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Abstract
The psychophysical RMS (root mean square) granularity ap is proposed for the evaluation of radiographic mottle, i.e., the granularity appearing in a radiograph made with an X-ray screen-film system. The value of is calculated from the Wiener spectrum of the radiographic mottle and the frequency dependence of the minimum perceptible contrast, which expresses a psychophysical property of human vision. Examples are presen ted for three typical X-ray films. The density dependence of a,, is in close agreement with the results of subjective rating that (V the most perceptible density is 1.0 and (2) the perceptibility at density 2.5 is much worse than that at 0.6. Whereas the Selwyn granularity coefficient, which relates to the physical RMS granularity, does not correlate with the results of subjective rating.