Characterization of slow concave stainless steel surfaces through off-axis lcd null-screens

The aim of this work is to instrument and characterize an optical test system to measure the quality of a slow stainless steel concave surface using an off-axis lcd null-screen. This configuration is particularly useful because it allows the entire surface to be effectively evaluated, providing a complete view of the shape of the surface under study. The null-screen method is considered an optical deflectometry method, since it provides detailed information about the local slope of the evaluated surface. The quantitative approach of the method consists in designing, by means of accurate ray tracing, a set of geometric objects resembling ellipses, such that when reflected on the concave surface under study, these objects produce an image of an ordered arrangement of points. If the surface is perfect, the mirror image accurately reproduces this dot pattern. However, if deviations in the reflected dot pattern are observed, they indicate imperfections, defocusing or misalignment of the test surface.