Abstract: For detecting changes of level of a moving surface, a sharply defined light pattern is directed on to the surface and an image of the pattern is formed on a multiple photodiode strip so that a predetermined point of reference of the image is variably located on the strip depending on variation in level of the surface, the photodiode strip is repetitively scanned to create a voltage waveform repetitively defining the variable position of the point of reference on the image in relation to the fixed position of a point of reference on the photodiode strip, and the waveform is analysed to determine the variations in level of the moving surface.