It is well known [1, 2] that the size of the focal spot and its shape are the most important parameters of an x-ray tube.
These geometric parameters of the focal spot are important, for example, for x-ray flaw detection where they serve as a means
of determining the resolution of the analysis . Knowledge of these parameters is also necessary in the use of x-ray optics
together with an x-ray tube, for example, when conducting testing of the x-ray optical systems of Kumakhov optics. That is,
in order to determine the transmission of the Kumakhov lens the size of the focal spot of the tube must be no greater than the
corresponding focal spot of the lens [2, 3].