Measuring unit for a humidity sensor for detecting humidity on a surface, rain sensor and method to detect humidity

The unit has a radiation-sensitive device (2) for receiving radiation dependent on the degree of moisture on the surface (12) and a measurement device connected to the radiation receiver and with a capacitor (3) that can be charged with a part of the measurement current dependent on the reflected radiation. The measurement device has a measurement control unit (4) for charging up the capacitor during the charging time, which depends on the measurement current level, and the measurement value depends on the capacitor charging voltage : Independent claims are also included for the following: (A) a moisture sensor (B) and a method of measuring moisture on a surface.