Necessity and Outlines of a New Theory of the Granularity of Medical and Industrial X-ray Films

Abstract

Tile analysis of film granularity of medical and industrial X-ray films is based on tile ideas of Rossmann formulated in 1963. Thsi relation seemed to be so well proved by many experiments and calculations in tile spectral range of 0-10 mm^-1 , tile visible range, that no attention was paid to the invisible rest of tile Wiener spectrum. But just tile measurements of Wiener spectra in a spectral region up to 250 mm^-1 (or higher) give results which contradict Rossmann’s theory. It is shown that neither the expression for quantum noise nor the assumption of a linear superposition of film noise and quantum noise is valid. The latter can only be seen as an approximation following out of the new theory, which is based on the idea that σ², as the integral over the Wiener spectrum, corresponds to the amount of silver in the film and is constant for a given density, film and developer. The shape of the Wiener spectrum is given by the |MTF|² of the system film +screen+X-ray-quantum under the boundary condition σ²=const. The new theory can explain all essential properties of the Wiener spectra as well as some special effects caused by the grain size distribution and chemical and physical developers.