PURPOSE. Quality of vision in patients who have undergone corneal refractive surgery depends
upon the optimal centration of the procedures used. The center of the pupil is used as a
reference point in some corneal ablation procedures. The achromatic axis would be a more
sensible option from an optical point of view, but it is not as readily detectable. As an
alternative, other refractive techniques, like the small aperture corneal inlay for presbyopia
correction, use the corneal reflex (first Purkinje image). To assess the relative position of
these two marks, we developed a new instrument to simultaneously measure both the first
Purkinje image (PI) and the intersection of the achromatic axis with the pupil plane.
METHODS. The apparatus records images of the pupil and the PI when illuminated with a circle
of infrared light-emitting diodes. A second optical path allows determination of the
achromatic axis by using a subjective method. Both the positions of the PI and the
achromatic axis intersection are determined simultaneously.
RESULTS. A series of data were obtained in 48 eyes. The mean location of the achromatic point
relative to the PI was [x ¼ 0.05 6 0.15 mm; y ¼ 0.09 6 0.18 mm]. Considered individually,
in 55% of eyes, the distance between locations is less than 0.2 mm, and in 95% of eyes,
distances are less than 0.4 mm.
CONCLUSIONS. On average, achromatic axis crossing of the pupil and PI locations coincides
within measurement errors. Although there was some intersubject variability, differences in
location were less than 0.6 mm in all measured eyes.