Abstract: The present invention relates to a method and apparatus for selective measurement of liquid and solid particles deposited on plate surfaces. The method is based on the transmission of radiation, which originates from total reflection from the interface between liquid droplets at the surface of the plate and surrounding gas, e.g., air. This radiation is transferred inside the plate as radiation undergoing total internal reflection between the surfaces of the plate. Such measuring radiation reaches a detector protected from background radiation, whereby intensity increase can be measured and used for the control, e.g., windshield wipers and washer equipment.