Headlamp - Wikipedia, the free encyclopedia
HID (xenon) light sources
Xenon projector low beam headlamp illuminated on a Lincoln MKS.HID stands for high-intensity discharge, a technical term for the electric arc that produces the light. The high intensity of the arc comes from metallic salts that are vapourised within the arc chamber. These lamps are formally known as gas-discharge burners, and produce more light for a given level of power consumption than ordinary tungsten and tungsten-halogen bulbs. Because of the increased amounts of light available from HID burners relative to halogen 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.
Automotive HID lamps are commonly called 'xenon headlamps', though they are actually metal halide lamps that contain xenon gas. The xenon gas allows the lamps to produce minimally adequate light immediately upon powerup, and accelerates the lamps' run-up time. If argon were used instead, as is commonly done in street lights and other stationary metal halide lamp applications, it would take several minutes for the lamps to reach their full output. The light from HID headlamps has a distinct bluish tint when compared with tungsten-filament headlamps.
History
Xenon headlamps were introduced in 1991 as an option on the BMW 7-series. This first system used an unshielded, non-replaceable burner designated D1 — a designation that would be recycled years later for a wholly different type of burner. The AC ballast was about the size of a building brick. The first American-made effort at HID headlamps was on the 1996-98 Lincoln Mark VIII, which used reflector headlamps with an unmasked, integral-ignitor burner made by Sylvania and designated Type 9500. This was the only system to operate on DC; reliability proved inferior to the AC systems. The Type 9500 system was not used on any other models, and was discontinued after Osram's takeover of Sylvania. All HID headlamps worldwide presently use the standardised AC-operated bulbs and ballasts.
Burner and ballast operation
HID headlamp bulbs do not run on low-voltage DC current, so they require a ballast with either an internal or external ignitor. The ignitor is integrated into the bulb in D1 and D3 systems, and is either a separate unit or integral with the electronic ballast in D2 and D4 systems. The ballast controls the current to the bulb. The ignition and ballast operation proceeds in three stages:
Ignition: a high voltage pulse is used to produce a spark — in a manner similar to a spark plug – which ionises the Xenon gas, creating a conducting tunnel between the tungsten electrodes. In this tunnel, the electrical resistance is reduced and current flows between the electrodes.
Initial phase: the bulb is driven with controlled overload. Because the arc is operated at high power, the temperature in the capsule rises quickly. The metallic salts vapourise, and the arc is intensified and made spectrally more complete. The resistance between the electrodes also falls; the electronic ballast control gear registers this and automatically switches to continuous operation.
Continuous operation: all metal salts are in the vapour phase, the arc has attained its stable shape, and the luminous efficacy has attained its nominal value. The ballast now supplies stable electrical power so the arc will not flicker.
Stable operating voltage is 85 volts AC in D1 and D2 systems, 42 volts AC in D3 and D4 systems. The frequency of the square-wave alternating current is typically 400 hertz or higher.
Burner types
HID headlamp burners produce between 2,800 and 3,500 lumens from between 35 and 38 watts of electrical power, while halogen filament headlamp bulbs produce between 700 and 2,100 lumens from between 40 and 72 watts at 12.8 V [15][16][17].
Current-production burner categories are D1S, D1R, D2S, D2R, D3S, D3R, D4S, and D4R. The D stands for discharge, and the number is the type designator. The final letter describes the outer shield. The arc within an HID headlamp bulb generates considerable short-wave ultraviolet (UV) light, but none of it escapes the bulb, for 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. "S" burners — D1S, D2S, D3S, and D4S — have a plain glass shield and are primarily used in projector-type optics. "R" burners — D1R, D2R, D3R, and D4R — are designed for use in reflector-type headlamp optics. They have an opaque mask covering specific portions of the shield, which facilitates the optical creation of the light/dark boundary (cutoff) near the top of a low-beam light distribution. Automotive HID burners do emit considerable near-UV light, despite the shield.
Color
The correlated color temperature of HID headlamp bulbs, at between 4100 K and 4400 K, is often described in marketing literature as being closer to the 6500 K of sunlight compared with tungsten-halogen bulbs at 3000 K to 3550 K. Nevertheless, 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. Moreover, the colour rendering index (CRI) of tungsten-halogen headlamps (≥0.98) is much closer than that of HID headlamps (~0.75) to standardised sunlight (1.00). Studies have shown no significant safety effect of this degree of CRI variation in headlighting.[citation needed]
Advantages
[edit] Increased safety
The HID headlamp light sources (bulbs) offer substantially greater luminance and luminous flux than halogen bulbs — about 3000 lumens and 90 mcd/m2 versus 1400 lumens and 30 mcd/m2. If the higher-output HID light source is used in a well-engineered headlamp optic, the driver gets more usable light. Studies have demonstrated drivers react faster and more accurately to roadway obstacles with good HID headlamps rather than halogen ones.[18] Hence, good HID headlamps contribute to driving safety.[19]
Efficacy and output
HID burners produce more light from less power than halogen bulbs. The highest-intensity halogen headlamp bulbs, H9 and HIR1, produce 2100 to 2530 lumens from approximately 70 watts at 13.2 volts. A D2S HID burner produces 3200 lumens from approximately 42 watts during stable operation.[15]. The reduced power consumption means less fuel consumption, with resultant less CO2 emission per vehicle fitted with HID lighting (1.3 g/km assuming that 30% of engine running time is with the lights on).
Longevity
The average service life of an HID lamp is 2000 hours, compared to between 450 and 1000 hours for a halogen lamp[20].
Disadvantages
Glare
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. In North America, ECE R48 does not apply and while lens cleaners and beam levellers are permitted, they are not required; [21] HID headlamps are markedly less prevalent in the US, where they have produced significant glare complaints.[22] Scientific study of headlamp glare has shown that for any given intensity level, the light from HID headlamps is 40% more glaring than the light from tungsten-halogen headlamps.[23]
Mercury content
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 which are in production since 2004 contain no mercury[24] [25], but are not electrically or physically compatible with headlamps designed for previous bulb types.
Lack of backward-compatibility
The arc light source in an HID headlamp is fundamentally different in size, shape, orientation, and luminosity distribution compared to the filament light source used in tungsten-halogen headlamps. For that reason, HID-specific optics are used to collect and distribute the light. HID burners cannot effectively or safely be installed in optics designed to take filament bulbs; doing so results in improperly-focused beam patterns and excessive glare, and is therefore illegal in almost all countries.[26]
Cost
HID headlamps are significantly more costly to produce, install, purchase, and repair. The extra cost of the HID lights may exceed the fuel cost savings through their reduced power consumption, though some of this cost disadvantage is offset by the longer lifespan of the HID burner relative to halogen bulbs.
Also running XenonKing HID's in ALL my vehicles... Bike has had them for five or so years and never one problem!
