The method is based on light coming through a first slit (light source), being converted to parallel light through a first lens (collimator), reflected at the surface of the test piece, focussed at a combed sliding shutter through a second lens (condensing lens). The signal measured by a photo-receiver behind the shutter is a measure of image clarity. Optical evenness of the test piece is shown by the ratio of the longitudinal and transverse values when the shutter is appropriately slid.