Describing the signal-transfer characteristics of asymmetrical radiographic screen-film systems

The measurement of modulation transfer function for radiographic screen-film systems depends critically upon a proper linearization of the measured line spread function. This is normally done by photographic photometry (i.e., using the measured density versus log exposure relationship to transform the density line spread function into an exposure line spread function). It has been long appreciated that this procedure may fail for asymmetrical dual screen systems that use film with emulsion coated on both sides of the support. The advent of asymmetrical and near-zero crossover films that can be used with highly asymmetric screen pairs has prompted a reinvestigation of these concerns about the definition and measurement of modulation transfer function. For such cases, it is useful to define the contrast transfer function, which is a function of exposure and spatial frequency. When normalized by its zero frequency value the contrast transfer function can serve as the "effective MTF" for low-contrast input signals in such systems. In the limit of symmetrical systems this quantity approaches the conventionally measured MTF. The utility of this approach is demonstrated by applying it to a commercially available asymmetrical screen-film combination.