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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

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

expressed in mm, is a measure of film

unsharpness.

Consequently, U

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

. 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

– defect image blurred

C. Increased focus size – s -: still larger Ug - defect image blurred and loss of contrast - C

is less than in A and B

= 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

= 0

(b) (c) and (d) density distribution due to film unsharpness

(b) theoretical; (c) with grain; (d) smoothed.

film

test plate

film density

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 113 114

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GE CRx25P Brochure Pagina 51

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