Precision Measurement for Aspherical Mirrors Based on Multiwavelength Point Diffraction Interferometry

Precision measurement of aspherical mirrors is critical for high-end optical systems, where surface accuracy directly impacts performance. However, there remain challenges in this task: 1) measurement precision in a large scope; 2) larger ranges and flexibility in complex surface test; 3) environmental robustness in applications; and 4) efficiency and accuracy in data processing. This study introduces a novel multiwavelength point diffraction interferometry (MPDI) method, significantly enhancing measurement flexibility and accuracy. Through the diffraction, a high-accuracy wavefront ( $\lambda $ / $10^{-4}$ ) is produced to perform relative measurement to aspherical mirrors. By integrating a triple-wavelength laser module and utilizing the beat frequency concept, the system generates at least nine equivalent wavelengths (245.8–3334.2 nm), enabling efficient testing of mirrors with both large surface deviations and high-resolution requirements. A key innovation is the random two-step phase retrieval method, which improves measurement efficiency by nearly 60% compared to conventional four-step methods, while minimizing the effects of environmental disturbances. The proposed system, equipped with machine vision alignment, ensures precise optical path alignment and enhances wavefront quality. Experimental results show a 5.3-fold increase in measurement range and rms errors as low as $0.6\times 10^{-3}\lambda $ ( $\lambda =633$ nm), with stitching-free testing. This MPDI method offers substantial improvements in measurement range, robustness, and efficiency, making it a valuable tool for precision aspherical mirror metrology.