Editing Headlamp (section)

==== Operation ====
HID headlamp bulbs do not run on low-voltage DC current, so they require a [[ballast (electrical)|ballast]] with either an internal or external ''ignitor''. The ignitor is integrated into the bulb in D1 and D3 systems, and is either a separate unit or part of the ballast in D2 and D4 systems. The ballast controls the current to the bulb. The ignition and ballast operation proceeds in three stages:
# Ignition: a [[high voltage]] pulse is used to produce an [[electrical arc]] – in a manner similar to a [[spark plug]] – which ionizes the xenon gas, creating a conducting channel between the tungsten electrodes. Electrical resistance is reduced within the channel, and current flows between the electrodes.
# Initial phase: the bulb is driven with controlled overload. Because the arc is operated at high power, the temperature in the capsule rises quickly. The metallic salts vaporize, and the arc is intensified and made [[spectral power distribution|spectrally]] more complete. The resistance between the electrodes also falls; the electronic ballast control gear registers this and automatically switches to continuous operation.
# Continuous operation: all metal salts are in the vapor phase, the arc has attained its stable shape, and the [[luminous efficacy]] has attained its nominal value. The ballast now supplies stable electrical power so the arc will not flicker. Stable operating voltage is 85 [[volt]]s [[alternating current|AC]] in D1 and D2 systems, 42 volts AC in D3 and D4 systems. The frequency of the square-wave alternating current is typically 400 [[hertz]] or higher.
[[File:High Beam Indicator.svg|alt=high beam indicator example|thumb|Headlight indicator example]]
The command is often near the steering wheel and a specific indicator is shown on the dashboard.