Basic Principle

The light (beam) emitted from light emitting diodes (LED) is fully reflected on the external surface of the windshield and returns to the photo diodes. When there is water on the external surface of the windshield, the light is optically separated and reflected partially and the remaining brightness is measured by the photo diodes. Water remaining on the windshield results in incomplete reflection of the light back to the photo diodes. The loss of brightness from this indicates how much the glass surface is wet.