The luminous efficiency of light entering the eye pupil at different points and its relation to brightness threshold measurements

Abstract

In previous papers (Crawford 1936) results have been given for the variation of pupil size and of brightness threshold (measured with small artificial pupil) with time after changing from a high to a low general light stimulus. These results cannot, however, be immediately combined to give the response of the eye as a whole (with natural pupil) until the possible effects of the efficiency of light entering the eye through different parts of the pupil have been investigated. It has been found (Stiles and Crawford 1933) that light entering the eye near the periphery of the pupil is less efficient in producing the sensation of brightness than light entering near the centre. Probably the same thing occurs for the threshold of the sensation of brightness, so that the threshold would be higher for light passing through the more peripheral parts of the pupil. The first part of this paper is concerned with the testing of this hypothesis. The results are expressed in the form of curves connecting brightness threshold with distance of the point of entry of the light from the centre of the pupil, these curves being called, for convenience, threshold efficiency curves. The measurements of this effect made hitherto have related to the foveal region of the retina and a fairly high field brightness. They are here extended, by the threshold brightness method, to regions of the retina more or less distant from the fovea and to field brightnesses extending down to zero. In order to complete the picture of the luminous efficiency effect, in so far as it relates to white light, it remains only to investigate the course of the variation in shape of the threshold efficiency curve with time after the field brightness is changed suddenly from one value to another. One case only has been dealt with, namely, the change from a high value of field brightness to zero. Finally, all these results may be combined with the corresponding measurements of pupil size and retinal sensitivity to give the response of the complete eye.