[Tatsfield]

Xenon bulbs take a moment or two to reach full light emission and that might be dangerous if the lights were need in a hurry, so when Xenon was the3 latest bright light output source, design had to make sure that the "flashable" lights worked immediately and the fall back was halogen bulbs.

Today with LEDs, which produce instant light, the designers can use the technology to provide all light sources but the Dept of Transport does not allow the replacement of the halogen bulbs with LED because the lens design for the two systems is different and the wrong bulb in the wrong housing with the wrong lens could cause too much light scatter which might dazzle oncoming drivers. Personally I think that this is overkill if the replacement LED is in the full beam housing as any full beam light can blind oncoming drivers and needs to be dipped so in practice are only used when there is no one there to be blinded. However the law in this matter is sadly clear even if it is not really achieving very much of practical use!