An alternative approach to fully automatic speckle-displacement measurement is described. Two speckle patterns of a specimen, one before and one after deformation, are captured by a CCD camera and registered by a frame grabber. Two series of small subimages are obtained by segmenting the two speckle patterns. The corresponding subimage pairs extracted from both series are analyzed pointwise. The interrogation of each subimage pair involves a two-step fast-Fourier transform. While the first-step fast-Fourier transform achieves a complex spectrum characterized by the local displacement information, the second-step one generates a signal peak in the second spectral domain that resolves the local displacement vector. A rough estimate of the displacement vector is achieved by detecting the maximum pixel of the discrete spectrum. A more accurate determination is attained by a subpixel-maximum determination through a biparabolic fitting near the signal peak. The u- and v-displacement fields are deduced by analyzing all subimage pairs. A large rigid-body displacement can be overcome by introducing an artificial rigid shift of the two speckle patterns toward each other before the numerical process. The technique retains all the advantages of optical speckle photography and provides an extended range of measurement. Dynamic incremental deformations can be inspected by registering more speckle patterns at many consecutive deformation stages by using a high-speed CCD camera. The system was applied successfully to the study of crack-tip deformation fields.

Digital speckle-displacement measurement using a complex spectrum method

Quantitative data can be extracted from holographic interference fringes using Phase-Measurement Interferometry (PMI) techniques. These techniques are used to determine the phase of the secondary interference fringe pattern and can be divided broadly into spatial and temporal techniques. Temporal techniques introduce a known phase shift between the object and reference beams in an interferometer and take a series of data over time as the phase shift is varied. Spatial techniques rely on encoding the phase shift information spatially in a single interferogram by using a large number of fringes as a carrier for the phase information which are generated by tilting the reference wavefront relative to the test wave front. Temporal techniques which process data in electronics are known as heterodyne techniques and were discussed in the last chapter. The processing of spatial phase-measurement data will be discussed in the next chapter. Temporal techniques which process the data analytically on a point-by-point basis will be the concentration of this chapter.

Phase-Shifting Holographic Interferometry

A fully automatic speckle metrology technique is developed. Two speckle patterns of a specimen, one before and one after the specimen deformation, are captured by a video camera. An equivalent double-exposure speckle pattern is obtained by superimposing the two digital images. The superimposed speckle pattern is then segmented into a series of small subimages. For each subimage a fast-Fourier transform is applied and a computer-generated Young’s fringe pattern is obtained. The fringe pattern, which characterizes the local displacement vector, is analyzed by a second fast-Fourier transform. The local displacement vector is determined by a cardinal interpolation and a crest searching around a signal peak in the second spectral domain. An artificial rigid shift between the two images is introduced in the cases of extremely large or small displacements. From analysis of all subimage pairs of the whole superimposed speckle pattern a complete two-dimensional displacement field is deduced. Experimental results using laser as well as white-light speckle patterns are demonstrated.

Computer-aided speckle interferometry using spectral amplitude fringes

Recent developments in digital speckle pattern interferometry

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This paper reports the use of a real-time arithmetic image processor in an electro-optic holography system. A speckle interferometer is used to combine an image of an object, lit by laser light, with a mutually coherent reference beam. A CCD TV camera detects the interference pattern, and the phase of the reference beam is advanced by 90° between frames. An image is generated from each set of four sequential TV frames by subtracting alternate frames, squaring, and adding the two results. The result is improved picture quality compared with the use of binary pixels and compared with electronic speckle pattern interferometry. Experimental results are shown.

Jawed Wahid

Papers

An electro-optic holography system with real-time arithmetic processing