In this method, the surface to be investigated is first of all exposed to collimated light and the radiation rejected therefrom supplied to a position-resolving image processing means. Then, a mask is generated on the basis of the image supplied by the image processing means, said mask having masked regions being defined by relatively bright areas of said image and unmasked regions being defined by relatively dark areas of said image. Now, the surface to be investigated is exposed to diffuse light and the radiation rejected therefrom supplied to the image processing means, with only the radiation from the unmasked regions being taken into consideration for analyzing purposes.