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  • Title: An improved low order method for corneal reconstruction.
    Author: Turuwhenua J.
    Journal: Optom Vis Sci; 2008 Mar; 85(3):211-7. PubMed ID: 18317329.
    Abstract:
    PURPOSE: An iterative and cubic arc-step method for corneal reconstruction developed previously, was retested including improved skew-ray compensation. The method was compared with a similar method described by Klein, and also with a least squares based approach using Zernike polynomials. METHODS: Images of an asphere, a cornea after photorefractive keratectomy (PRK), an ellipsoid and radially keratotomized (RK) cornea were generated. Three reconstruction methods were applied: (i) the cubic method (without skew-ray compensation; with old and improved compensation), (ii) the method due to Klein and (iii) a least squares approach based on Zernike polynomials (order taken up to 25). Errors were recorded for all conditions tested. RESULTS: The root mean square errors for the improved method were well below micron level, and consistently lower than the method of Klein. The Zernike-based method produced lowest errors for aspheric and ellipsoidal surfaces, when order was > or =(10, 8) respectively. However, the improved method produced the lowest errors for the PRK and RK examples. In this case, the Zernike-based method produced submicron errors for orders > or =14 (both surfaces), but errors comparable with the arc-step methods could not be achieved for polynomial orders < or =25. The improved method completed in three to four iterations (abs. height error <1 x 10(-7) mm) in all cases. CONCLUSIONS: Compensation for skew-rays was incorporated in a straightforward way, yielding an efficient and effective low-order method. The Zernike method produced the lowest root mean square errors for asphere and ellipsoid, provided order was > or =(10, 8). However, the arc-step methods (the cubic and Klein methods) were more accurate for RK and PRK surfaces, at least for Zernike orders < or =25. The results suggest that low order methods provide a good solution to the reconstruction of corneal shape, at least for Placido disk videokeratography applications.
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