In general, the pupil center is located nasal and slightly superior to the geometric corneal center, and there is a significant shift in the pupil center’s location with increasing dilation, with its lateral position coming closest to the geometric center of the cornea at its maximal dilation. The vertical motion is less consistent, with natural dilation moving the average position only 0.004 mm toward the center of the pupil, but pharmacologic dilation then moving the pupil away from the center (0.04 mm superiorly). The average absolute change in position we measured was between the values reported by Wyatt14
but less than those reported by Wilson et al.,13
with an average movement of the pupil center from mesopic to photopic conditions of 0.13 ± 0.07 mm. One difference in the studies was the use of the achromatic axis in the study by Wilson et al. This axis is variable among observers,29,30
and this difference could explain the difference in inferred direction of the shifts. However, for a small Maxwellian entrance pupil, the location of the achromatic axis should be stable, and this cannot explain the difference in size of motions measured. Individual differences could explain this difference in the measured change in pupil centration, although we found few large motions. Histograms of the distribution of number of eyes versus the movement of the pupil center between conditions are shown in . Whereas we found one eye where the motion was more than 0.5 mm, 85% of the eyes had motions of their pupil centers of less than 0.25 mm, and none had movements larger than 0.4 mm from mesopic to photopic conditions. The number of subjects showing a greater than 0.3-mm change in the location of the pupil center increased in the pharmacologically dilated conditions, with seven showing a change of center location of 0.3 mm or more. Although there was a trend for the pupils with the largest relative change in size to have the largest motion, as was suggested by Walsh,12
it was not significant (P
> 0.25), and even restricting the analysis to younger ages (< 40 years) did not cause the trend to reach significance. Overall, we conclude that although occasional large shifts in pupil center occur, such movements are not the rule.
FIGURE 4 The distribution of distances in pupil movements between three conditions in the 130 eyes in the study. Distance distribution of the movement of the pupil center from (top) photopic to mesopic, mesopic to pharmacologically dilated (middle), and photopic (more ...)
The Purkinje image has often been suggested as a landmark for aligning the eye in optical systems. The first Purkinje image is formed by reflection of light from the anterior corneal surface. When the cornea is illuminated with collimated light, the curvature causes the formation of an image at the focal point of the corneal curvature.27,28,31
Theoretically, there should be no change in the relative positions of the first Purkinje image and the geometric center of the cornea with dilation of the pupil, as long as there is no change in the direction of gaze relative to the optical axis of the pupillometer. Although the locations of the Purkinje images across conditions were very similar, with no significant change in center location, we found a small but significant difference between measurement conditions, in the distance from the corneal center to the Purkinje image. The first Purkinje image moved slightly to the temporal side under the pharmacologically dilated condition. These dilated measurements were made more than 20 minutes after measurements in the other two conditions. This suggests that some of the change in location could arise from slight changes in fixation strategy of the subjects.
In our data sample, it was necessary to eliminate data from two eyes, because after data collection was completed, it was realized that the subjects had not fixated correctly in at least one of the conditions. These conditions were recognizable in the images as a slight rotation of the eye and a large discrepancy between the positions of the Purkinje images across conditions. Eliminating these two outliers decreased the change in distance from the Purkinje image to the corneal center under dilated conditions, but did not eliminate it (mean distance moved 0.04 mm, P
< 0.01, t
-test). It is difficult to account for this movement, because, in general, small misfixations should be randomly distributed around the mean. It is not likely that these changes are attributable to changes in the point spread function of the optics with changes in pupil size, because any realistic change in the centroid of the point spread function would be quite small relative to the rotations required to move the Purkinje image. However, it is important to note that although the change in location is statistically significant, it was slight. The motion of the pupil center was larger and was in the same direction in most subjects. Thus, although the location of the Purkinje image is relatively stable in the cornea, it appears to move slightly, most likely because of small eye movements. We concluded that the limbus probably provides a more reliable coordinate system for the eye; however, we could not test this, because the present study assumed that the edge of the limbus provides a stable reference, a supposition supported by the small change in measured diameter (approximately 1% of the total diameter) across measurement condition. This conclusion is potentially important when considering the need for relating measurements of the optical quality of the eye to planning corneal surgery. Current recommendations are to base measurements of wave aberrations on the center of the pupil.10,23
However, these measurements must then be related to physical positions of the cornea. There is some tolerance for misalignment between surgery and the pupil, even for the correction of high-order aberrations,21,22
the possibility of inadvertent large changes in fixation could seriously influence references based on the location of the Purkinje image.
In summary, we confirmed that there are systematic changes in pupil diameter with age15,26
and that there are changes in the location of the pupil center between mesopic, photopic, and pharmacologically dilated conditions, but these motions are relatively small. Finally, it is of note that although most individuals had only slight changes of pupil location with dilation, occasional eyes showed large changes in centration, and these changes tended to be more common under pharmacologically dilated conditions.