Method to evaluate the impact of residual roughness after corneal ablation in perception and vision

Purpose: Theoretical models have also been proposed for achieving smoothness with laser systems used for ablation processes (Invest Ophthalmol Vis Sci. 2017;58:2021–2037). However, methods do not exist for evaluating the impact of the residual roughness after corneal ablation, in perception and vision. We propose a computational method to convert wavefront aberrations with a varying degree of roughness, to a description of the human visual optical point spread function (PSF), to calculate the polychromatic retinal image.

Methods: A simulation program was developed in SCILAB (Scilab Enterprises, Versailles, France). Patient specific information like Zernike Coefficients (up to 8th Zernike order) and Pupil diameter were used to calculate the wavefront including chromatic compensation for the Red, Green and  Blue channels (Optom Vis Sci 2003;80:6–14). A random noise was added to the calculated wavefront to simulate the roughness in the cornea within user-defined limits. PSF of the eye was calculated for the Red, Green and Blue channels and the retinal image was determined, as a weighted combination of different color channels. Different corneal roughness conditions (Zero, Typical (X µm) and High Roughness (X µm)) were compared in terms of Michelson Contrast (MC) and Modulation Transfer Function (MTF).

Results: The simulated roughness in ablation reduced the perceived retinal image quality drastically (Zero Roughness MC = X, MTF = X; Typical Roughness MC = X, MTF = X; and High Roughness MC = X, MTF = X) 

Conclusion: The proposed model can be used for quantifying the impact of residual roughness in ablation processes at relatively low cost. This method can help compare different refractive laser platforms in terms of their associated roughness in ablation, indirectly improving the quality of results after Laser vision correction.