Automated measurement of strabismic deviations using a remote haploscope and an infrared television-based eye tracker

The conventional strabismus examination of infants and children requires apparatus such as prisms and occluders before the eyes. Such apparatus often distracts the child, complicating the examination. To avoid this distraction, we have developed an optical system using a large concave mirror to form an aerial image of the child in a remote location, and the examination can be performed on the image of the child rather than directly on the child. A large, gimbal-mounted, concave spherical mirror images the patient's eyes into a fixed, remote haploscope. The unrestained patient is monitored with an infrared-sensitive video camera, and alignment with the apparatus is maintained by tilting the spherical mirror appropriately under stepper motor control. The patient's eyes are totally dissociated, viewing wide-angle superimposed fields. Visual targets are generated and displayed under computer control, being projected by the haploscope eyepieces onto the surface of the concave mirror at a fixation distance of 80 cm. Strabismic deviations in the nine diagnostic positions of gaze were originally measured in adults using Hess-Lancaster, subjective cover, and objective cover techniques. Incorporation of a television-based eye tracker into the system now provides automatic neutralization of eye movement during alternate cover testing. The purpose of this study was to determine the efficiency and repeatability of testing older children and adults in the automatic eye-tracker mode, prior to application to young children and infants.