonhalogen limit of 75,000 candelas 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 US halogen headlamp bulb, the 9004, is a transverse-dual-filament design that produces 700 lumens on low beam and 1200 on high beam. The original European bulbs H1 and H3 were eventually permitted in US headlamps beginning in 1997.
The first halogen filament polyellipsoidal "projector beam" automotive lamp was the Super-Lite 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 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. Projector and CAD technology allowed the development of reflector headlights with non parabolic, complex-shape reflectors. First made by Valeo under their Cibie brand, these headlights would revolutionize automobile design. The 1987 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.
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 catch on in North America. 1997's Lincoln Mark VIII was an early American effort at HIDs; it was also the first and only car with DC HIDs.
See also
Automotive headlights
Modern headlights are electrically operated, positioned in pairs, one or two on each side of the front of a moving 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 due to reflection from fog, rain and snow due to the refraction of the water droplets. Low beams 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.
Laws and regulations
US laws required sealed beam headlamps on all vehicles between 1940 and 1984, 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 have been the norm. There are two different beam pattern and headlamp construction standards in use in the world: The ECE ("European") standard, which is allowed or required in virtually all industrialized countries except the United States, and the Society of Automotive Engineers 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 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. 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.
Some countries require automobiles to be equipped with automatic daylight 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, Hungary and most Scandinavian countries.
Headlights 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.
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. Elsewhere in the world, internationalized European ECE regulations are in force.
Types of headlights
Most headlights use incandescent bulbs (usually halogen), either with separate high and low beam bulbs, or a single bulb on each side with dual filaments. More and more are using high-intensity discharge (HID) lamps, similar to the Mercury-vapor lamps used in white street lights. These have a distinct purplish or bluish cast to them, which also causes fluorescence in certain materials. HID bulbs are also extremely energy-efficient, using over 80% less power, and drawing less electrical current from the alternator and improving fuel efficiency.
Traditionally, headlamps have consisted of a light source (filament) placed at or near the focus of a parabolic reflector, with an optically-faceted glass or polycarbonate lens to direct and distribute the light per the beam specifications. Computerized optical development tools have allowed the creation of non parabolic complex-shape reflectors, either with optically-faceted or completely clear lenses.
Lens optics
Image:Headlight lens optics schematic.png Lens optics, side view. Light is dispersed vertically (shown) and laterally (not shown). | Image:Headlight lens optics.jpg Lens optics, sealed beam example. The frensel patches disperse the light in a precisely defined way. |
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 headlight 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 archive the desired results. Most sealed-beam headlights have lens optics.
Reflector optics
Image:Headlight reflector optics schematic.png Reflector optics, side view | Image:Headlight reflector optics.jpg Reflector optics |
The optics required to give the proper light distribution pattern is designed into the reflector itself. The reflector design starts as a parabola, and the optical engineers replace the entire surface with individual complex-shape patches. The precise shape of each patch is designed such that their cumulative effect produces the required distribution pattern.
Reflector optics are usually made from injection molded plastic. Heat from the bulb is usually one of the main design constraints. The reflective surface is vapor deposited aluminum with a clear overcoating. Extremely tight tolerances must be adhered to in the design, tooling and production of complex-reflector headlamps. Nevertheless, a large number of facets help to even out optical errors.
Projector optics
Image:Headlight projector schematic.png Projector optics, side view | Image:Headlight projector optics.jpg Projector headlamps |
Projector optics are considered superior to reflector and lens optics for headlight systems. This is because of the fine control inherent in the design, allowing very accurate light dispersion. In this system a filament is located at one focus of an elliptical and 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 be stationary in which case separate high-beam lamps are required. The condenser lens may have slight fresnels to reduce cutoff sharpness.
HID ("Xenon") Headlamps
Xenon HID headlamps are the bright automobile headlights that use xenon short-arc lamps to produce the light rather than ordinary incandescent light bulbs (including quartz halogen lamps). (HID refers to High-Intensity Discharge, the technical term for the electric arc that produces the light.) The light from xenon lamps has a distinct bluish tint when compared with incandescent light. The high intensity of the arc comes from metallic salts that are vaporized within the arc chamber. The Xenon gas is present so that minimally adequate light is produced immediately upon headlamp activation. If the Xenon weren't present, these headlamps would have a long warmup time similar to arc-type street lamps.
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 2100 lumens from between 40 and 65 watts. Because of the increased light available from HID bulbs, Xenon 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.
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 ramp.
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 into the bulb design. 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 (blacklight).
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.
External links
HID Retrofitting — optical physics
LED headlights
Automotive headlight applications using LEDs are not yet in volume production, but prototypes now exist that give performance roughly equal to existing halogen headlamps. These prototype designs currently require a large number of the most powerful LED emitters available. The relatively high expense, regulatory delays, glare concerns and LED operational concerns (especially thermal handling) have so far prevented them from entering the market, though LEDs are increasingly being adopted for signaling functions such as brake lamps and turn signals.
External links
New-generation LED headlamp prototype with performance equal to HID
VW Golf V LED headlamp study and prototype
Directional headlights
These provide improved lighting for cornering. Some automobiles have their Headlights connected to the steering mechanism so the lights will follow the movement of the front wheels. The Citroën DS was one car equipped with such a system. Also, some automobiles have vertically adjustable lights, to compensate for dipping when carrying heavy loads.
Dual-beam headlights
Night driving has long been dangerous due to the glare of headlights from oncoming traffic which temporarily blinds drivers approaching from the opposite direction. Therefore, headlights 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 headlights 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 headlight 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 energized, the light beam is widened and shifted slightly downard 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.
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 the H4 bulb (renamed HB2 for the U.S. market). Similar 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 permits a greater overall amount of light within the low beam, since the entire reflector area is used. However, 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 generally produces 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.
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.
Care
Headlamps require very little care. Sealed beam headamps are modular. When the filament burns out, the entire module is replaced. Newer cars provide bulb "capsules" which are removable from the rear of the reflector housing for replacement purposes. Manufacturers vary the mechansim for retaining the bulb capsule in the reflector.
Over time, the outer cover, most often formed of polycarbonate, can become cloudy and discolor, turning yellowish. This is a result from oxidation and etching of the plastic surface. The effects of this oxidation can be removed by wetsanding and buffing the outer surface of the cover.
The reflector can become oxidized and lose its specular reflective properties. This can happen if the housing of the headlamp becomes damaged or unsealed and water vaper enters. Once the reflector loses its reflectivity, the entire headlamp should be replaced.
