HID stands for High Intensity Discharge. HID lighting system (also known as Xenon) uses a special quartz bulb that contains no filament and is filled with xenon gas and a small amount of mercury and other metal salts. Inside the bulb are two electrodes separated by a small gap (about 4 mm). When high voltage current is applied to the electrodes, it excites the gases inside the bulb and forms an electrical arc between the electrodes. The hot ionized gas produces a “plasma discharge” that generates an extremely intense, bluish-white light.
Though the color of the light is often perceived as having a bluish tint when viewed at night, most of the light that is produced by HID headlamps is actually very close in color to natural noontime sunlight – though some of the light produced is also in the blue and ultraviolet spectrum. Halogen headlamps, by comparison, are more yellowish in appearance but are brighter and whiter than older incandescent style headlamps.
The near-white light produced by HID headlamps improves visibility and makes it easier to see distant objects.
The color of light can be measured in “degrees Kelvin,” which refers to the “temperature” (shade) of light. Natural sunlight at noon is 4870 degrees K. Light produced by a HID xenon bulb is 4100 degrees K. Light from a standard halogen bulb is 3200 degrees K, and that from an ordinary incandescent bulb is 2800 degrees K. The lower the temperature rating, the more yellowish the light appears.
Blue-white light is better for visual perception, but yellow light is actually somewhat better for reducing glare in fog, rain and snow (that’s why fog lights are yellow).
A car HID system consists of the following:
- HID projector
- HID bulb
- Shroud (Optional, used to cover the back part of the projector as decoration)
- HID ballast (adjusts the voltage and current frequency to operating requirement)
- Wiring harness