In Situ Coherence Scanning Interferometry Measurement System for Large-Scale Rotationally Symmetric Aspheric Surfaces

Large-scale aspheric surface (LSAS) plays a vital role in a variety of fields due to its excellent optical performance. In situ noncontact measurements for LSASs could realize efficient and nondestructive inspection for further improving the machining results. Among the typical optical measurement methods, coherence scanning interferometry (CSI) is the most promising one due to its superior noise management capability. However, the existing in situ CSI measurement system could only measure discrete sampling areas manually, which is inefficient and unable to characterize the profile of the whole surface. In this article, an in situ CSI measurement system is proposed to evaluate trans-scale features by establishing the kinematic model of the system. A convenient calibration method using a sphere artifact is proposed to identify the intrinsic structural parameters in the model. The workpiece localization and point cloud registration algorithms are derived according to the kinematic model so that the LSAS can be measured automatically and the measured points can be unified in a coordinate system to evaluate the trans-scale features. Validation experiment on a spherical surface demonstrates that the proposed method achieved an accuracy of $0.12~\mu $ m. In addition, validation on an LSAS confirms its capability to automatically and accurately perform null interferometric measurements at different positions on a curved surface. The effectiveness of the proposed method is further verified by comparing these results with nonnull interferometric measurements at the same location.