X-ray collection using cylindrical monocapillary. Dependence of the collected flux on capillary working distance WD at constant sample detector distance. The detection through a capillary increases the collection solid angle. (a) Detection through a pinhole. For short capillary length (b), the signal magnitude S1 is higher than S0 detected in case (a); (c) if WD is shortened until WDc, the signal magnitude S2 increases until θ2 = θc; (d) for WD lower than WDc, the signal remains constant.