Abstract: A detector of stains on a transparent plate surface is arranged on an internal surface of the plate and includes an emitter for emitting a modulated light signal towards the inside of the plate and a receiver for receiving the signal after it has been reflected on an external surface of the plate. An optical unit made with a material having an index substantially higher than the plate, is arranged on the internal surface of the plate. The optical unit is made up of at least three faces returning the light signal and an interface with the internal surface of the plate through an optical coupler. A deflector deflects the signal from the emitter toward the plate and from the optical unit towards the receiver. At least two reflections on the external surface of the plate are produced through the optical coupler without reflection on the internal surface of the plate.