Operating Manual

Three factors govern the discernibility of defects in a radiograph:

1. Geometrical effects:

• Size of the source

• Source-to-object distance

• Defect-to-film distance

2. Film properties (governing image quality):

• Graininess

• Contrast

•Fog

• Inherent unsharpness

3. Quality of radiation applied.

11.1 Unsharpness

Geometric unsharpness

X-ray tubes and radioactive sources always produce radiographs with a certain amount

of blurring – the “geometric unsharpness”, U

in fig. 1-11, because of the finite dimen-

sions of the focal spot or source size.

The magnitude of this unsharpness, U

, is given in the following equation:

In which:

s is the effective focus (or source) size

F is the focus-to-film (or source-to-film) distance

a is the defect-to-film distance

The maximum value of U

related to a defect situated at a maximum distance from the

film (and for which a = t) can be calculated from the formula:

In which: t = the thickness of the object

Defect discernibility

and image quality

9796

Fig. 1-11. Geometric unsharpness.

The source diameter, S, is shown very

large for clarity.

source

anode

film

(ffd)

density across film

F-a

(max)

F-t