摘要
This communication gives the asymmetric factor b and deviation parameter η describing the diffraction geometry in a more general way including very asymmetric scattering arrangements. Darwin's parameter Y defined on the dispersion surface and the associated deviation parameter ηY given by the Takagi–Taupin equations are proposed to characterize the angular deviation from the exact resonance of Bragg diffraction. With respect to ηY of a high accuracy, the validity of conventional forms of the deviation parameter is further discussed.