Absolute accuracy of Placido-based videokeratographs to measure the optical aberrations of the cornea.

Advances in excimer laser spot size and ablation pattern capabilities, together with the development of Hartmann-Shack sensors for high-resolution eye aberration measurements, have taken refractive surgery to a new stage. Nevertheless, studies have shown that the lens contributes differently to total aberration and depends on accommodation and age factors. Therefore, the choice of which aberrations to correct is also related to the internal optics of the eye, although only the cornea undergoes surface changes. A simple model is to subtract the total aberrations of the eye from the aberrations of only the cornea, which are calculated based on its three-dimensional profile. This has been done successfully, in practice, for certain commercial videokeratographs (VKS), but it is important to analyze the consequences of general flaws associated with VKS systems when used to estimate the corneal aberrations. A computer simulation of Placido-based VKS has been developed and applied to several theoretical surfaces resembling corneal profiles. The resulting Placido images were used to retrieve the corneal profiles. Corneal surface data were input into an algorithm that calculates the wave-front aberration as the difference in optical path length of the principal ray of an on-axis object point and several marginal rays. Wave aberration was then fit by a set of Zernike polynomials using the least-square method. From the Zernike coefficients, the correlation of different corneal profiles to their optical aberrations could be estimated. Misalignment (decentralization), improper focusing, and tilt were also implemented to the VKS simulation, and a quantitative analysis of the consequences on precision of wave aberration estimation was undertaken.