![background image](http://html1.mh-extra.com/html/rti/slit-camera/slit-camera_user-manual_1497645012.webp)
12
4. X-Ray Tube Focal Spot Measurements
Slit Measurement Using Film and Magnifier Lens
RTI Slit Camera User’s Manual
1.0B
Nominal focal spot
Enlargement factor
value, f
E = n / m
f
0.4
E
3
0,5
f
1.0
E
2
1.1
f
E
1
4.3.
Slit Measurement Using Film and Magnifier Lens
1. When the alignment described above is acceptable, the evaluation of the focal
spot shall be done. Expose the cassette at the selected technique factors.
Film density should be between 0.8 and 1.2 above the base-plus-fog of the
film.
2. On the radiograph of the slit holder, measure the distance, in mm, between
the front edge and the open part of the slit holder, using a ruler (X
2
in the
figure below).
3. Calculate the magnification using the following formula:
34
34
X
m
n
E
2
(Eq. 4.1)
where:
n = slit to detector distance
m = slit to focal spot distance
X
2
= image edge distance measured in the x-ray image in mm
(34 mm is the true physical distance from the front edge to the edge
of the open part of the slit holder.)
Figure 4-2. Slit Holder showing the distance X
2
between the front edge and the open
part of the slit holder. The true physical distance is 34 mm.
For example, assume the image edge separation was measured as
68 mm. The magnification (E) is then
5
.
1
34
34
85
m
n
E
(Eq. 4.2)
X
2