Headlight: Encyclopedia II - Headlight - Types of headlights

Headlight - Types of headlights

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.

The optics required to give the proper light distribution pattern is designed into the reflector itself, with such a unit being known as an "optic reflector". The reflector design starts as a parabola standing in for the size and shape of the completed package. The optical engineer replaces the entire surface with individual segments of specifically calculated, complex contours. The precise shape of each segment is designed such that their cumulative effect produces the required 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.

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 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.

HID stands for high-intensity discharge, the technical term for the electric arc that produces the light. Automotive HID lamps are commonly called Xenon headlights because of the Xenon gas used in the lamps. The Xenon gas allows the lamps to produce minimally adequate amounts of light 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 headlights use a small, purpose-designed metal halide lamp and produce more light than ordinary incandescent light bulbs (including quartz halogen lamps). The light from HID headlights has a distinct bluish tint when compared with normal headlights. The high intensity of the arc comes from metallic salts that are vaporized 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.

An HID headlamp requires a ballast. The ballast converts the 12 volts used in automotive electrical systems to the several thousand volts required to strike and maintain the arc.

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).

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.

HID headlamps are subject to the extra ECE regulations for any headlamp brighter than 2,000 lumens; a headlamp cleaning system must be fitted, and any beam level adjustment must be automatic rather than manual.

HID retrofit kits, like these, that simply place an HID capsule in a lamp designed for an incandescent light source are dangerous. With a U.S. style beam pattern, the result can be significantly increased glare for oncoming drivers. With either U.S. or European style lamps, the light from an HID capsule will not be distributed properly, ultimately resulting in less visibility for the driver. Further, a kit that simply places an HID capsule into a lamp with no further modifications will quite obviously lack any manner of leveling equipment.

Automotive HID FAQ - Headlamp Systems

HID Retrofitting — optical physics

Headlight - 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.

New-generation LED headlamp prototype with performance equal to HID

VW Golf V LED headlamp study and prototype

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.

Adapted from the Wikipedia article "Types of headlights", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki