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HID stands for "High Intensity Discharge". These bulbs use an electric arc rather than the glowing filament in a Halogen bulb. HID headlamp bulbs do not run on low-voltage DC current like a Halogen Bulb, so they require a ballast, which controls and regulates the flow of current. This ballast will require an ignitor to light the bulb. This is either integrated directly into the bulb or as a separate unit which is occasionally part of the ballast, similar to fluorescent bulbs in your home. In a vehicle, HID bulbs tend to be much more expensive to replace than a Halogen bulb.

Replacing stock Halogen headlights with HIDs would require some modifications, especially where the wiring is concerned. While the lamp can sometimes fit into the opening on the front of the vehicle, your stock lamps will use different bulbs, sockets, fuses, relays, and wiring than most HID bulbs. You may want to check with your local mechanic for advice if you have never attempted this type of conversion before.

HID lights can also be referred to as Xenon bulbs (though not all Xenon bulbs are HID). Some vehicle manufacturers also use Bi-Xenon headlights, headlights which use an HID low beam bulb in combination with a halogen high beam bulb.