To assess the curvature of a fast free-form convex surface, such as the cornea of the human eye, a smartphone corneal topographer based on the null-screen method has been developed. The null screen consists of a semiradial distribution of ellipse-shape dots arranged on a conical surface so that, when reflected from a test surface, it forms an ordered set of circular dots if the surface is perfect. Any deviation from this geometry is indicative of surface defects. To demonstrate that the corneal topographer provides reliable results on the topography of the human cornea, in this work, we set out to perform a calibration of the corneal topographer using a reference sphere with geometric parameters 7.8 mm radius and 12 mm diameter. We use a normal iterative method to reconstruct the surface shape and obtain the corneal topography.
In this project we propose a new corneal topographer that improves the accuracy of measurement from previous versions, by gathering all those details that made them to have great accuracy. The corneal topographer uses the null-screen method which is based on the idea that a reflected image in a surface contains information about the form of the reflective surface. The topographer uses a long cone, and it is aimed to evaluate corneas removing the symmetry of revolution of the algorithms. At first, the corneal topographer is utilized in a spheric reference surface, where the results obtained show that the radio of the reconstructed surface varies 0.11 % from the radio of the tested surface. This project is still in process since the evaluation algorithms must be adapted to evaluate corneas without symmetry of revolution. Keywords: null screen, corneal topography, topographic maps.
We propose the design of a conical null-screen with quasi-elliptical targets drawn on it so that its image, which is formed by reflection on a biconical convex surface, becomes a precise radial set of circular targets if the surface under test is perfect. This null-screen avoids alignment difficulties of the test system due to the contour of the face (eyebrows, nose or eyelids). In addition, the proposed method prevents the targets from overlapping and touching each other. We discuss how to integrate the system to calibrate it by testing a spherical and a biconical surface on which we obtain geometrical parameters such as radius of curvature and conic constant, as well as elevation, and sagittal and meridional curvature maps with a smartphone-based corneal topographer.
In this work we report the design of a conical corneal null-screen compact topographer, which uses a mobile device to capture null-screen reflection produced by the posterior corneal surface. The instrument features a head holder like those of virtual reality headset with the aim of align the topographer. For corneal topography the device is calibrated by testing a reference surface where the geometrical parameters such as the radius of curvature and the conic constant, are obtained. We present examples of surface topography measurements on some human corneas.
In this work, the topography of human corneas is evaluated with a conical corneal topographer based on the null-screen method. Geometrical parameters such as the radius of curvature and the conic constant, are obtained. Additionally, elevation, sagittal curvatures and meridional curvature maps can be calculated with the proposed method. Here, it is assumed that the shape of the cornea surface is an aspherical surface. To validate our proposal, we compare the results with those obtained by a commercial corneal topographer.
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