Importance of fixation, pupil center, and reference axis in ocular wavefront sensing, videokeratography, and retinal image quality.

PURPOSE: To examine the impact of the location of the fixation target, pupil center, and reference axis of ophthalmic aberrometers and videokeratographers on the measurement of corneal aberrations relevant to vision.
SETTING: Clinical Research, Visual Optics Institute, College of Optometry, University of Houston, Houston, Texas, USA.
METHODS: The design features of a generic aberrometer and videokeratographer and their interaction with the eye were examined. The results provided a theoretical framework for experimental assessment of pupil translation errors on corneal aberrations relevant to vision and their correction in 129 eyes.
RESULTS: Two key principles emerged. First, the aberrometer's measurement axis must coincide with the eye's line-of-sight (LoS). Second, the videokeratographer's measurement axis (the vertex normal) must be parallel with the eye's LoS. When these principles are satisfied, the eye will be in the same state of angular rotation and direct comparison of measurements is justified, provided any translation of the pupil from the vertex normal is taken into account. The error incurred by ignoring pupil displacement in videokeratography varies between eyes and depends on the type of aberration and amount of displacement, with the largest residual correction root-mean-square wavefront error being 1.26 mum over a 6.0 mm pupil, which markedly decreases retinal image quality.
CONCLUSION: Translation of the pupil center with respect to the vertex normal in videokeratography should not be ignored in the calculation of the corneal first-surface, internal aberrations of the eye relevant to vision, or the design of refractive corrections based on videokeratography.