The scattering theory of the effect of glare on the brightness difference threshold

The introduction of light sources into the visual field results, as is well known, in an increase in the brightness difference threshold for foveal vision. Several extensive investigations of this effect have been carried out in the last few years, and the values of the threshold under different conditions of glare have been related to the physical variables, such as the angular seperation between glare source and test object. Holladay, to whom one of these experimental investigations is due, puts forward the theory, following a suggestion of Raman, that the threshold is raised owing to an increased illumination of the fovea centralis by light scattered in the media of the eye, by the pencil of light from the glare source, in its passage from cornea to retina. Holladay applies Rayleigh's scattering formula to compute the illumination scattered onto the fovea in this way, and argues therefrom as to the way in which the threshold will vary with change of glare angle. Holladay's discussion is subject to criticism on the following grounds: (1) it fails to make clear the part played by the pupil aperture; (2) it treats white light as monochromatic in applying Rayleigh's formula; (3) it neglects to make a collateral deduction of the magnitude of the scattering coefficient which the eye media must posses in order to produce enough scattered light to explain the observed increases in the threshold. The object of the present paper is to give a brief sketch of the scattering theory, with the above defects removed, and to compare the results of recent experimental measurements, with the predictions of the theory.