Automatic wipers

[nigel4]

How do the automatic wipers work (what are they detecting)?
How well do they work?
Are they affected by car wash residues, wax or ceramic?

 

[Alb]

The sensor detects spots of water on the windscreen.

 

[MikeJS]

Mine work very well and I have a ceramic treatment on the glass which does not seem to degrade the wipers in any way.