In this work we report the design of a null-screen for corneal topography. Here we assume that the corneal surface is an ellipsoid with a diameter of 12 mm and a radius of curvature of 7.8 mm. To avoid difficulties in the alignment of the test system due to the face contour (eyebrows, nose, or eyelids), we designed a compact conical null-screen with spots drawn in such a way that its image, which is formed by reflection on the test surface, it becomes an exact semi-radial array of circular spots if the surface is perfect. To validate our proposal, we perform topography measurements of a reference surface and some human corneas using a probabilistic algorithm. The results obtained with our algorithm were consistent and we can recover the shape of the surface with accuracy.
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