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4. X-Ray Tube Focal Spot Measurements
15
Slit Measurement Using Digital Image
1.0B
RTI Slit Camera User’s Manual
X
2
= 199.78 - 117.61 = 82.17 mm
The true distance, X
1
, between the front edge and the edge of the open part of the slit
holder is 34 mm.
X
1
= 34 mm
4.4.1.6.
Measure the focus size, f
m
To measure the focus size one
need the line spread function
(LSF) of the image of the focal
spot through the slit. This is
done from the menu
Analyze/Plot Profile,
(or Ctrl+K).
ImageJ will plot the profile of the
area marked in the image with
the rectangular tool.
In the image of the line spread
function the width can now be
measured. Either using the
rectangular tool and read the
width, or reading the cursor x-values on the left and right edge. The cursor x-value is
show in the lower part of the window. In this example the width is measured as FWHM
(Full Width Half Maximum), and is 3.32 mm.
f
m
= 3.32 mm
4.4.1.7.
Calculate true focal spot size, f
Using Eq. 4.1, and the values above gives following:
mm
34
2
32
.
3
0
.
34
17
.
82
0
.
34
f
f
1
2
1
.
X
X
X
m
This is the final result for a measurement of the length of the focal spot.
4.4.1.8.
Measure the width of the focal spot
Rotating the Focus Slit 90 degrees in the holder (The stand stand should not be
moved), and then repeating the above procedure, will give the width of the focal spot.
In this case the result was 1.86 mm.
f = 1.86
4.4.1.9.
Nominal focal spot value
The IEC 60336 standard specifies the Nominal focal spot value in a look-up table, see
Appendix A. Using the results above (width = 1.86, length = 2.34) and the look-up
table, gives a nominal focal spot value of 1.6.