Sophisticated models of the stochastic processes involved in screedfilm image formation have been developed in the last few yem. These models allow one to predict overall system performance and to compare this with lhe performance of real system.In the case of screens these stochastic processes involve the probability of x-ray absorption, the x-ray to light photon conversion efficiency governed by the phosphor's scintillation spectrum and the subsequent scattering or absorption of the emitted light photons in the Screen. io the case of systems using two screens the film's contribution to the final photographic image involves degradation of image resolution due to print-through. Our models allow us to quantify lhe degree to which this occurs and to establish an upper limit on image resolution in the absence of print-through and as a function of the peak emission wavelength of the phosphor which influences light absorption in the screen.

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A filmkcreen system is described where the light emission of the intensifying screen is in the ultraviolet (w), which is more efficiently absorbed by lhe film, and hence print-through is vinualiy eliminated. In addition, because of significantly higher uv absorption within the screen, the modulation lransfer function (Mm) is superior to that of conventional systems based on visible light. Thus the overall uv fildscreen system has superior imaging efficiency and quality, as measured by the detective quantum efficiency (WE) and the noise equivalent quantum efficiency (NEQ)), to those of conventional systems, These imaging chancteristics of the novel systems are described here and are compared with those of some conventional systems,

As new generations ofscreen-film systems tend toward idealpeifonnance when evaluated by fundamental information theoretic criteria, there has been a natural increase of interest in identifying and controlling those remaining statisticalprocesses which are present during imaging but which have no essential role in image transfer, and which are thus manifest in an increase in the output image noise level.In the present contribution we have investigated the nature and role of the probability distribution function governing the fluctuations in image density about a mean level, and especially the relationship of this distribution to the Wiener spectrum associated with image noise. Experimental measurements are shown confirming that, at a given mean density level, the probability distribution function is independent of exposure statistics regardless of whether thefilm is exposed to uncorrelated light photons or to correlated light via an intensifying screen. We therefore conclude that djfferences in the associated Wiener spectrum are entirely attributable to changes in the extent ofthe autocorrelation interval and not due to changes in the shape or extent ofthe probability distribution function.

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