The magnitude of a measurement of the noise power spectrum (NPS) of a radiographic screen-film system is affected by the light-scattering properties of the film studied and the optical characteristics of the microdensitometer used to sample the image. To facilitate absolute NPS intercomparisons among laboratories, NPS in terms of instrument density must be converted to diffuse density. Conversion in terms of a Callier Q factor was found to be inadequate due to nonlinearity of the density response of the microdensitometer. By establishing instrument-to-diffuse density characteristic curves for the microdensitometers at two laboratories and correcting the NPS by the square of the slopes of these curves at the density of the image, good agreement was achieved for independent NPS measurements for a given film sample.
Keywords: noise power spectrum, diffuse density, microdensitometer calibration, Callier Q factor, derivative of Callier factor, Kodak XRP film, Dupont Hi-Plus screen, Dupont Detail screen
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