Operating Manual

A steeper gradient means an increase in
density difference at equal radiation dose
and so a greater contrast, resulting in better
defect discernibility. If one requires high
contrast, it is therefore necessary to use the
highest possible density radiograph, while
remaining within the acceptable density
range of the viewing screen so as not to
impede film interpretation.
Most codes of good practice ask for densities
between 2.0 and 3.0 in the relevant area of
the image.
Table 1-7 shows the loss in contrast on
typical film as density values obtained fall
below 3.0 .
The specimen in figure 6-7 containing a
small step is radiographed with an exposure
time resulting in a density difference of 0.5
(B minus A). If now, using the same type of
film and the same tube voltage, a longer
exposure time is given, the density difference
is 0.9 (D minus C). The second radiograph,
therefore, shows more contrast.
Fig. 5-7. Gradient/density curves
for three types of film:
A low sensitivity: very fine grain film (D2)
B average sensitivity: fine grain film (D4)
C high sensitivity: medium grain film (D8)
density
6362
Gradient of the density curve
The density curve shows one of the most important characteristics of a film. The slope of
the characteristic curve at any given point is equal to the slope of the tangent line at this
point. This slope (a/b in figure 3-7), is called the “film gradient” G
D
, “film contrast” or the
“film gamma”.
Average gradient
The straight line connecting two points on a characteristic curve, as figure 4-7 shows, is
equal to the “average gradient” of the segment of the curve linking these two points. This
gradient (G
D
) is the average of all gradients in the segment between density values 3.50
and 1.50, and is a standard characteristic of a particular type of radiographic film.
In all films (for example D2 through to D8) the gradient (a/b) increases with increasing
density within the for conventional viewing screens useful density range of D<5.
The various types of films are not identical. This becomes clear if plotting the values of
gradient G
D
against the density resulting in the gradient/density curves, as shown in
figure 5-7. At higher film sensitivity the gradient is lower and, hence, the density curve
less steep.
Fig. 3-7. Gradient of an X-ray film
D
log.rel.exp.
Fig. 4-7. Average gradient (a/b)
of an X-ray film
Table 1-7. Contrast loss with reduced film density
D
log.rel.exp.
Density D Film contrast as a %
of the value at
D = 3.0
3.0 100
2.5 85
2.0 71
1.5 54
1.0 35
Fig. 6-7. Illustration of enhanced contrast
at increasing density
density