A dual speckle method for eliminating air distortion in high temperature optical measurement experiments

High-temperature deformation measurements based on optical measurement techniques are important for studying material properties. Due to the existence of temperature gradients between the high-temperature subject and the imaging system, the uneven air density distribution leads to random changes in the refractive index of air along the beam transmission path, which triggers random jitter in the images captured by the imaging system, resulting in large errors in the measured displacement field. In this paper, a dual speckle method is developed for eliminating air distortion in high-temperature experiments. The main experimental setup consists of two cameras, where one camera is focused on the speckles sprayed on the specimen surface that can measure the coupled displacement field, and the other camera is focused on the semipermeable speckles, which can measure the pure displacement caused by the change in the refraction index of air. Then, the true displacement of the tested specimen can be obtained by an arithmetic operation after the calculating point meets the one-to-one relationship. A simplified yet effective test was conducted and verified the feasibility of the method proposed, as the error analysis was analyzed in detail.