A method of qualifying a diffraction grating comprises performing plural measurements by illuminating a region of the grating with a beam of measuring light and detecting an intensity of measuring light diffracted by the grating into a 0th diffraction order. A wavelength of the measuring light or a polarization of the measuring light or an angle of incidence of the measuring light onto the diffraction grating is varied between subsequent measurements. A shape parameter of diffracting elements forming the grating comprises a pitch, height or width of structural features of the diffracting elements. The shape parameter is advantageously used in analyzing interferometric measurements performed on optical surfaces during manufacture of optical elements of a high accuracy.