Basic Principle
WARNING: This page is about a different variant/trim than selected.
Emitted Beam from luminosity diode is reflected entirely against the windshield exterior, and then turn into photo diode. If there is water on the windshield exterior, beam separates optically, and the degree of remained beam is measured in the photo diode.
What there is water in the windshield, it means beam is not reflected all, so the degree of lost beam indicates the degree of glass surface wet.
NOTE:
Rain sensor consist of two luminosity diode, two photo diode, optic fiber and coupling pad.