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.
Basic Overview
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.
Functionality
In headlights two filaments are necessary to provide for a main and a dip beam function. These must be positioned correctly in relation to the highly polished reflector. This is called focusing and is carried out during manufacture.
The main or high beam filament is positioned at the focal point of the reflector to project the maximum amount of light forward and parallel to the reflector axis. This light is then shaped by the lens which is made up of many small glass prisms fused together. These prisms bend the light horizontally and vertically to achieve the desired pattern for road illumination.
The dip or low beam is placed above and slightly to one side of the main filament. Mounting the dip filament in this position produces a beam of light that is projected downwards and towards the kerb side.
With this arrangement the main filament produces the best possible light output while the dip filament gives a downward and dispersed beam which should not dazzle oncoming drivers.
A semi-sealed beam headlight uses a replaceable bulb with a pre-focus collar. The collar locates the bulb in the headlight and also controls the correct positioning of the filaments.
A sealed beam headlight has a highly polished aluminized glass reflector which is then fused to the optically designed lens. This forms a completely sealed unit which has the filaments accurately positioned in relation to the reflector. When a filament fails in a sealed beam light, the whole unit must be replaced.
Some headlight bulbs have a partial shield below the dip filament. This shield stops light from the filament striking the lower part of the reflector. The shield provides the primary shape of the dip beam. The final shaping of the beam is carried out by small cylindrical shaped prisms in the headlight lens. This provides a dipped beam that is asymmetrical.
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
Light is dispersed vertically and laterally.
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 molded 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
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
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 plastic 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.
LED headlights
Automotive headlight applications using LED's are not yet in volume production, but prototypes now exist that give performance roughly equal to existing halogen or HID headlamps. These prototype designs currently require a large number of the most powerful LED emitters available. The relatively high expense, regulatory delays and LED operational concerns (especially with heat removal) have so far prevented them from entering the market, though LED's are increasingly being adopted for signaling functions such as brake lamps and turn signals.
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.
Source: CDX Global & Wikipedia - en.wikipedia.org
