A novel in situ calibration of object distance of an imaging lens based on optical refraction and two-dimensional DIC

Abstract

The strain results of two-dimensional digital image correlation (2D-DIC) are known to be markedly affected by out-of-plane motion in practical measurements. To correct strain results, it is necessary to measure the object distance of the camera imaging lens. Due to the difficulty of conventional calibration using a translation stage, a novel in situ calibration based on optical refraction is proposed herein to determine the object distance. In our calibration, recorded images show a radial outward displacement field when a glass plate is placed in front of a planar specimen. This study presents a formula derivation of the displacement and strain distributions theoretical model. Based on the strain model and the calculated strain fields obtained using a 2D-DIC algorithm, the object distance can be obtained with a high measurement accuracy. Experiments were conducted to verify the validity and reliability of the proposed calibration. Comparison results between a conventional translation calibration and the proposed calibration indicate that the proposed calibration can reach a measurement accuracy of 0.19% (1.2 mm in this paper). This experiment demonstrates that the proposed calibration is an easily implemented method for the in situ computation of the object distance of an imaging lens.