Processing technique for digital speckle photogrammetry

摘要

An improved numerical technique is disclosed for processing digital photo-graphs for measuring surface strain by speckle photogrammetry. The data to be processed is obtained by recording two digital photographs of speckle patterns of the image of the surface of an object where one photograph is recorded before a stress is applied and the other recorded afterward. In the case of a vibratory stress, the recordings are made by illuminating the object stroboscopically first at one extreme of its vibration cycle for the first photograph and then at the other extreme for the second photograph. Each photograph is divided into sectors and digital Fourier transforms are performed on each sector. The phase of these transforms is calculated for each image sector on both photographs, the resulting phase functions from the unstressed recording are subtracted from the corresponding phase functions on the stressed recording, and the results wrapped into a numerical range of −pi to plus pi. The slopes in the x and y directions of the resulting difference functions are calculated for each sector and related by a scale factor to the displacement of the speckles in that sector of the recording. The x and y speckle displacements in the neighboring sectors forming two-by-two arrays are subtracted to obtain relative speckle displacements from which to calculate x strain, y strain, shear, and rotation about the surface normal.