A photographic technique for measuring horizontal and vertical eye alignment throughout the field of gaze

We present a photographic method based upon corneal light reflections for the measurement of binocular misalignment. Our procedures allow for the measurement of eye alignment errors to fixation targets presented at any distance throughout the subject's field of gaze, and allow for the measurement of errors in the horizontal and vertical directions. Furthermore, estimates of the alignment state can be made simultaneously from both eyes while fixation targets are presented monocularly or binocularly. This photographic method represents an enhancement of typical clinical prism and cover methods because: (1) it can provide extensive information efficiently about patterns of misalignment across a large number of fixation locations; (2) it also can provide information about the scatter in addition to the magnitude of convergence error; and (3) it can be easily applied to noncooperative subjects such as animals and young children. Furthermore, the procedure requires relatively inexpensive equipment and technical expertise that are readily available in most clinical or animal research laboratory settings. The method is validated by comparing the results obtained photographically to standard prism and cover assessments of macaque monkeys with strabismus. This comparison demonstrates that results obtained by the two methods are in good agreement and that the degree of accuracy is similar for the two methods. An estimate of the angle of deviation based on 10 photographs has a 95% confidence interval of about two degrees.