Photorefraction: a technique for study of refractive state at a distance.

Photography of the fundus reflections of a point source of light from a human subject facilitates estimation of the refractive disparity about any desired axis, between the plane of focus of the subject’s eyes and that of the camera. The method employs a special attachment to a 35-mm reflex camera, consisting of a fiber-optic light guide mounted in the center of an array of pie-shaped cylinder lens sectors and placed in front of the camera’s wide-aperture lens. The light guide supplies a 1/4 second flash of filtered tungsten light of irradiance less than 1 μW/cm2 at the corneas of the subject, who is seated 1–2 m distant. The reflected light emanating from the subject’s pupils is transformed by the array of cylinder lenses into a star-shaped pattern at the film plane; the lengths of the arms are proportional to the dioptric disparities about the corresponding axes. Theoretical intensity distributions of star patterns for spherical and astigmatic errors have been computed upon the assumption that the retina is a diffuse reflector. They are shown to agree well with experiments. The technique provides an objective method for estimating the refractive states of both eyes of a subject simultaneously, under more-or-less-natural circumstances, and may find practical application in the visual screening of very young children.