It has been observed that some stray light (x-rays) is thrown into the neighborhood of the focus obtained if x-rays are focused by a bent optical flat. Most of this extra light is due to imperfections of the optical flats, while halation, Fresnel diffraction at the slits and air scattering make only minor contributions. An analysis of the distribution of this extra light, a study of the effect of controlled irregularities and pin hole images of surfaces taken with x-rays lead to definite conclusions about the structure of such surfaces.
W. EHRENBERG, "X-Ray Optics: Imperfections of Optical Flats and Their Effect on the Reflection of X-Rays," J. Opt. Soc. Am. 39, 746-751 (1949)