IMAGE SEPARATOR FOR STEREOSCOPIC FUNDUS PHOTOGRAPHY*t BY

THE recent publication of stereoscopic manuals of fundus photography (Blodi and Allen, 1964; Hoyt and Beeston, 1966) has demonstrated the great potentials of this form of investi-gation. A stereoscopic photograph is not only an excellent medium for teaching, but is also of great diagnostic value in presenting a magnified three-dimensional image of the fundus, which may be examined in detail at leisure and compared readily with the ophthalmoscopic appearance or correlated with the fluorescein angiogram. In addition, it can provide an objective record of the progress of ocular lesions which are themselves three-dimensional in nature. An instrument is described that has been designed to be used in conjunction with a fundus camera to take stereoscopic photographs. Although the history of stereoscopic fundus photography dates back to the work of Thorner (1909), it is only recently that advances in photographic equipment, in particular the electronic power-pack, have made this form of photography more practicable. It is conceded that the only truly quantitative method of stereophotography entails the use of simultaneous exposures, but to produce a suitable fundus camera to give simultaneous exposures is technically difficult; although such cameras do exist (Norton, 1955; Donaldson, 1965), they are very expensive and, being in a limited field of photography, are at present beyond the range of most photographic units. Stereoscopic photographs can be obtained more easily though less accurately by taking successive photographs and altering the optical axis of the camera between exposures. This dual-exposure method, known as relative stereophotography, has been made possible by the advent of the rapidly recharging electronic flash unit which reduces the time between exposures; the pictures obtained by this method give a qualitative stereoscopic effect although their quantitative value is variable. Several different methods for altering the optical axis of the camera have been devised; these include rotating the camera (Stenstrom, 1960) moving it laterally (Allen, 1964), moving the patient's fixation (Norton and Sullivan, 1958), and altering the direction of the optical system by an image-separating device (Allen, Kirkendall, Snyder, and Frazier