Optical microscopy image restoration and its application in strain measurement by digital image correlation method

As a direct and effective non-contact, full-field optical measurement method, digital image correlation (DIC) method has been widely used in 2D/3D displacement and strain measurement of materials and structures in various fields. Combined with advanced micro-scale speckle preparation technology, in-situ loading experiments can be carried out under the microscope, which allows micro-scale displacement and strain measurement by DIC. However, out-of-plane displacement is inevitable during the loading process. Due to the limited depth of field of the optical microscope, even a slight out-of-plane displacement can result in significant blurring of the speckle pattern. This will bring negative effects to the deformation measurement. In order to reduce the error caused by the image blurring, the blind deconvolution method was used in this paper to restore the blurred speckle image. Furthermore, Gaussian filtering was applied to solve the noise problem. The effect of the image restoration on the measurement accuracy of the DIC was analyzed quantitatively. A tensile experiment was carried out on the indium tin oxide (ITO) films. The experimental results showed that the error of the measured elastic modulus was reduced by 13.91%. The results indicate the measurement accuracy was improved effectively after image restoration.