4. X-Ray Tube Focal Spot Measurements
13
Slit Measurement Using Digital Image
1.0B
RTI Slit Camera User’s Manual
4. Measure across the middle of the slit image using the magnifier lens (with a
built-in graticule).
5. To determine focal spot size, divide the measured value by the enlargement
factor. For example, if the measured length (f
m
) of the slit image is 1.76 mm
and the enlargement factor is 1.5, then:
f
17
.
1
5
.
1
76
.
1
(Eq. 4.3)
4.4.
Slit Measurement Using Digital Image
4.4.1.
Evaluation using ImageJ
Independently of which software tool is used to make the evaluation there are a few
“musts”.
1. The image has to be calibrated in terms of size. I.e. one has to be able to
measure the size of an object in the image plane in units of mm.
2. One has to be able to get the intensity profile across the image of the focus
slit.
In example below the image size has been calibrated by knowing the true size of the
digital detector and measure the number of pixels from edge to edge in the evaluation
software. An alternative way is to place an object of known size directly on top of the
image detector, and use that for the calibration.
Note:
In the example below the image of the RTI Slit Camera shows a former version of the
Slit Camera and holder. Even if the images are not recognized, the principles for the
evaluation are the same.
4.4.1.1.
Preparing the image
The figure to the left shows the original image of the focal
slit as it was exported from the work station. In this case a
jpg file. To be able to measure the intensity profile with
ImageJ one has to calibrate the pixel values, rotate, and
invert the image. The procedure is shown below.
