Operating Manual

11.2 Selection of source-to-film distance
Preceding paragraphs of this chapter described the effects of geometric unsharpness and
the possibility to influence this by adjusting the source-to-film distance.
This section will expand on this.
To obtain a radiograph which is as sharp as possible, so as to show maximum detail, the
total unsharpness should be kept to a minimum. The radiation energy level selected for
making the radiograph, see chapter 9, can serve as a lead. It is, after all, determined by
the thickness of the material to be radiographed, but is at the same time also responsi-
ble for film unsharpness U
f
, which can be extracted from table 1-11 and or figure 4-11.
It is no use to try and keep geometric unsharpness U
g
far below the value of U
f
, as in that
case U
f
determines the total unsharpness anyhow.
If the aim is to make geometric unsharpness U
g
equal to the value of U
f
, the source-to-
film distance (F) required can be calculated from the following formula :
In which:
F = source-to-film-distance
U
t
= total unsharpness
t = thickness of the object
s = effective source size
All measurements in mm.
Instead of calculating F, various
code-based procedures and guidel-
ines provide graphs from which
minimum distance (F
min
) can be
determined. Figure 5-11 shows a
nomogram on the basis of EN 1435,
from which the minimum focus
distance for two quality levels (cate-
gory A and B) can be extracted.
Table 1-11 and figure 4-11 show experimentally determined values of inherent uns-
harpness for film exposed at various radiation energy levels.
These values are based on the use of filters and thin lead intensifying screens; thicker
screens produce slightly higher values. If no lead screens are used, U
f
is 1.5 to 2 times
smaller. U
f
is influenced mainly by radiation intensity and the type of intensifying
screens used; the type of film is hardly of any consequence.
The distance between film and intensifying screen is of great importance for the value of
U
f
. Good contact between film and intensifying screen is imperative and can be achie-
ved by vacuum-packing of film and screens together.
From the above information it can be deduced that U
f
increases at higher radiation energies.
Total unsharpness
Total film unsharpness U
t
is determined by the combination of U
g
and U
f
. The two values
cannot be just added up to arrive at a figure for U
t
. In practice, the following formula pro-
duces the best approximation for film unsharpness U
t
:
Broadly, if one value of unsharpness (U
g
or U
f
) is more than twice the value of the other,
the total unsharpness is equal to the largest single value; if both values of unsharpness
are equal, total unsharpness is about
 2 = 1.4 times the single value.
If necessary, U
g
can be reduced by increasing the focus-to-film distance. This can only be
done to a limited extent because, due to the inverse square law, exposure times would
become extremely long. As a compromise an optimum focus-to-film distance F is chosen
whereby U
g
= U
f
.
101100
Radiation energy U
f
in mm
50 kV 0.05
100 kV 0.10
200 kV 0.15
400 kV 0.20
2 MeV 0.32
8 MeV 0.60
31 MeV 1.00
Se75 (320 keV) 0.18
Ir192 (450 keV) 0.25
Co60 (1.25 MeV) 0.35
Table 1-11. Empirical values of film uns-
harpness Uf at various radiation energies
using lead intensifying screen
Fig. 4-11. Graphical representation of table 1-11.
Values of U
f
for X - and Gamma radiation at increasing radiation energies
U
t
=
U
2
g
+U
2
f
F =
t(U
t
+1.4s)
U
t

Fig.5-11. Nomogram for minimum source-to-film distance f
min
according to EN 1435-criteria.
Catagory A - less critical applications (general techniques)
Catagory B - techniques with high requirements
of detail discernability
The above graph appears enlarged in the appendix.
distance f
min
for catagory B
distance f
min
for catagory A
distance (b)
f
min
= minimum distance
source to source-side object (mm)
s = source size (mm)
b = distance source-side object to film (mm)