High-intensity discharge (HID)
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Automotive HID may be 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 start, and shortens the run-up time. The usage of argon, as is commonly done in street lights and other stationary metal-halide lamp applications, causes lamps to take several minutes to reach their full output.
The light from HID headlamps exhibits a distinct bluish tint when compared with tungsten-filament headlamps.
Retrofitment
When a halogen headlamp is retrofitted with an HID bulb, light distribution and output are altered.[32]In the United States, vehicle lighting that do not conform to FMVSS 108 are not street legal.[32] Glare will be produced and the headlamp's type approval or certification becomes invalid with the altered light distribution, so the headlamp is no longer street-legal in some locales.[33] In the US, suppliers, importers and vendors that offer non-compliant kits are subject to civil fines. By October 2004, the NHTSA had investigated 24 suppliers and all resulted in termination of sale or recalls.[34]In Europe and the many non-European countries applying ECE Regulations, even HID headlamps designed as such must be equipped with lens cleaning and automatic self-leveling systems, except on motorcycles. [33] These systems are usually absent on vehicles not originally equipped with HID lamps.
History
Xenon headlamps were introduced as an option on the BMW 7-series in 1991 for Europe, and in 1993 for US models. 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, since reliability proved inferior to the AC systems.[citation needed] The Type 9500 system was not used on any other models, and was discontinued after Osram's takeover of Sylvania in 1997.[citation needed] All HID headlamps worldwide presently use the standardised AC-operated bulbs and ballasts.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 part of the 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. Electrical resistance is reduced within the tunnel, 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.[35][36][37]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.
Colour
The correlated colour temperature of factory installed automotive HID headlamps is between 4100K and 5000K[citation needed] while tungsten-halogen lamps are at 3000K to 3550K. The spectral power distribution (SPD) of an automotive HID headlamp is discontinuous and spikey while the SPD of a filament lamp, like that of the sun, is a continuous curve. Moreover, the color-rendering index (CRI) of tungsten-halogen headlamps (98) is much closer than that of HID headlamps (~75) to standardised sunlight (100). Studies have shown no significant safety effect of this degree of CRI variation in headlighting.[38][39][40][41]Advantages
Increased safety
Automotive HID lamps offer about 3000 lumens and 90 Mcd/m2 versus 1400 lumens and 30 Mcd/m2[disputed ] offered by halogen lamps. In a headlamp optic designed for use with an HID lamp, it produces more usable light. Studies have demonstrated drivers react faster and more accurately to roadway obstacles with good HID headlamps than halogen ones.[42] Hence, good HID headlamps contribute to driving safety.[43] The contrary argument is that glare from HID headlamps can reduce traffic safety by interfering with other drivers' vision.[original research?]Efficacy and output
Luminous efficacy is the measure of how much light is produced versus how much energy is consumed. HID burners give higher efficacy than halogen lamps. The highest-intensity halogen lamps, 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.[35] 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.[44]Disadvantages
Glare
Vehicles equipped with HID headlamps (except motorcycles) are required by ECE regulation 48 also to be equipped with headlamp lens cleaning systems and automatic beam leveling 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 levelers are permitted, they are not required;[45] HID headlamps are markedly less prevalent in the US, where they have produced significant glare complaints.[46] 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.[Published by Wyatt Olsen
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