Pupil location under mesopic, photopic, and pharmacologically dilated conditions.

The pupil of the eye is a critical limiting factor in the optics of the visual system. The pupil, when it changes size, controls not only the amount of light available to the visual system, but also, and just as important, the optics of the eye. The eye has numerous optical aberrations1–6 that increase as the pupil dilates.7–9 The exact size and location of the pupil are important in corneal refractive surgery.9–11 For instance, in a typical LASIK procedure, the cornea is reshaped to provide an appropriate refractive correction in the optical zone, usually 5 to 6.5 mm in diameter. Beyond this optical zone is a transition zone to the peripheral cornea that has not been ablated. The relationship between the size and location of the optical zone and the pupil is critical for a successful surgical outcome. For instance, if the corneal ablation is well centered over the pupil of small diameter but becomes decentered with pupils of large diameter, then the patient may experience glare, halos, or ghost images in low illumination. There have been several studies of the change in centration of the pupil with changing size. Walsh12 used a photographic technique to measure the changes between light-adapted, dark-adapted, and pharmacologically dilated conditions. In 39 subjects, he found an average movement of approximately 0.19 mm, with the center of the pupil moving slightly nasally and superiorly as the pupil constricted. Wilson et al.13 used a video apparatus to examine the horizontal changes in the pupil center with natural dilation in eight subjects. They found larger shifts in most subjects (up to 0.59 mm), with movement occurring in different directions in different subjects, although they used the achromatic axis of the eye as the reference position, rather than the limbus center used by Walsh,12 and therefore an exact comparison of direction is not possible. Wyatt14 used a modified slit lamp to examine the change in centration with natural dilation in 23 subjects and similar to Walsh12 found that the center of the pupil moved nasally and superiorly to the limbus, with an average movement of approximately 0.1 mm with constriction. These studies did not investigate the role of refractive status or age on the movements measured, and it is possible that these factors can explain some of the differences in results. Younger subjects with their larger absolute change in pupil size15 with dilation may show larger shifts. If there are large changes in the pupil center with changes in size, there could be relatively large changes in the refractive state of the eye,16 because the curvature of the cornea changes with location. Similarly, with the advent of modern wave-front sensing technology for determining the aberrations of the eye,5,17–20 the location of the pupil becomes an important factor in determining effective optics.21,22 It is now generally accepted23 that the line from the fovea through the center of the pupil should be used in calculating the aberrations of the eye.23,24 However, if there are large changes in the location of the pupil center with changes in pupil size, this axis becomes less valuable, because its location depends on the measurement conditions. In the current study, we measured the location of the pupil, relative to the corneal limbus center of normal eyes under mesopic, photopic, and pharmacologically dilated conditions, by using an infrared pupillometer that is incorporated into the InterWave Scanner, a modification of the spatially resolved refractometer (SRR)18,25 wave-front sensing device. We also determined the location of the first Purkinje image (the reflection of anterior corneal surface) within the same coordinate system and using the same images as the limbus and pupil measurements.