title: Surface shape evaluation with a corneal topographer based on a conical null-screen with a novel radial point distribution.
abstract: In order to measure the shape of fast convex aspherics, such as the corneal surface of the human eye, we propose the design of a conical null-screen with a radial point distribution (spots similar to ellipses) drawn on it in such a way that its image, which is formed by reflection on the test surface, becomes an exact array of circular spots if the surface is perfect. Any departure from this geometry is indicative of defects on the evaluated surface. We present the target array design and the surface evaluation algorithm. The precision of the test is increased by performing an iterative process to calculate the surface normals, reducing the numerical errors during the integration. We show the applicability of the null-screen based topographer by testing a spherical calibration surface of 7.8 mm radius of curvature and 11 mm in diameter. Here we obtain an rms difference in sagitta between the evaluated surface and the best-fitting sphere less than 1 μm.
authors.forename: Manuel
authors.initials: M
authors.lastname: Campos-García
authors.forename: Cesar
authors.initials: C
authors.lastname: Cossio-Guerrero
authors.forename: Víctor Iván
authors.initials: VI
authors.lastname: Moreno-Oliva
authors.forename: Oliver
authors.initials: O
authors.lastname: Huerta-Carranza
mesh_terms: D000465:Algorithms; D002138:Calibration; D003315:Cornea / Q000473:pathology*; D019781:Corneal Topography / Q000379:methods*; D004867:Equipment Design; D005123:Eye / Q000473:pathology*; D006801:Humans; D007909:Lenses; D016014:Linear Models; D008962:Models, Theoretical; D015203:Reproducibility of Results; D013499:Surface Properties