HID light sources (xenon and bi-xenon)

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', although they are actually metal halide lamps that contain xenon gas. 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 metal halide lamp applications, it would take several minutes for the lamps to reach their full output. HID headlamps use a small, purpose-designed burner 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 12.8 V [13][14][15]. 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 85 V in conventional D1 and D2 systems, or 42 V with mercury-free D3 and D4 systems.

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 color 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.

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.[16]