The Pupil is a Moving Target: Centration, Repeatability, and Registration

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Abstract
To evaluate the ability to accurately center and register wavefront data to provide effective custom wavefront application for vision correction. Centration images were obtained of 40 undilated pupils at 5 different illumination levels using the Alcon LADARWave aberrometer (Alcon Laboratories Inc, Fort Worth, Tex). Pupil position relative to the limbus was determined for all eyes under all levels of illumination. Variability in centration of the pupil was based on difference in magnitude of offset between pupil center and limbal center. Human operators were compared to a computerized limbus recognition system through repeatability testing using 10 different images of one eye of a single patient. As determined by human operators, the pupil center in undilated pupils shifted a mean of 175 microm between the lowest level and the highest level of illumination (range 34 to 335 microm [standard deviation 84 microm]). Repeatability testing suggests human measurements can accurately position the pupil to within approximately 50 microm using current technology. Computer image processing using limbus recognition software appears equal or superior to manual pupil centration. The pupil center can have a significant change in position with changing illumination. Wavefront data must be centered on a fixed eye structure rather than the pupil center to ensure that wavefront data captured in scotopic light are appropriately matched to the pupil in the photopic light used at surgery. The goal is to avoid clinically significant decentration of the wavefront between capture and excimer laser application.

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