(A) A reciprocal lattice point intersects the Ewald sphere. The inset shows the coordinate system used in cctbx.xfel and prime. The vector S0 represents the direction of the incident beam (–z-axis) and forms the radius of the Ewald sphere of length 1/λ. The reciprocal lattice point i is expressed in reciprocal lab coordinates using Equation 5 as represented by the vector xi. The Ewald-offset distance, rh, is the difference between the distance from the Ewald-sphere center to the reciprocal lattice point (length of Si) and 1/λ. The inset shows the definition of the crystal rotation axes; they are applied in the following order: θz, θy, θx. (B) Shown is the volume of a reciprocal lattice point with radius rs. The offset rh defines the Ewald-offset correction Eocarea, which is the ratio between the area intersecting the Ewald sphere, Ap, and the area at the center of the volume, As.