The physiological optics of the visualisation of X ray pictures

Abstract

I. The reduction of the visual image obtained by observing a transilluminated X-ray picture into a conception of the anatomical object under observation is a complicated process involving the following stages: A. Homogeneous X-ray beam. B. Anatomical object. C. Inhomogeneous X-ray beam (shadow-image). D. Photographic image. E. Retinal image. F. Visual image. Radiologists have studied intensively the various stages between A–D. The subsequent stages between D–F leading from the photographic to the visual image have been largely ignored. II. The relation between the transilluminated X-ray picture and the subjective visual image of the observer is analysed on a detailed description of the successive steps; photographic image, retinal image and visual image. III. In this analysis a major difficulty is caused by the fact, that physical quantities and the corresponding psychical quantities are incommensurable. The efforts of Fechner and others to establish a mathematical relation between intensity of stmulus and intensity of sensation are discussed and the problem of „visibility” is considered. IV. The lower limit of visibility (differential threshold, least perceptible contrast) appears to be of great importance. The value of the least perceptible contrast Cmin is dependent on many factors. An experimental study on the influence of these factors is described and reviewed in the light of the work of other investigators. V. The significance of visual acuity is stressed and the close connection between visual acuity and differential sensitivity is shown. VI. Conclusions of a pratical character are drawn mainly as to optimum brightness of the viewing-box and the surroundings, optimum distance of observation, elimination of glare-sources, colour of the viewing box, etc.