Issues with measuring the focal spot size
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Author:  Klaus [ 05.12.2019, 23:08 ]
Post subject:  Issues with measuring the focal spot size

Up to the 2008 focal spots were measured with the EN standards and all was okay. But with the steeper target angles (11°) some problems occur which needed to be addressed. The small foci of this tubes produce "shadow images", especially the small foci:

In one direction there seem to be three foci active; duplex wires show sometime four wires instead of two; hole type penetrameter with high contrast sometimes have three holes. What is the reason?
Due to the technology inside the tube the central beam have two side wings

which are responsible for the ghost images.

Here are examples of two tubes with eleven degree target angle, first the large focal spot which was announced to be 1mm measured with EN12543-2 (Pinhole Method):

This standard required to measure the area of the focal spot with more than 10% of the background value. The measurement shows that the sidewing of the left tube hurts the standard, the right tube is allright within the specification.

With the small foci it is even worse. The left tube does not meet the standard even when the threshold is rised to 30%. The right tube needs 30% to rise the threshold above the signal level of the side wings.

That is the reason that the manufacturor added a footnote for this focal spot that it is measured with the higher threshold level in deviation to the standard.

In practice the side wings were not so dominant as they show up with the measurement method in EN12543-2 or ASTM E1165. As EN12543 offers several methods we compared part 2 (pinhole) and part 5 (edge) with different tubes and focal spot sizes from 5mm down to 0.25mm and found that there are big differences within both methods - sometimes factor two :O :

One of the reasons is, that the threshold level is fine for tubes with high uniform distribution of the focal spot intensity as the left tube shows

The right tube with the lower gradient shows much higher influence of the threshold level in the result:

Several methods were tested to get the "best practice value" for the focal spot size. The basic idea was to get the total intensity value, which would be the volume above the threshold and not the projected lines in two directions. As the edge methods showed values much closer to reality the edge method of EN12543-5 was tested with plate hole penetrameter, 1mm thick plate of Tungsten with several holes, edges of the EN462-5 duplex wire no. 4 (first platin wire) and finally measured with the duplex wire EN462-5 (or ASTM E2002). As an example here are the values of a standard 20 degree tube which should have a 1mm focal spot due to EN12543-2. The tests were done with several energies (no really influence) and different magnifications; the result of last variance is shown in the next graph:

The practically focal spot size is in the range of 450µm to 470µm in both directions. The same tests were done with different focal spots from 5mm down to 0.25mm with different magnifications and energies. The graph show that the three edge methods are inside the tolerance level of ASTM E1165 (dotted red lines)

The duplex wire method is quite sensitive to the magnification and very sensitive to "anomal" focal spots like in the first picture of this thread and the standard values from EN12543-2 are far away from practice - and the nominal values also :-( .

A new method would be required, based on the edge method because this method is closer to practice (shows the real unsharpness in the image). It could be shown that the pinhole method could be transfered to an edge method if the intensity would be integrated column-wide in both directions. The method is called: Integrated Line Profile (ILP) and is in the standard ASTM E1165-12 now and in the next version of EN12543-2 (probably released in 2020).
The best edge method - the one with the hole type penetrameter - did also find its way into the standard (ASTM E1165-12) as appendix and is called the "user method" as it could be done simply with a proper hole type penetrameter by all users without buying expensive equipment.

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