Basically. Minor dust particles or dirt particles on the lens can actually cause the light beams from the xenons to glare off at different angles which could affect an on coming driver. Hence why the headlight washers are law in the UK.
Not only is the lens cover about a cm away from the light (which already changes the aspect ratio of the beam) there is also a possibility of dirt becoming trapped under the lens cover which will as said, make the dirt particles refract and cause misery to oncoming drivers.
Heat apparently is not an issue with regards to the Xenons.
Thats about all I remember from a Post that Teutonic Tamer wrote up. Dont believe everything I say purely because I cant remember exactly what was said.
Mike
