Phase measuring deflectometry (PMD) is widely employed in automotive intelligent manufacturing for high-precision, large-area inspection of specular and free-form surfaces. However, the conventional three-frequency four-step phase-shifting method requires multiple projected patterns, which reduces measurement efficiency and exhibits poor robustness on highly curved surfaces. To overcome these limitations, this study proposed a novel encoding and decoding strategy that utilized only four phase-shifted fringes and one additional unequal phase-shifted fringe for phase retrieval and unwrapping. Moreover, an amplitude symmetry-based segmentation algorithm was introduced for background segmentation, and a phase self-correction approach was developed to mitigate jump errors in the fringe order map caused by surface imperfections. Experimental results demonstrated that the proposed method significantly improved the measurement efficiency compared with the conventional approach while improving measurement accuracy.
Wang et al. (2026) studied this question.