Calibration of Hirschberg test photographs under clinical conditions

The feasibility of determining the angle of ocular deviation from clinical photographs was assessed. Corneal reflex photographs were obtained from 30 strabismic patients and 5 orthophoric control subjects, using a centered electronic flash, with a millimeter ruler placed across the forehead for calibration. Measurements of corneal reflex displacement were obtained from the photographs and compared with standard prism-cover results. The value of 20.89 prism diopters (PD) per millimeter of corneal reflex displacement was obtained for the factor of conversion from reflex displacement to strabismic angle. This value of the Hirschberg coefficient is valid only for measurements of reflex displacement along a true frontal plane, as provided in a photograph. Detailed algorithms for the calculation of strabismic angle from photographic data are provided in an appendix.