Abstract

Much attention has been paid to methods tor measuring the plane of focus of the eye with little or no cooperation of the subject (1,2). We present here a simple arrangement to do this using infrared light emitting diodes (LEDs). Long wavelength light (approx. 900 nm) does not induce a pupil response, thereby allowing measurement at large pupil sizes and increasing the sensitivity of the retinoscopy. The method is a modification of one popularized by Kaakinen (1). By contrast, however, in our method a) we use infra-red light rather than visible light and b) the light source is positioned at different distances (eccentricities) from the optical axis of the camera while the fundus reflex is observed. Whether or not a reflex can be detected depends on the defocus of the eye relative to the camera, D, the pupil size, P, the excentricity, E, of the light source from the optical axis, and the camera to subject distance, A. In a theoretical treatment of photoretinoscopy by Howland (3,4) the defocus of the eye relative to the camera is shown to be given by the equation: (1) where DF is the Dark Fraction or ratio of the portion of the diameter of the pupil which is dark to that which is illuminated by the fundus reflex.

© 1986 Optical Society of America