Purpose: Quality of vision in patients with small aperture corneal inlays to correct for presbyopia depends on its proper centration. In practice, the inlay is positioned on the 1st Purkinje image but if this location is far from the foveal achromatic axis, a significant transverse chromatic aberration could degrade retinal images. We developed a new instrument to simultaneously measure both the 1st Purkinje image and the intersection of the achromatic axis with the pupil plane.
Methods: The apparatus records images of the eye’s pupil and the 1st Purkinje reflection when illuminated with a semicircle of infrared LEDs. In addition, a liquid crystal spatial light modulator produces a small aperture conjugated to the subject’s pupil plane with a location that can be controlled by the subject. Subjects perform a Vernier-type alignment task by moving a 1-mm aperture over the eye’s natural pupil to align a red target to a blue grid. Both the positions of the 1st Purkinje reflection and the achromatic axis intersection are determined simultaneously. Series of data in 33 eyes with a range of +/- 4 D refractive errors were obtained.
Results: For each subject the procedure was repeated 10 times. The standard deviation in the measurements was below 0.18 and 0.04 mm for the achromatic axis and 1st Purkinje positions respectively. The average location of the achromatizing pupil, relative to the subject’s natural pupil, was: x = 0.29 ± 0.19 mm (nasal); y = 0.09 ± 0.19 mm (superior). These values should be compared to the average location of the 1st Purkinje image: x = 0.34 ± 0.19 mm; y = 0.07 ± 0.07 mm. Considered individually, the two positions were statistically different in 8 and 16 eyes for the horizontal (x) and vertical (y) directions respectively. The differences in the two locations were smaller than 0.4 mm for both directions in all the measured eyes.
Conclusions: We have designed and built a new instrument that allows to measure simultaneously both the locations of the 1st Purkinje image and the achromatic axis intersection with the pupil plane. On average, both locations coincide within the measurement errors. Although there is individual variability, the maximum differences in location did not exceed 0.4 mm in any eye. This value should induce a modest amount of transverse chromatic aberration, indicating that centration of the inlay on the 1st Purkinje image should be adequate for most patients.