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V. D. Sicam, K. A. Vermeer, J. J. Snellenburg, M. Mensink; Stimulator Pattern Density and Precision in Next Generation Corneal Topography. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5081.
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© ARVO (1962-2015); The Authors (2016-present)
Recent developments in customized laser refractive surgery and customized contact lens applications demand more accurate corneal topography measurements (Curr Opin Ophthalmol 2007; 18:325-333.) At the VU University Amsterdam in the Netherlands a skew ray error free corneal topography method was developed over the past 10 years to address this issue, especially in improving higher order aberration measurements (Optom Vis Sci. 2006; 83(12):910-8.) . This method uses a grid pattern instead of concentric circles. In this study, the minimum number of stimulator points in skew ray error free corneal topography that will produce sufficient precision in reconstructing the anterior corneal surface is determined.
Source and image points are simulated for different surface types encountered in corneal topography (sphere, toric, human eye with no corneal abnormality). A new raytracing method based on forward raytracing has been applied in order to evaluate the precision for the three test surfaces. This is an upgrade of a method previously developed (OptomVis Sci2007; 84(9):915-23) . This is better than previous methods because it uses no approximations in the underlying equations used for derivation. Gaussian noise (20 µm s.d.) was added to the image points to simulate actual experimental noise. This was applied to different number of source points (ranging from 100 to 800) that will produce an equal spread of image points over a 10 mm corneal zone. The precision for central radius of curvature and refractive corneal aberrations were assessed over a 7 mm corneal zone (Zernike convention - rms values up to order 8).
A minimum number of about 700 stimulator points covering a 10 mm corneal zone is enough to achieve a precision of better than 0.125 Equivalent Diopter central radius of curvature. For this configuration the precision in corneal astigmatism, trefoil, coma and quadrafoil aberrations is 0.08, 0.05 , 0.04 and 0.05 Equivalent Diopter respectively.
Although it appears that 700 stimulator points will give good performance in reconstructing the corneal surface this is only applicable to skew ray error free corneal topography.
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