As an example of the effect of signal-to-noise ratios caused by film grain size, Figure 6-13 shows a microdensitometer
trace across the radiographic images of a series of small wires made through one inch of aluminum. All exposure
parameters were a constant except exposure time, which was varied to compensate for the three different film speeds.
The ratios between the trace amplitudes for the wires and the respective backgrounds indicate the signal-to-noise ratios
for Class 1, Class 2, and Class 4 radiographic films. Note the higher frequency content of the Class 1 film, indicating
its greater detail resolution capability.
Figure 6-13. Microdensitometer Tracings of Images of DIN Wire Penetrameters.
Another way to classify film is according to film speed. Table 6-5 lists the approximate relative speeds of radiographic
films exposed to radiation energies between 100 and 150 keV.