Schematic illustrations of the beam geometry in a plane-parallel single crystal (a) in an unbent crystal, (b) in a bent crystal, and (c) in a crystal for a two-beam MBL interferometer. L is the distance between the incident point of the X-rays and the crystal edge, and H is the crystal thickness. and are the electric fields of the incident beam and the diffracted beam. is the electric field of the diffraction beam in the direction of the diffracted beam at the exit point (L,z) from the lateral surface. The refracted beams in the BL and BBL modes are denoted by the Poynting vectors and , respectively. In (c) the second subscript 1 (2) of the electric field and the Poynting vectors and denotes the beam passing through the upper (lower) side of the platinum wire.