HID stands for high-intensity discharge, the technical term for the electric arc that produces the light. Automotive HID lamps are commonly called 'xenon headlamps', because of the xenon gas used in the lamps. The xenon gas allows the lamps to produce minimally adequate amounts of light immediately 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 headlamps use a small, purpose-designed metal halide lamp which produces more light than ordinary tungsten and tungsten-halogen bulbs. The light from HID headlamps has a distinct bluish tint when compared with tungsten-filament headlamps. The high intensity of the arc comes from metallic salts that are vapourised within the arc chamber.

HID headlamp bulbs 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 12.8v [1][2][3]. 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.

HID headlamp bulbs do not run on low-voltage DC current, so they require a ballast with either an internal or external ignitor. The ballast controls the current to the bulb. When the headlamps are switched on, the ignitor provides rapidly pulsed current at several thousand volts to initiate the arc between the electrodes within the bulb. Once the arc is started, its heat begins to vapourise the metallic salts within the arc chamber, and the ballast gradually transitions from startup operation to arc-maintenance operation. Once the arc is completely stabilised, the ballast provides 85v in conventional D1 and D2 systems, or 42v with Mercury-free D3 and D4 systems.

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.

European 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.

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 contain no mercury, but are not electrically or physically compatible with headlamps designed for previous bulb types.

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.

Automotive headlamp applications using LEDs are not yet in volume production, but have been undergoing very active development, and present prototypes give performance roughly equal to existing halogen headlamps. These prototype designs currently require large packaging and a large number of the most powerful LED emitters available. As LED technology continues to evolve, the performance of LED headlamps is predicted to improve to approach, meet, and perhaps one day surpass that of HID headlamps. LED headlamps, foglamps and other forward illumination devices have so far generally been featured only on manufacturers' concept cars, but the first series-production LED headlamps will be factory-installed in 2007 on the Lexus LS 600h / LS 600h L. They will also appear on the version of the 2007 Audi R8 sports car sold outside North America.

The limiting factors with LED headlamps presently include high system expense, regulatory delays and uncertainty, glare concerns related to the output spectrum of white LEDs, and logistical issues created by LED operating characteristics. LEDs are commonly considered to be low-heat devices due to the public's familiarity with small, low-output LEDs used for electronic control panels and other applications requiring only modest amounts of light. However, LEDs actually produce a significant amount of heat per unit of light output. Rather than being emitted together with the light as is the case with conventional light sources, an LED's heat is produced at the rear of the emitters. The cumulative heat of numerous high-output LED emitters operating for prolonged periods poses thermal-management challenges for plastic headlamp housings. In addition, this heat buildup materially reduces the light output of the emitters themselves. LEDs are quite temperature sensitive, with many types producing at 30?? C (85?? F) only 60% of the rated light output they produce at an emitter junction temperature 16?? C (60?? F). Prolonged operation above the maximum junction temperature will permanently degrade the LED emitter and ultimately shorten the device's life. The need to keep LED junction temperates low at high power levels always requires additional thermal management measures such as heatsinks and exhaust fans which are typically quite expensive.

Additional facets of the thermal issues with LED headlamps reveal themselves in cold ambient temperatures. Many types of LEDs produce at -12?? C (10?? F) up to 160% of their 16?? C (60?? F) rated output. The temperature-dependency of LEDs' light output creates serious challenges for the engineering and regulation of automotive lighting devices, which are in some cases required to produce intensities within a range smaller than the variation in LED output with temperatures normally experienced in automotive service.

Cold weather also brings another thermal-management conundrum: Not only must heat be removed from the rear of the headlamp so that the housing does not deform or melt and the emitters' output does not drop excessively, but heat must in addition be effectively applied to the front lenses of the lamps?awhich are not heated by the cold light beam produced by LEDs?ato provide rapid and complete thawing of snow and ice accumulation.

LEDs are increasingly being adopted for signalling functions such as parking lamps, brake lamps and turn signals as well as Daytime Running Lamps, as in those applications they offer significant advantages over filament bulbs with fewer engineering challenges than headlamps pose.