Apparatus for cleaning the interior surface of a window in an inline optical sensors which is subject to contamination and fouling by a product stream in contact therewith. The window surface is wiped by a blade driven by a pneumatic operator, and the amount of air supplied to the operator is adjusted to control the speed at which the blade moves across the window surface. When a cleaning operation is initiated, the blade is wiped across the window surface a predetermined number of times, and measurements utilizing data from the sensor are suspended until the cleaning operation is completed and the data has had time to restabilize.