Rain Sensor
A rain sensor detects water on the windshield so the vehicle can start the wipers and adjust their wiping interval or speed.
How the common optical type works
An optoelectronic rain sensor is usually coupled to the inside of the windshield near the rear-view mirror. Infrared light is directed into the glass. A dry outer surface reflects most of it back toward a photodiode; water droplets change the optical path, so less light returns. The control unit interprets that change rather than measuring rainfall like a weather station.
The sensor is often combined with light, solar-load, humidity, or display-brightness sensing in one module. Automatic wiping still depends on correct optical coupling and software calibration. Dirt, ice, wax, damaged glass, air bubbles in a coupling pad, or incompatible replacement glass can affect operation.
A rain sensor reduces repeated manual wiper adjustment, but it does not prove that the driver's view is clear. The driver must override it when spray, snow, contamination, or another condition demands a different response.
For wipers, washers, heating, visibility, and service considerations, see Windshield Wipers, Washers, Heating, and Visibility.