基于数字全息和数字图像相关方法的面内位移测量

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

针对一套数字全息装置只能测量离面位移这一个维度位移的局限性问题,将数字全息与数字图像相关方法相结合,利用数字全息重建的强度图像,实现了面内另外两个维度位移的测量,拓宽了一套数字全息装置的测量能力。该方法对待测物体位移前后的全息图数值重建得到两幅强度图,然后对变形前后的强度图应用数字图像相关方法得到面内两个维度位移的测量结果。该方法中采用的数字全息的强度图本质上是激光散斑图,而不是传统数字图像相关使用的白光下或自然光下的表面喷涂散斑或者物体自然纹理,因此是对数字图像相关方法应用于激光散斑图像的有益探索。最后通过二维位移装置对平面施加面内方向位移的实验验证了数字图像相关方法对于数字全息重建的激光散斑图在面内位移测量的有效性。最后,讨论了基于数字全息对曲面物体利用数字图像相关进行面内位移测量的可行性。传统的二维数字图像相关方法由于有限焦深导致散斑退相关现象,因而无法对曲面物体进行面内位移测量。但是,借助数字全息技术在一幅全息图中能将物体聚焦到不同深度的能力,可以将曲面物体分割为不同深度部分开展二维数字图像相关测量。本文从理论上讨论了该方法的可行性,并在实验上遍历了不同深度的DH重建对曲面物体面内位移的测量准确性的影响,为数字图像相关方法借助数字全息技术的辅助对曲面物体面内位移的测量进行了有益探索。

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 of2D-DIC for in-plane displacement measurement of curved objects are explored and demonstrated feasible.