Operating Manual

99
In this situation the unsharp images of each of the two edges of the defect may overlap,
as shown in example C. The result is that image C not only becomes unsharp, but also
suffers a reduction in contrast compared to image A, made with a point source and
image B made with a relatively small source.
Inherent unsharpness
Not only the silver halide crystals directly
exposed to X-radiation are formed into
grains of silver, but also (albeit to a lesser
degree) the surrounding volume of emul-
sion. This cross-sectional area represents
the “inherent unsharpness” or “film uns-
harpness” U
f
.
So, even in the absence of geometric uns-
harpness, if the radiation energy is high
enough, film unsharpness can occur: the so
called “inherent unsharpness”. If a steel test
plate with a sharp thickness transition is
radiographed with high energy X-rays,
there will be a gradual transition of film
density across the image of the “step” from
A to B.
Without inherent unsharpness, the film
would show an absolutely sharp transition
between the two densities, as shown in figu-
re 3a-11. In practice, the density change
across the image is as shown in figures 3b,
3c and 3d-11.
The width of this transitional area (U
f
),
expressed in mm, is a measure of film
unsharpness.
98
Consequently, U
g
can be reduced to any required value by increasing the source-to-film
distance. However, in view of the inverse square law this distance cannot be increased
without limitation, as extremely long exposure-times would result. The formula also
indicates that geometric unsharpness assumes more and more importance as the distan-
ce between defect and film increases.
A special case arises, however, when one uses a micro focus X-ray tube with a focal spot
size in the range 10-50
 m. With such a small focus size, the image can be deliberately
magnified (see section 17.1) by using a short source-to-specimen distance, and a large
specimen-to-film distance, and still retain an acceptably small value of U
g
. The advan-
tage of this technique, called the “projective magnification method”, is that the grainin-
ess always present in a photographic image is less of a disturbing factor in the discerni-
bility of very small defects.
Figure 2-11 shows the effect of geometric unsharpness on the image of a defect smaller
than the focus size.
film film film
Fig. 2-11. Geometric unsharpness: effect on the image of a small defect.
A. Point focus size - s -: no geometric unsharpness - defect image sharp
B. Small focus size – s -: geometric unsharpness U
g
– defect image blurred
C. Increased focus size – s -: still larger Ug - defect image blurred and loss of contrast - C
o
is less than in A and B
C
o
= contrast
Fig. 3-11. Inherent (film) unsharpness for X and
Gamma-radiation.
For clarity, the density curves are magnified along the X-axis.
(a) density distribution across image of sharp edge,
assuming U
f
= 0
(b) (c) and (d) density distribution due to film unsharpness
(b) theoretical; (c) with grain; (d) smoothed.
film
test plate
film density