Operating Manual

9.6 Relative exposure factors
“Relative exposure factors” can be used to convert an exposure chart for one type of film
to another film, although still for the same radiation energy.
These factors are not constant for different radiation energies and should, therefore, be
used with some caution. Some examples of relative exposure factors for Agfa films are
shown in table 1-9.
These are the factors by which to increase or decrease the exposure-time when using the
types of film other than those for which the exposure charts have been prepared. In view
of the widely-varying quality of the radiation emitted by different types of X-ray equip-
ment and the appreciably different characteristics of the various types of X-ray films made
for industrial use, caution should be exercised in applying these relative exposure factors
generally.
Darkroom technique too, plays an important role and a uniform manual or automatic
development process is, therefore, essential.
With radioactive sources, which give a constant quality (hardness/energy) of radiation, the
relative exposure factors listed can be used quite safely.
9.7 Absolute exposure times
Table 2-9, derived from reference [2], lists the widely varying absolute exposure times
when different radiation sources are used for radiography on steel of varying thickness.
The relative exposure factors from table 1-9 for both types of film can be recognised
in this table.
9.8 Use of the characteristic (density) curve
with an exposure chart
In the following examples the tube voltage and focus-to-film distance (FFD) are assumed
to be constant, and automatic development is for 8 minutes in G135 developer at 28°C.
Example 1:
Effect of the thickness of the
object on the density of the radio-
graphic image
It is required to radiograph, on
D7 film, a steel object comprising
two sections of different thick-
ness of 12 mm and 15 mm.
The exposure chart figure 7-9
shows that at 160 kV and an
FFD. of 70 cm, using 10mA.min,
a density of 2 behind the section
measuring 15 mm in thickness
will be obtained.
Question: What image density
will be obtained behind the sec-
tion measuring 12 mm under
these given conditions?
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Type of film Relative exposure factors
100 kV 200 kV 300 kV Ir192 Co60
(1) (2) (3) (4)
D2 9.0 7.0 8.0 9.0
D3 4.1 4.3 5.0 5.0
D4 3.0 2.7 3.0 3.0
D5 1.7 1.5 1.5 1.5
D7 1.0 1.0 1.0 1.0 1.0
D8 0.6 0.6 0.6 0.6
RCF+F6 (5) 0.174 0.132 0.389 0.562
RCF+F8 (5) 0.03 0.022 0.035 0/040
Table 1-9. Relative exposure factors. For (1) to (5) refer table 1-8.
Table 2-9. Absolute exposure times for steel of varying thicknesses, derived from [ref. 2]
X-ray tube Gamma source Linac
Energy 100 kV 250 kV 300 kV 450 keV 1,25 MeV 8 MeV
mA 3 10 10
Exp. C/Kg.s 1.8 4.7 5000
FFD. mm 500 700 700 1000 2000 2000
Film type D4 D7 D4 D7 D4 D7 D4 D7 D7 D7
Mat.thickness Exposure time in seconds
15 mm 50 10
25 mm 100 20 70 15 80
50 mm 1080 210 660 210 6300 1980 1680
100 mm 6300
150 mm 32400 10
200 mm 30
400 mm 4200
Fig. 7-9 Exposure chart for D7
Material = steel; density = 2; ffd = 70 cm;
screens = 0.02 mm lead;
Automatic processing, with developer G135 at 28°C,
8 min. cycle.
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