In-plane Displacement Measurement Based on Digital Holography and Digital Image Correlation

We propose an in-plane displacement measurement method based on combination of Digital Holography (DH) and Digital Image Correlation (DIC). It utilizes intensity images reconstructed by DH, and extends the capability of a single DH setup to in-plane displacement measurement. The methdology of the proposed method is illustrated. And the holograms before and after the in-plane displacement of the object are recorded by CCD. Then the intensity images are numerically reconstructed from holograms. And finally the DIC algorithm is applied to the intensity map to obtain the in-plane displacements in u and v components, respectively. The method we proposed solves the problem that one single DH setup can only measure out-of-plane displacement, but not in-plane displacement. Furthermore, the intensity images of DH used in this method is essentially laser speckle images, rather than the surface spray speckle or natural texture of the object under white or natural light used in traditional DIC. Therefore it is a meaningful exploration of DIC method for the type of laser speckle images. Real experiments of in-plane displacement measurements of diffuse planar objects are performed. The results presents an average relative measurement error of 1.17%, verifying the validity and high accuracy of the proposed method. Finally, this paper discusses the feasibility of proposed method for in-plane displacement measurements of diffuse curved objects. Conventionally, 2D-DIC is unable to perform in-plane displacement measurements of curved objects due to the speckle decorrelation phenomenon caused by the limited depth of focus. However, with the ability of the DH technique to focus objects to different depths in a single hologram, it is possible to segment curved objects into different depth parts with each depth part focused for DIC measurements. This paper theoretically discusses such feasibility and experimentally traverses the DH reconstruction distance to evaluate the accuracy of the proposed method for in-plane displacement measurements of curved objects. This is also a meaningful exploration of DIC measurements of in-plane displacement of curved objects with the assist of DH. The advantages of this technique over the existing methods are as follows: 1) The setup is simple and has high information utilization, making full use of the intensity map information that has been discarded in previous holography method, enabling in-plane displacement measurements with only one DH setup; 2) Extend DIC to the type of laser speckle images and prove its effectiveness; 3) With the help of DH capablity in mutlti-depth focusing, the possibility of 2D-DIC for in-plane displacement measurement of curved objects are explored and demonstrated feasible.