FREQUENCY-DEPENDENCE OF MINIMUM PERCEPTIBLE CONTRASTS AND OPTIMUM FILM DENSITIES OF RADIOGRAPHS

The frequency dependence of minimum perceptible contrasts (differences in diffuse density units) for various viewer brightnesses was obtaine d by observing noiseless sinusoidal patterns made on fine-grain photog raphic films. For each frequency, the contrast is more easily percepti ble with an increase in the viewer brightness and a decrease in the av erage density. Under the best conditions a contrast of 0.0043 is perce ptible. The best perceptibility was attainned at 0.8 to 1.0 mm-1. The optimum densities, at which the detectability and resolution became be st, were obtained from the maxima of the psychophysically modified gra dient as defined by Kanamori in 1964. The optimum density decreases as the viewer brightness decreases and the frequency increases. For a ty pical medical x-ray film OG, the optimum densities are less than 1.3, while those of industrial x-ray film Industrial-100 are less than 2.4.