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High-intensity discharge (HID) headlights, better known as xenon lights, offer more pure white light on the road than standard halogen bulbs while drawing less power. However, installing xenon bulbs as an aftermarket upgrade, as opposed to having them fitted on the production line as standard, is not legal in the eyes of the transport department. It says you must change the whole headlamp unit, because the lens and reflector for a halogen bulb will give the wrong beam pattern if used with an HID bulb.
The Driver and Vehicle Standards Agency, for its part, publishes a raft of safety regulations that all gas-discharge headlights must adhere to. For instance, they must be fitted with a washing system (unless the output is less than 2,000 lumens) and an automatic self-levelling system (unless it can be proved that the dipped beam is aimed correctly, as tested in the MoT). They must also be set up so that the dipped beam stays on when the main beam is activated.