HID lighting systems use 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 or 3/16th inch). 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. 

Like street lamps and fluorescent bulbs, HID headlamps require a high voltage ignition source to start. It typically takes up to 25,000 volts to start a xenon bulb, but only about 80 to 90 volts to keep it operating once the initial arc has formed. The normal 12 volts DC from the vehicle's electrical system is stepped up and controlled by an igniter module and inverter (ballast), which also converts the voltage to AC (alternating current) which is necessary to operate the HID headlamps. 

The Digital Ballast adjusts the voltage and current frequency to operating requirements. The AC ballast frequency is usually in the 250 to 450 Hz range. 

When HID headlamps are first turned on, the light appears more bluish but quickly brightens as the bulbs warm up. Because there is no brittle filament inside a xenon HID bulb to break or burn out, the headlamps typically last up to ten times longer than halogen headlamps.

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