For detecting oscillations occurring during knocking detonations in an internal combustion engine, optical detectors are located in the combustion chamber. In order to minimize the contamination of the window of the sensor system on the side of the combustion chamber, the surface of the window is provided with contours defining points and/or edges. In order to receive the largest amount of light, the window is so formed as to bulge into the combustion chamber and may be coated with a contamination-resistant layer.