A Transparent Optical Sensor for Moisture Detection Integrated in a PQ-PMMA Medium

A transparent optical sensor for detecting water on a windscreen surface of vehicles, a so-called rain sensor (RS), is presented. The RS is based on a light source, coupling holographic diffraction elements (HDEs), a waveguide (WG), and a detector. The advantage of the novel RS is its transparency, which allows it to be located in the driver´s view area. The HDEs that couple the light signal into and out of the WG are manufactured as a laminate of a Plexiglas polymethylmethacrylate-Phenanthrenequinone photopolymer composite integrated into a windscreen. In the detecting area the light signal is coupled out of the WG onto the windscreen surface under total internal reflection angles [between 42^° for glass and air border (for the transition from glass to air), and 62^° for glass and water]. Depending on the moisture deposited on the windscreen surface, the light signal will be reflected back into the WG and will either propagate to the detector or will be coupled out of the glass.