In ASTM E1165-12 and newer is an Annex A1 for end users with a more simple method to measure the focal spot size. Please refer to the thread about the .

Annex A1 starts with tutorial information which ends up with a formula which is used for calculation of the unsharpness in the image due to the focal spot:


The object with an edge to create the unsharpness is prefered a plaque hole type IQI, e.g. ASTM E1025 or E1742

where the unsharpness of both edges is measured. Alternatively a wire could be used, e.g. the largest wire from ASTM 2002 duplex wire gauge.
For evaluation a line profile function with measurement functionality is required. If your software is not so comfortable you may use the tool ISee! from the BAM, which could be downloaded for free. Please see the of ASTM E1165.

For image capture you should use a large magnification to reduce the influence of the detector unsharpness ( term 2*SRb/v in the formula in the beginning of this thread). E1165 requires a minimum magnification of 5* (detector spatial resolution / anticipated focal spot size). E.g. for a detector with 200µm pixel size and 240µm SRb the minimum magnification for a 400µm focal spot is 5*(240µm/400µm) = 3. This may lead to an error of about 20% ... .Due to my experience I recommend instead of the 5 a value between 10 and 40.
ASTM E1165 recommend to put the penetrameter on a shim block with two times the thickness of the IQI. If your object for inspection has a similar thick and sufficient flat area, you may place the IQI there.
Let us start with the "manual evaluation using a line profile" in A1.5.1. First the pixel size has to be calibrated.

In ISee! open the line profiler (F3) and draw a line from the left edge to the right edge. In the dialog is a "Calibrate Pixel Size" button (1);

if you press this button two boxes will open and you should give in the diameter of the hole of the IQI in the left box and system pixel size (detector pixel size devided by magnification) and system spatial resolution (detector spatial resolution devided by magnification) in the right box.
Then draw a short line profile on the left edge of the hole; increase the width of the line (2) to about 5 and select the 50%-84% edge unsharpness (ASTM) (3) feature. In the diagram you will see the unsharpness - already extrapoleted to the 100% value - here 0.19mm:

Do the same for the falling edge - where we get 0.22mm:

The horizontal unsharpness is the sum of both unsharpness values (as they are measured from 50% to 84% they only show half of the unsharpness - therefore both values have to be added to get 100%). Here we get 0.41mm as horizontal focal spot size.

Same procedure has to be repeated for the vertical size and here we get

0.2mm and 0.21mm which is in total 0.41mm. We see a quite round focal spot - very fine.

The Annex A.1 in E1165 allows also an "Automatic Evaluation Using a Software Function". If you software supports that feature the task could be much more easy - but you also have to set the values for hole diameter of the IQI, spatial resolution of the detector and detector pixel size.
In the following the measurement procedure is explained with Image.3500 from YXLON .


After setting the required system parameter an ROI is drawn of about double the size of the hole diameter. With FR Tools ->Circular Focal Spot Size Measurement you will see a red dot in the center of the hole and a second red dot surrounding the hole. When 360 degrees are done you will get a new image with the results of the focal spot measurement in horizontal and vertical direction:

The "deformated" circle around the hole shows with factor of 10 exaggerated the shape of the focal spot. You may see that the maximum unsharpness is at an angle of about 300 degrees (and if you look in the final picture of the line profile method above you will see that the right edge and the lower edge show higher unsharpness values).
For special purposes Image.3500 allows to change the display of the results to min and max values:


Did I already mention that this method is much more easy to use ?

The user methods in the annex A1 are somehow more simple to do - so, what is the difference to the main method with the pinhole and ILP?
For nine different X-Ray tubes with focal spot sizes from 250µm up to 5mm with several tubes of any type the deviaton of both methods were compared.

ASTM E1165 has a confidential interval (red dotted line) and all results are by far within this interval in width and length.

For the Precision and BIAS statement in A 1.7 ASTM requires an evaluation following E691. The user method with the IQI penetrameter hole was tested with more the hundred penetrameter hole images (focal spot length) using different positions and DDAs; different evaluations were done proving the method being constend.

Agenda:
Y200µm Software Evaluation with 200µm detector, 3.05mm Penetrameter, Magnification 7
Y400µm Software Evaluation with 400µm detector, 3.05mm Penetrameter, Magnification 5
Y6.1 IQI Software Evaluation with 200µm detector, 6.1mm Penetrameter, Magnification 4
Y man. Visual Evaluation with 200µm detector, 4 different positions, Magnification 7

Sometimes the user method may give some unexpected values. Before complaining you should check the quality of your plaque hole IQI ...