Frequency Dependence of Minimum Perceptible Contrasts and Optimum Film Densities of Radiographs

Abstract

The frequency dependence of minimum perceptible contrasts (differences in diffuse density units) for various viewer brightnesses was obtained by observing noiseless sinusoidal patterns made on fine-grain photographic films. For each frequency, the contrast is more easily perceptible zvith an increase in the viewer brightness and a decrease in the average density. Under the best conditions a contrast of 0.0043 is perceptible. The best perceptibility was attainned at 0.8 to 1.0 mm-1.The optimum densities, at which the detectability and resolution became best, were obtained from the maxima of the psychophysically modified gradient as defined by Kanamori in 1964. The optimum density decreases as the viewer brightness decreases and the frequency increases. For a typical 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.