The Bosch rain sensor works according to the following optical functional principle: A luminous diode (LED) emits light in the yellow spectral range. If the windshield is dry, all the light is transported along the windshield, which acts like an optical waveguide. Finally, the light is coupled out and measured with the help of a photographic diode. If the windshield is wet or dirty, the measurement changes. The wetter or dirtier the windshield is, the less LED light will reach the diode. Using the difference in the incoming signals, the microprocessor calculates the quantity of water on the windshield and adjusts the speed of the windshield wiper system. If torrential water covers the windshield completely (as often happens when passing a truck), the mechanism immediately switches to maximum wiping speed. A SCHOTT filter embedded in the electronic system takes care to filter out all unneeded wavelengths so signals are accurately measured. The sensor reacts faster than the driver and allows both hands to stay on the steering wheel.