AbstractIt is shown that the parasitic radiation arising from a KRATKY‐type collimator for the investigation of the small‐angle scattering, also under ideal conditions, is caused by the diffraction of thex‐rays on the edges of the slits. Calculations for the relative intensity of the parasitic scattering, using the FRESNEL‐diffraction on half‐planes and slits, give values approximately ten times greater than the measured values. A refined model leads to a better approximation. Analogues considerations for other collimators are given. The diffraction as a physical cause for the parasitic radiation yields principal limits for the measurement of the small‐angle