The schematic illustration of the X-ray beam reflection from a curved surface is provided in Figure . The width of the spread on the detector l depends on the sample curvature R, the incidence angle αi, and the sample-detector distance L. Let us consider two parallel rays impinging on a cylindrically curved surface. The first one, r0,i, hits the apex of the surface at the αi equal to the effective grazing angle of the beam. The second one, rh,i, is separated by a distance h (Figure ). After reflection on the spherically curved surface with the curvature radius R, they will propagate further as rays r0,s and rh,s, respectively, with an angular separation 2Δαi,h. Here ∆𝛼 𝑖,ℎ is the difference between the grazing angles of two rays r0,i and rh,i, and can be calculated, following Konovalov et al. as