A method is presented to determine contour maps of the first and the second derivatives of the deflection of an elastic bending plate. The deflection of a rough surface object is recorded once as a modulated diffraction grating. One of the diffracted waves is spatially filtered and brought into a shearing interferometer. The resultant shearing interferogram depicts contours of the first derivative of the deflection. The second derivative is obtained from the second modulated diffraction grating which is made with the interferogram of the first derivative. By recording the grating non-linearly, it is possible to obtain a substantial increase in the sensitivity of the interference fringes.