A focused light beam is directed onto a surface patch of a mask and decomposed into partial beams by diffraction at a structure formed on the surface of the mask. Detectors are set such that the intensity of at least two orders of diffraction can be measured. The measured intensities are compared with one another. By way of example, a quotient can be ascertained. The operations are repeated for adjacent surface patches. If the absolute values of the measured intensities fluctuate with a constant quotient, then a variation of the reflection or transmission over the surface of the mask is inferred. If the quotient varies as well, then line width fluctuations within the structure on the mask are inferred.