Abstract
Reflection-based topography systems use the cornea as a convex mirror and view the first Purkinje image. Keratometers use two pairs of images in perpendicular meridian at a diameter of about 3 mm and is a highly accurate measure of the central curvature of regular corneas. Photokeratoscopes view the reflected image of the black and white concentric rings of a Placido disc, providing qualitative shape information across a large area of the cornea.
Videokeratoscopes analyse the position of the edges of the Placido rings in meridians around 360°, giving about 10,000 data points. Reconstruction algorithms applied to each point on the two-dimensional mire image generate a three-dimensional interpretation of the corneal shape. The accuracy centrally is approximately 0.15D, which drops more peripherally and in irregular corneas.
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Corbett, M., Maycock, N., Rosen, E., O’Brart, D. (2019). Videokeratoscopes. In: Corneal Topography. Springer, Cham. https://doi.org/10.1007/978-3-030-10696-6_2
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