Videokeratoscope-line-of-sight misalignment and its effect on measurements of corneal and internal ocular aberrations.

A growing number of research laboratories are using the new technologies of videokeratoscopy and Shack-Hartmann aberrometry, in combination, to study the optical structure of the human eye. A potential source of error arises, however, because the two instruments are designed to measure the human eye along different reference axes. The Shack-Hartmann aberrometer is usually aligned coaxially with the line of sight, but videokeratoscopes usually are not. Thus far, corneal optics research has not adequately addressed the problem of videokeratoscope-line-of-sight misalignment and its effect on the computation of corneal and internal ocular aberrations. We measured corneal, ocular, and internal aberrations for three normal human eyes, developed a method to compensate for videokeratoscope-line-of-sight misalignment, and analyzed the importance of compensating for the misalignment. Our results show that when the value of angle lambda (the angle between the line of sight and the pupillary axis) is larger than 2 degrees-3 degrees, the misalignment, if ignored, can lead to incorrect estimates of corneal and internal aberrations as well as the corneal/internal aberration balance.