Optimal Aperture and Digital Speckle Optimization in Digital Image Correlation

Digital image correlation (DIC) method is a non-interference and non-contact full-field optical measurement technology. DIC’s measurement accuracy is largely determined by the image acquisition system and the quality of the speckle pattern. Although the optimal speckle pattern has been designed theoretically, the optimization process does not consider the influence of the imaging system. Furthermore, the optimal aperture has not been yield. The objective of this paper is to improve the measurement accuracy through optimization of aperture and speckle pattern. In this paper, speckle images with different apertures and different speckle generation parameters were captured, and the corresponding measurement errors were evaluated. (1) The influence of aperture and the optimization of speckle are independent of each other. The optimal speckle generation parameters (speckle size and density) are determined experimentally, which turn out to be consistent with existing theoretical models. (2) The optimal aperture is 5.6, because excessively large F-number causes the loss of high-frequency information and excessively small F-number leads to significant lens aberrations, both of which will reduce the measurement accuracy. The optimal aperture and speckle generation parameters (speckle diameter and density) are found and proved to be uncorrelated through actual experiments in practical conditions. These results provide experimental basis for the selection of parameters in actual engineering applications.