text: Another type of topographic contrast, extinction contrast, is slightly more complex. While the two above variants are explicable in simple terms based on geometrical theory (basically, the Bragg law) or kinematical theory of X-ray diffraction, extinction contrast can be understood based on dynamical theory.
  Qualitatively, extinction contrast arises e.g. when the thickness of a sample, compared to the respective extinction length (Bragg case) or Pendelloesung length (Laue case), changes across the image. In this case, diffracted beams from areas of different thickness, having suffered different degrees of extinction, are recorded within the same image, giving rise to contrast. Topographists have systematically investigated this effect by studying wedge-shaped samples, of linearly varying thickness, allowing to directly record in one image the dependence of diffracted intensity on sample thickness as predicted by dynamical theory.
  In addition to mere thickness changes, extinction contrast also arises when parts of a crystal are diffracting with different strengths, or when the crystal contains deformed (strained) regions.
  The governing quantity for an overall theory of extinction contrast in deformed crystals is called the effective misorientation
  where  is the displacement vector field, and  and  are the directions of the incident and diffracted beam, respectively.
  In this way, different kinds of disturbances are "translated" into equivalent misorientation values, and contrast formation can be understood analogously to orientation contrast.
  For instance, a compressively strained material requires larger Bragg angles for diffraction at unchanged wavelength. To compensate for this and to reach diffraction conditions, the sample needs to be rotated, similarly as in the case of lattice tilts.
  A simplified and more "transparent" formula taking into account the combined effect of tilts and strains onto contrast is the following: