Photographic calibration of the Hirschberg test

A series of direct photographic measurements of corneal reflex positions was taken to reconcile discrepant interpretations of the Hirschberg test. A value of 21 prism diopters per millimeter was obtained for the conversion factor relating ocular rotations to reflex displacement, valid over a range of 200 prism D. Quantitative interpretation of routine clinical photographs of strabismic patients can thus be easily obtained by including a millimeter scale in each picture. This value for the conversion factor is in agreement with a simple optical model for the corneal reflex. The discrepancy with the traditional value (about 14 prism D/mm) appears to arise from an attempt of most observers to measure reflex displacement from the corneal apex along the surface of the cornea, rather than to project the reflex and pupil onto a true frontal plane, as is done in a photograph.