Improved computing scheme for measuring eye alignment with Purkinje images I and IV

Abstract

This study introduces an improved computing scheme for determining eye rotation from Purkinje images I and IV. The original computing scheme systematically underestimated eye rotation. Paraxial raytracing calculations revealed that this error resulted from failure to account for the fact that Purkinje images I and IV fall at different distances behind the cornea. The error could be overcome with a correction factor derived from paraxial raytracing calculations. A series of experiments were carried out to test the validity of this correlation factor, involving exact raytracing calculations as well as measurements on physical model eyes and human eyes. The influence on the correction factor of ocular surface asphericity, accommodation, age and ocular component variations were examined. The new method was also compared to Hirschberg's technique, which makes use of Purkinje image I alone, as a means of screening for strabismus.

Phakometric measurement of ocular surface radius of curvature and alignment: evaluation of method with physical model eyes

“…The in¯uence that inter-and intra-individual variations in the ocular surface radii of curvature and alignment have upon the size and position of Purkinje images is particularly relevant for the measurement of visual axis alignment while screening for strabismus (Barry et al, 1997a).…”