Operating Manual

14
Film exposure
and handling errors
119
Before a particular difference in density in a radiograph is attributed to a defect in the
object examined, it must be sure that it is not the result of incorrect handling- or proces-
sing of the film. It is, therefore, essential to be able to recognise such faults when exami-
ning the film in order to prevent their recurrence. It is often possible to identify faults due
to wrong processing by looking obliquely at the surface of the film while facing towards
the light, and comparing the two emulsion surfaces. The X-ray image usually is identical
on both sides of the film, while a fault in processing will frequently affect only one surfa-
ce, and can be seen as a change in reflection on the surface.
The most common faults, and their possible causes, are listed below:
Insufficient contrast
a: with normal density:
1. radiation too hard
2. over-exposure compensated by reduced developing time
3. unsuitable or wrongly mixed developer
4. prolonged development in too cold a developing bath
b: with insufficient density:
1. insufficient development
2. exhausted developer
3. unsuitable or wrongly mixed developer
Excessive contrast (i.e. lack of intermediate tones)
1. radiation too soft
2. under-exposure, compensated by prolonged developing
3. unsuitable or wrongly mixed developer
General lack of density
1. radiation too soft
2. under-exposure, compensated by prolonged developing
3. unsuitable or wrongly mixed developer
General excessive density
1. over-exposure
2. prolonged development or developing temperature too high
3. unsuitable or wrongly mixed developer
Insufficient sharpness
1. source-to-focus distance too short
2. source or object moved during exposure
3. film-to-object distance too great
4. dimensions of source or focus too big
5. poor contact between film and screens
6. wrong type of foil used
118