The linear dimension of the spot, radiation from which passes through the capillary (effective size of capillary), is
determined from the formula D = 2l tanqcr + d, where l is the distance of the source of the radiation to the inlet to the capillary;
qcr = 4·10–3 is the critical angle of the total external reflection of CuKa radiation; d, distance between the walls of the
capillary. Thus, with a distance l = 0.5 mm, the effective size of the capillary D = 2·500·4·10–3 + 5 = 9 mm.