contain an inert gas (Xenon), which emits light when it comes in contact with a high-voltage electrical arc.

HID lamps produce up to twice as much light as halogen lamps, with a bluish-white cast and minimal fading at the periphery of the beams. However, the brightness perceived by an individual depends on the distance from the light source, beam direction, relative brightness of the surrounding environment, and other factors.

Source:LTA