Characterization of concave surfaces using dynamic polychromatic off-axis null-screens

In this work, we present a methodology based on dynamic polychromatic off-axis null-screens, extending the principles of deflectometry to achieve accurate slope measurement and surface reconstruction of concave surfaces. The null-screen is designed to generate structures that are dynamically modulated and displayed in an off-axis configuration. The procedure consists of creating a pattern of points similar to ellipses which, when reflected on an ideal surface, produce a virtual image formed by a perfect circule of points distributed in a semi-radial array. Any deviation in the geometry or distribution of the points indicates the presence of defects or irregularities on the surface, making it possible to assess whether the test surface approximates the designed shape. In this work, the proposed configuration allowed accuracy of up to 10 µm rms to be achieved using dynamic polychromatic null-screens, with which two concave mirrors were characterized and their surface deformations determined.