Optical surface measurement based on reverse Hartmann test using coaxial illumination

Interferometry with high measurement accuracy is a universal method for optical mirror surface measurement, but its dynamic range is small. When local slope gradients on the test surface excess a critical threshold, interferometers capture excessively dense fringe patterns that preclude reliable phase unwrapping. In contrast, reverse Hartmann test, as a geometric measurement approach, delivers comparable theoretical accuracy to interferometry while exhibiting greater dynamic range. We propose integrating the reverse Hartmann illumination module into the Fizeau interferometer optical path. This system compensates for interferometry's measurement gaps in high-gradient surface measurement without employing compensating optics. Leveraging the null-test configuration inherent to Fizeau interferometers, a beam splitter establishes a coaxial configuration among the display, detector, and test mirror, facilitating precise geometric parameter calibration. Based on the theoretical model, an actual measurement system was built. The large dynamic range and high precision of the proposed coaxial surface measurement system are verified by quantitative comparative experiments with the interferometry. By axially displacing the test mirror, controlled surface errors were introduced. The experimental results show that proposed coaxial surface measurement system's dynamic range far exceeds interferometry, with 9.31μm root mean square (RMS) accuracy. The integration of proposed coaxial measurement path with interferometric path promises a compact, large dynamic range, and high accuracy surface measurement system, advancing cost-effective precision optics manufacturing.