Phase error analysis and reduction in near optical coaxial phase measuring deflectometry

Phase-measuring deflectometry (PMD) is a crucial technology for measuring the forms of specular surfaces. However, existing stereo-PMD techniques have noticeable weaknesses when it comes to measuring structured specular surfaces. This limitation arises because the optical axis of the imaging system must intersect significantly with the optical axis of the display system, following the law of reflection. In contrast, near optical coaxial phase measuring deflectometry (NCPMD) offers several advantages over conventional PMD techniques. These advantages include a compact configuration, lightweight design, and minimal measurement errors due to the shadows of surface structures. NCPMD achieves this by utilizing a plate beamsplitter. With the assistance of the plate beamsplitter, the optical axis of the display screen can be configured much closer to the optical axis of the imaging system. As a result, the system becomes more compact and significantly reduces volume compared to the conventional PMD configuration. However, the introduction of the plate beamsplitter can impact on the measurement accuracy of the system. Specifically, the refractive effect of the beamsplitter can reduce the measurement accuracy. To address this challenge, a refraction error model is proposed for the NCPMD system. This model considers the influence of the plate beamsplitter’s refraction, allowing for the determination of measurement errors caused by this effect. Additionally, a virtual simulation system is established to analyze the shape reconstruction error resulting from the plate beamsplitter’s refraction. According to the experiments and results, the measurement accuracy can be effectively improved after the refractive error compensation.