A method for measuring blowing structures of a prestressed disc is described. The method has the steps of: (a) irradiating at least one analysis area of the disc with linearly polarized light from a radiation source at an angle of incidence and recording an image at least of the analysis area at an angle of observation using at least one detector, (b) supplying the image to an evaluation unit, and (c) using the evaluation unit to read a brightness profile along an analysis line on the image, to determine the local maxima and the local minima of the brightness profile, and to determine an intensity index by means of the difference between a brightness mean of the local maxima and a brightness mean of the local minima.