Operating Manual

83
Method and answer
The characteristic curve (fig. 9-9) shows that at the measured densities of 1.5 and 0.5
respectively, the corresponding logarithm of relative exposures are 2.15 and 1.65.
Since density 3.0 should not be exceeded, the area which is most important for inter-
pretation, which showed density 1.5 on the first exposure, must now display 3.0
Characteristic curve, figure 9-9, shows that density 3.0 corresponds with log.rel.exp.
2.45 and the difference between the two values amounts to 2.45 - 2.15 = 0.3.
This means that the exposure time must be doubled (10
0.3
= 2), resulting in a radiation
dose of 30 mA.min. This answers the first question.
If the exposure time is doubled, the log.rel.exposure of the lowest density value
originally measured will increase by 0.3, i.e. 1.65 + 0.3 = 1.95. The corresponding
density will be 1.0 (fig. 9-9).
The average gradient between the upper and lower densities on the original radiograph
was (1.5 - 0.5) / (2.15 - 1.65) = 2.0.
The average gradient on the new radiograph is (3.0 - 1.0) / (2.45 - 1.95) = 4.0, so the
average contrast has doubled.
82
Method and answer
The exposure chart (fig.7-9) shows that under the conditions mentioned above density
D = 2 is obtained on D7-film through the 15 mm thick section, using an exposure of
10 mA.min, point A on the chart.
Under the same conditions the
12 mm section would require an
exposure of 5 mA.min (point B in
the chart), which means an expo-
sure ratio of 10/5.
The exposure through the 12 mm
section is two times greater than
through the 15 mm section.
The logarithm of this ratio equals:
0.3 (log 2 = 0.3).
The characteristic curve (fig. 8-9)
of the D7-film shows that density 2
corresponds to log relative exposu-
re 2.2 (point C in fig. 8-9).
At 12 mm the log. rel. exposure is
2.2 + 0.3 = 2.5.
The corresponding density is then
3.5 (point D in fig. 8-9)
Example 2:
Effect of exposure on contrast
Assume that when an exposure
of 15 mA.min is used for a
radiograph on D7-film, both
average density and contrast
prove to be too low after pro-
cessing. The highest and lowest
density in the most relevant
section of the image are only
1.5 and 0.5.
The intention was to make a
radiograph with a maximum
density of 3.0.
Questions:
What exposure time would be required for the same radiation intensity and
what contrast increase would be achieved?
Fig. 8-9. Characteristic (density) curve of the D7-film
Fig. 9-9. Characteristic density curve of the D7-film
film
base
log.rel.exp.
log.rel.exp.
density
density
film
base