Characterization of the inherent error in a spherically-biased corneal topography system in mapping a radially aspheric surface.

BACKGROUND

Accurate measurement of corneal topography is crucial for many clinical applications. Also, the human cornea is known to be an asphere. Therefore, the purpose of this study was to quantitatively evaluate the accuracy of the EyeSys Corneal Analysis System in measuring a radially aspheric test surface and characterize the error function.

METHODS

Curvature of a calibrated ellipsoid was determined using three techniques: 1) calculating theoretically, 2) modeling with a spherically-biased algorithm, and 3) measuring experimentally using the EyeSys system, both with the surface aligned and under conditions of misalignment.

RESULTS

The inherent error steadily increased from center to periphery, with a maximum error greater than 3.00 diopters at a radius of 4 mm for an eccentricity of 0.5 and apical radius of curvature of 7.5 mm.

CONCLUSIONS

The EyeSys Corneal Analysis System does not accurately measure the instantaneous radii of curvature of an ellipsoid. Misalignment error is small compared to the inherent error due to a spherically-biased reconstruction.