Diffraction image of primary narrow $X$-ray beam in weakly deformed crystal

Huygens–Fresnel principle for dynamical diffraction of $X$-rays in a crystal with weak deformation field of its space lattice is stated with a view to construction of the wave functions of diffracted beams in the crystal lattice at an arbitrary distance $r$ of the point source of primary radiation from the crystal. In particular, the well-known approximations of an incident spherical $(r = 0)$ and plane $(r\to\infty)$ waves result as the limiting cases of the problem under consideration. Some features of the interference absorption (the Borrmann effect) of $X$-ray wave packages in a crystal with weak field of lattice displacements were discussed.