The oblique imaging of the CCD Moiré method and its application to displacement measurement in large-scale structures

• Published in: Optics and laser technology 2025-02, Vol.181, p.112031, Article 112031
• Main Authors: Shangguan, Zhihao, Li, Boda, Zhuang, Shuge, Sun, Jiaming, Zhou, Shichao, Ma, Shaopeng
• Format: Article
• Language: English
• Subjects: CCD Moiré; Measurement error compensation; Oblique imaging; River-crossing bridge; Variable-period grid
• ISSN: 0030-3992
• DOI: 10.1016/j.optlastec.2024.112031

•The oblique imaging law of CCD moiré is summarized systematically.•The measurement error and limit oblique angle of oblique CCD moiré are analyzed.•A novel oblique imaging CCD moiré measurement scheme is proposed.•The displacement of a 30-m-long river-across bridge is measured by the scheme. Using the periodic pixels on a Charge-coupled Device (CCD) or Complementary Metal-Oxide-Semiconductor (CMOS) imaging sensor of digital camera as the reference grating, the fringes or grid can be directly visualized as Moiré by the camera, which is uniformly called CCD Moiré. As a vision-based non-contact full-field displacement measurement technology, CCD Moiré demonstrates considerable promise for high-resolution measurement across large field-of-area (FOA) as it employs super-resolution imaging of fringe/grid grating and breaks the constraint between FOA and resolution in the visual measurement techniques. The previous CCD Moiré method necessitates that the camera’s optical axis be aligned perpendicularly to the surface of the specimen for measurement. However, the engineering application often encounters non-perpendicular oblique measurement scenarios, which not only affect the generation of CCD Moiré images but also introduce measurement errors. This study explores the imaging law and error mechanism of the oblique CCD Moiré method and proposes an experimentally examined compensation scheme for oblique measurement. Specifically, the oblique imaging law of CCD Moiré is investigated before analyzing the measurement error and the limited oblique angle of the non-uniform CCD Moiré caused by the oblique. Then, the oblique imaging CCD Moiré measurement model based on the variable-period grid is proposed to realize high-precision measurement at various oblique angles, and the compensation scheme is verified by simulation and four-point bending experiments. Lastly, a large FOA oblique measurement of multi-point deformation based on the proposed model is conducted on a large-scale river-crossing bridge, 30 m in length, during the traversal of two types of vehicles, where the bridge displacement recorded demonstrates closely aligned results with the laser Doppler Vibrometer sensor. As a low-cost and high-resolution measurement technology, the research presented herein has the potential to significantly broaden the scope of CCD Moiré method in practical engineering applications.