The tilted-wave-interferometer: freeform surface reconstruction in a non-null setup

The measurement of aspheres and freeforms poses several challenges to interferometric and other optical testing methods which are well established for spherical surfaces. Accuracy, measurement time and flexibility are requirements encountered in a production environment. The approach of tilted-wave-interferometry is to illuminate the specimen with a whole ensemble of wavefronts. Each of these wavefronts has a different amount of tilt with respect to the optical axis. The surface under test is completely covered by interferogram patches where rays from one source hit the surface such that resolvable fringe densities result. Yet, one still deviates from the null setup which requires that the interferometer is calibrated precisely. This is obtained by recording interferograms of a known reference object placed in the test space. The aberrations of the interferometer are described within a black box model. A sophisticated set of algorithms is used to reconstruct these coefficients with high accuracy. The non-null configuration prohibits a direct evaluation of the measurement from the interferograms. Instead, the surface is reconstructed by the solution of an inverse problem. A second step gives access to the remaining high-frequency errors. The key ideas and implementations in the process of measurement and calibration are explained and the differences to other common concepts in optical testing are elaborated. As an example, results of a freeform measurement, including both form and high-frequent deviations from design, are discussed.