@conference {407, title = {Method to evaluate the impact of residual roughness after corneal ablation in perception and vision}, booktitle = {European Association for Vision and Eye Research}, year = {2017}, month = {09/2017}, abstract = {

Purpose: Theoretical models have also been proposed for achieving smoothness with laser systems used for ablation processes (Invest Ophthalmol Vis Sci. 2017;58:2021{\textendash}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{\textendash}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 {\textmu}m) and High Roughness (X {\textmu}m)) 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.\ 

}, keywords = {Verma2017_2}, author = {Verma, S and Hesser, J. and Arba-Mosquera, Samuel} } @article {Verma2017, title = {Optimum Laser Beam Characteristics for Achieving Smoother Ablations in Laser Vision Correction}, journal = {Investigative Opthalmology {\&} Visual Science}, volume = {58}, year = {2017}, month = {apr}, pages = {2021}, abstract = {

Purpose Controversial opinions exist regarding optimum laser beam characteristics for achieving smoother ablations in laser-based vision correction. The purpose of the study was to outline a rigorous simulation model for simulating shot-by-shot ablation process. The impact of laser beam characteristics like super Gaussian order, truncation radius, spot geometry, spot overlap, and lattice geometry were tested on ablation smoothness. Methods Given the super Gaussian order, the theoretical beam profile was determined following Lambert-Beer model. The intensity beam profile originating from an excimer laser was measured with a beam profiler camera. For both, the measured and theoretical beam profiles, two spot geometries (round and square spots) were considered, and two types of lattices (reticular and triangular) were simulated with varying spot overlaps and ablated material (cornea or polymethylmethacrylate [PMMA]). The roughness in ablation was determined by the root-mean-square per square root of layer depth. Results Truncating the beam profile increases the roughness in ablation, Gaussian profiles theoretically result in smoother ablations, round spot geometries produce lower roughness in ablation compared to square geometry, triangular lattices theoretically produce lower roughness in ablation compared to the reticular lattice, theoretically modeled beam profiles show lower roughness in ablation compared to the measured beam profile, and the simulated roughness in ablation on PMMA tends to be lower than on human cornea. For given input parameters, proper optimum parameters for minimizing the roughness have been found. Conclusions Theoretically, the proposed model can be used for achieving smoothness with laser systems used for ablation processes at relatively low cost. This model may improve the quality of results and could be directly applied for improving postoperative surface quality.

}, issn = {1552-5783}, doi = {10.1167/iovs.16-21025}, url = {http://www.ncbi.nlm.nih.gov/pubmed/28384723 http://iovs.arvojournals.org/article.aspx?doi=10.1167/iovs.16-21025}, author = {Verma, Shwetabh and Hesser, Juergen and Arba-Mosquera, Samuel} } @article {Arba-Mosquera2016, title = {Minimising residual aberration in the presence of cyclotorsion uncertainty in refractive surgery}, journal = {J Emmetropia}, volume = {3}, year = {2016}, month = {01/2016}, pages = {139{\textendash}144}, abstract = {

PURPOSE: To formulate a simple valid method to minimize the residual aberration magnitudes due to uncertainty in cyclotorsion measurement. SETTING: Optics research laboratory. METHODS: Assuming that cyclotorsion error can be estimated and compensated for with a certain level of uncertainty, we formulate a function to modify the wavefront error in order to minimize the residual aberration magnitudes due to cyclotorsion measurement uncertainty. A modal optimum nomogram factor is computed for minimization of the residual aberration for each Zernike mode. We also calculate the optimized residual aberration using the minimization factor. RESULTS: For modal optimum, the minimization factor was dependent only on Zernike meridional order and cyclotorsion uncertainty. Th e value of the minimization function ranged between {\^a}{\texteuro}{\textquotedblleft}1 and +1. CONCLUSION: In a perfectly performing system, an " overplanned " treatment will never minimize residual wavefront aberration, independent of the cyclotorsion uncertainty. An " underplanned " treatment calculated using the presented approach will minimize the residual aberration magnitude imposed by the higher order Zernike terms, and will reduce its relative orientation to the original aberration pattern. Th e gains using this method are undeniably modest, but at such low implementation costs can still be valuable.

}, issn = {2171-4703}, url = {http://www.journalofemmetropia.org/numeros/pdf/7-3/Journal-article-3.pdf}, author = {Arba-Mosquera, Samuel and De Ortueta, Diego and Verma, Shwetabh} }