Camera Vans Simple Qn

[AdamS]

When a camera van is operating do the darkened windows need to be open or do the cameras work through the window?

Have to admit that I will be slightly peeved today, road should be dual carriageway but isnt so 60, lovely clear sunny day only one other car and no pavements etc there I was overtaking another car I(the only one there on the road)probably hit 80 and there he was on the bridge. He had reversed behind the brick of the bridge far enough that the window was visable but all the other markings were hidden.

Slowed as soon as I saw him but probably too late