Department for Transport (DoT) wrote:It is not acceptable to convert a headlamp approved for use with a halogen filament bulb to use a Xenon gas discharge bulb. In our opinion the only acceptable way to convert a vehicle to use Xenon headlamps is to replace the entire headlamp unit for one which has been approved for use with gas discharge bulbs. Such headlamps will be approved to UNECE Regulations 98 and 'E' marked to show compliance

Our main concern is for the potential for modified headlamps to produce an dincorrect beam pattern and to cause glare to other road users. Halogen headlamps are designed to produce a defined beam pattern using a filament wire with a shape and dimensions which are tightly controlled by regulations. A gas discharge bulb produces an electrical discharge which has a very different shape and dimensions to a filament wire adn so the beam pattern produced will be incorrect

If headlamps approved to UNECE regulation 98 are retro fitted to a vehicle then they must be self levelling (either as part of the headlamp unit or some vehicles have self levelling suspension which is adequate) and headlamp washers must be installed.