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Xenon Headlamps
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. Xenon 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 Xenon 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.
Xenon 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).
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.
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.
Dynamic Headlamp Beam Control
Headlight Levelling Control
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 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. 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 high-intensity portion of the beam below the horizontal without any action required by the driver.
