THE LINE SPREAD-FUNCTION AND THE MODULATION TRANSFER FUNCTION OF X-RAY FLUORESCENT SCREEN-FILM SYSTEMS—PROBLEMS WITH DOUBLE-COATED FILMS

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Abstract
The line spread-function (LSF) of the screen—film system has been defined as the sum of the spatial distribution of illuminance in the front and the back emulsion caused by a beam of x rays which passes through a narrow slit. Fourier transformation of the LSF gives the corresponding modulation transfer function (MTF). In the past, the LSF has been obtained by scanning slit images on double-coated film. This method can be justified for symmetric screen—film systems, but is, in general, invalid for asymmetric screen—film systems, although the errors introduced may be small, even for grossly asymmetric systems, as is shown in this paper. In practice, many screen—film systems are essentially symmetric and yield nearly the same LSF's and MTF's by either method. For this reason, it is valid to obtain the LSF and MTF of such systems by treating the double-coated film as a unit. A mathematical analysis indicates that the MTF of the screen—film system is the sum of weighted MTF's derived from the illuminance distribution in the front and the back emulsion. The weighting factors are the ratios of the light exposure in each of the two emulsions to the total light exposure produced by a uniform x-ray exposure.

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