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V. D. Sicam, R. G. L. Van der Heijde, J. J. Snellenburg, I. H. M. Van Stokkum; New on Screen Diagnostic Method in Cornea Topography. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3529.
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A ray-tracing algorithm is developed to evaluate surface reconstruction in cornea topography. The method traces feature points on the cornea topographer stimulator to the reconstructed surface onto the reflected image captured on the CCD camera resulting into a display of residues. This enables onscreen evaluation on the accuracy of the corneal surface reconstruction.
The method was applied using the VU topographer. This topographer uses color coded pattern as a source for reflection on the cornea. Application of the method to this instrument is facilitated by easy registration of the one-to-one correspondence between feature points on the stimulator and the appropriate feature points on the image captured by the CCD camera. Performance of the algorithm was evaluated using spherical test surfaces, a toric test surface, a typical cornea and a cornea with deformation.
The ray-tracing method was able to demonstrate that the CCD residue increase with complexity of the measured surface. using Zernike radial order 6 for the corneal surface reconstruction, Mean CCD residue in pixels (1 pixel = 0.01 mm spatial resolution at the corneal apex plane) was smallest for the artificial surfaces (< 0.90) and greatest for the irregular cornea (~3). Increasing the Zernike radial order improves the accuracy of the corneal surface reconstruction. In the case of the irregular cornea the accuracy improves by more than a factor of two from order 6 to order 20 Zernike radial order. Using the CCD residue to provide weight information in an additional least squares fitting procedure further improved the pixel accuracy of the surface reconstruction with corresponding error correction in the local curvature greater than 0.25 Diopter.
This new ray-tracing method can be used to evaluate accuracy of surface reconstruction in cornea topography. The residue information obtained from this algorithm is useful in further improving the accuracy. The method can provide accurate corneal shape diagnostics useful for laser refractive surgery applications especially because the procedure provides residue information that can be displayed simultaneously on screen with the captured image of the cornea.
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