Operating Manual

7.5 Film speed (sensitivity)

In radiography the relationship between exposure (in C/kg) and resulting density is com-

monly referred to as film speed. Other than in normal photography where film speed is

indicated by a DIN or ASA number, films for industrial radiography do not carry an inter-

nationally recognised speed number.

The generally accepted method of measuring the film speed of radiographic films is to mea-

sure the exposure required to achieve a density of 2.0 above base and fog, using a specific

processing technique. The various relative exposure values are shown in table 1-8.

7.6 Graininess

When a developed X-ray film is viewed in detail on an illuminated screen, minute density

variations are visible in a grainy sort of structure. This visual impression is called “graininess”

and a measurement of this phenomenon establishes a degree of “granularity”.

Effect of developing conditions on the density curve

The characteristic curve of an X-ray film is not only determined by the emulsion characte-

ristics but also by the way the film is developed. Parameters which can influence the cha-

racteristic curve are: developing time and its temperature, developer concentration and

agitation.

The effect of, for example, the developing time on speed (relative exposure factor),

contrast and fog, has been made visible in figure 7-7. Initially, up to approx. 4 minutes,

speed and contrast are low but increase rapidly with developing time.

From 8 minutes on, a further increase in developing time increases the background fog,

and eventually a decrease in contrast will occur.

Although it is possible to compensate, to a certain extent, for minor variations from the cor-

rect radiation exposure by adapting the developing time, normally a fixed time is maintai-

ned. In manual developing the standard time is 5 minutes. Developer type, film agitation

in the tank and temperature also influence density. That is why the overall developing pro-

cess should preferably be standardised or automated. In most cases, deviating from the

optimum developing conditions leads to reduced image quality.

Fig. 7-7.

Film characteristics at various

developing times

developing time (min)

relative exposure factor contrastdevelopment fog

industrial X-ray film developer G128 at 20ºC

speed

development fog

contrast effect

(average gradient for

densities between

1.5 and 3.5)