This work proposes the design of an off-axis null-screen that is displayed on an LCD screen and allows characterizing the surface quality of a sliding seismic isolator. For this purpose, we design a null-screen with a pattern of quasi-elliptical dots drawn on it, so that its image, which is formed by reflection on the concave surface, becomes a precise set of circular targets if the surface under test coincides with the designed surface. It is discussed how to integrate the off-axis measurement system to calibrate it by testing a reflecting concave surface, as a result optical parameters are obtained, at the surface vertex, such as radius of curvature, conic constant, and some deformation coefficients necessary for the characterization of the deformations associated to the test surface with respect to the design surface. Finally, based on the preliminary results, the advantages and disadvantages of the proposed test are discussed.
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