PSYCHOPHYSICAL RMS GRANULARITY FOR THE EVALUATION OF RADIOGRAPHIC MOTTLE

The psychophysical RMS (root mean square) granularity sigma(p) is prop osed for the evaluation of radiographic mottle, i.e., the granularity appearing in a radiograph made with an X-ray screen-film system. Tile value of sigma(p) is calculated from the Wiener spectrum of the radiog raphic mottle and the frequency dependence of the minimum perceptible contrast which expresses a psychophysical property, of human vision. E xamples are presented for three typical X-ray films. The density depen dence of sigma(p) is in close agreement with the results of subjective rating that (1) the most perceptible density is 1.0 and (2) the perce ptibility at density 2.5 is much worse than that at 0.6. Whereas the S elwyn granularity coefficient, which relates to the physical RMS granu larity, does not correlate with the results of subjective rating.