This document specifies a method for the measurement of focal spot sizes within the range of 5 µm to 300 µm of X-ray systems up to and including 225 kV tube voltage. This determination is based on the evaluation of an image with a dedicated focal spot that has been radiographically recorded using an edge and evaluated with a digital method.
The imaging quality and the resolution of X-ray images depend highly on the characteristics of the effective focal spot, in particular its size and the two-dimensional intensity distribution as seen from the detector plane.
For the characterization of commercial X-ray tube types (i.e. for advertising or trade), the nominal values of Annex A are preferred.
NOTE            The same procedure can be used at higher kilovoltages by agreement but the accuracy of the measurement can be poorer.