Research on the influence of measurement accuracy of the CCD moiré method

Abstract

The CCD moiré method is an emerging tool for full-field measurements due to its simple optical setup and high resolution in topography and deformation measurements. It takes the periodic pixel on the image sensor of the digital camera as the reference grating, effectively “magnifies” the specimen grating pasted on the specimen surface. Different from ordinary moiré, CCD moiré has an image sensor that serves as both a reference grating and a recording pixel, its interval of parameters for measurement is narrow, and moiré quality is affected by multiple factors. This paper presents a systematic study of the factors affecting the accuracy of CCD moiré measurement. Firstly, a theoretical deduction of the formation process and measurement principle of CCD moiré is conducted, and then four classical single-moiré image-based phase extraction algorithms are summarized. Secondly, the imaging model of CCD moiré is established and all the factors that affect the accuracy of CCD moiré are studied, including lens aperture-coefficient FN, the pith of grating (ps, pr), pixel's duty-ratio dr, and camera acquisition mode. The results have identified the following optimal parameter interval: (1) determining the optimal pitch-ratio (n, or, ps / pr) is crucial for achieving high image quality and measurement accuracy, and the recommended interval isn∈[1.1,1.25]; (2) the pixel's duty-ratio is preferred to be set atdr∈[0.5,0.7], as it enables a favorable tradeoff between the image signal-to-noise ratio(SNR) and image contrast; (3) the recommended aperture-coefficient is FN∈[4,8]; (4) the Skip and Normal camera acquisition mode is preferred based on actual measurement situations. Finally, the measurement accuracy and advantages of the CCD moiré method are effectively verified through two experiments: one on four-point bending in comparison with the DIC method, and the other on the ship vertical arch displacement measurement under a large field of view (FOV).