The title of IEC 60336 is
Medical electrical equipment – X-ray tube assemblies for medical diagnosis – Characteristics of focal spots

The measurement method is similar to EN12543-2; beside the pinhole diaphragm a slit diaphragm may be used as the focal spot length is evaluated only.
Another (important) difference is the threshold of 15% (instead of 10% in EN) of the isodose contour line - with is a simple threshold in IEC 60366 as there is one dimension only.
(taken from 60336 (c) IEC:2005)

And there is a second reason that focal spots are nominated much smaller when measured with IEC60366 - for a nominal focal spot value the dimension may be significant larger. Here is as example the value for a 1mm nominal focal spot value

where the length is double the value of the nominal focal spot value

The IEC 60366 is not really relevant for NDT applications - but you should look how the focal spot is measured when you buy a system and calculate the required system-resolution. More information about the influence of the focal spot size in the image could be found at .