T.O. 33B-1-16-256.4.1.3.2As an example of the effect of signal-to-noise ratios caused by film grain size, Figure 6-13 shows a microdensitometertrace across the radiographic images of a series of small wires made through one inch of aluminum. All exposureparameters 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 ratiosfor Class 1, Class 2, and Class 4 radiographic films. Note the higher frequency content of the Class 1 film, indicatingits greater detail resolution capability.Figure 6-13. Microdensitometer Tracings of Images of DIN Wire Penetrameters.6.4.1.3.3Another way to classify film is according to film speed. Table 6-5 lists the approximate relative speeds of radiographicfilms exposed to radiation energies between 100 and 150 keV.
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