<title>Statistical processes limiting the noise of a screen-film system: II</title>

Abstract

In a recent publication we demonstrated that the increase in image noise which results from exposing a film via a phosphor screen can be attributed entirely to the increased extent of the autocorrelation interval introduced by the screen, and not to any change in the shape or scale ofthe probability distribution function which governs the fluctuations about the mean level. This result implies that the (0,0)-value of the autocovariance function is independent of the degree of so-called quantum mottle and since the autocovariance function, ACV(x,y), and the Wiener Spectrum, WS(u,v), are Fourier transform pairs, it follows that the integral of the Wiener Spectrum over all spatial frequencies (u,v) must share this independence.Since this result was counterintuitive to existing assumptions ofthe role ofscreen and film in defining the Wiener Spectrum (i. e., a simple additive combination of quantum mottle and film grain), we now investigate this relationship in more detail in order to provide a more complete insight. For this purpose we have simulated a set of controlled images which correspond to a wide range of screen correlation intervals, from 192 jim down to uncorrelated film noise. Included in this simulation we have also explored the role of the overall amplification factor, i. e., the average number of image grains associated with an x-ray quantum. The results of these simulations are presented here, and confirm our previous findings, by establishing the invariance ofthe scale (0,0) value ofthe ACV.