The lens equation for Bragg diffraction optics. The general case of asymmetrical reflection

The lens equation for Bragg diffraction of monochromatic X-rays and neutrons in the case of a cylindrically curved crystal is derived for the general Bragg- and Laue-case geometries. The resulting equation is identical to the formula for polychromatic focusing only in the limits of symmetrical reflection and/or in Rowland geometry.