A Schlieren microscope is described which can be used to measure profiles of optical quality surfaces. The intensities in the image field are measured with a photo-diode array. By measuring a reference sample and the object, both with and without knife it is possible to correct for different errors. Shape and roughness can be measured with accuracies of 10 nm and 1 nm respectively with a horizontal resolution of about 3μm. The system is fast and relatively insensitive to vibra-tion.