Method for operating a detector of irregularities on a transparent object

The raindrop and dirt particle sensor (1) has first and second LED's (3,4) and a photocell (5) for receiving reflected light, mounted on a conducting circuit board (6). A transparent optical coupling block (7) oscillator glued to the inside surface of the windscreen (2) has several lenses. It directs a beam of light from the first LED onto the outer surface of the windscreen at a shallow angle, and it is totally internally reflected from a clean surface. Some light escapes if a drop of water is present. The second LED provides a beam of light which may be reflected off dirt particles.