The optical speckle-displacement correlator based on hybrid optical-digital joint transform correlator architecture with
digital first and optical second stage is used to determine correlation peak position with subpixel accuracy without usage
of intricate interpolation algorithms. Experimental setup for realization of the optical speckle-displacement correlation
technique was constructed on basis of a digital Fourier processor allowing joint power spectrum median and ring median
binarization and an optical Fourier processor. Speckle patterns of steel beam specimen (steel 45) with different maximum
spatial frequencies were recorded. As one of the joint transform correlator main parameters is distance between fringes at
correlator frequency plane, comparison of optical speckle-displacement correlator performance for different values of
speckle pattern maximum spatial frequency for the given joint power spectrum modulation was performed. Experimental
results have shown that the signal-to-noise ratio (SNR) increases steadily while the maximum frequency of speckle
pattern multiplied Fourier spectrum is reaching the Nyquist frequency fN. The analysis of the speckle pattern with
frequencies higher than Nyquist frequency has shown that the SNR growth is continued to the some boundary frequency
fB>fN after which the SNR is fallen sharply. Thus, the influence of aliasing on the correlator performance was studied and
the best correspondence between value of speckle pattern maximum spatial frequency and distance between fringes at
the correlator frequency plane was found.
The optical speckle-displacement correlation technique was developed to increase the reliability of surface displacement
field recovery near stress concentrators. The performance of optical speckle correlators based on joint transform correlator
(JTC) architecture and a joint power spectrum nonlinear filtering (median thresholding, adaptive median thresholding,
ring median thresholding) is studied by using computer models of these correlators. The design of hybrid joint transform
speckle correlator is detailed. Example results of correlation signal using computer models of digital speckle correlation
and optical speckle-displacement correlation techniques and created hybrid joint transform speckle correlator setup are
described.
The comparative analysis of the digital speckle-displacement measurement technique and the optical speckle-displacement correlation technique is carried out. Some versions of a joint transform correlator (JTC) architecture, namely a conventional JTC, a JTC with median thresholding, a JTC with subset median thresholding, and a fringe adjusted filter JTC, are used for study of in-plane rigid body displacements. The systematic and random errors of speckle pattern's mean displacement calculated by using the mentioned above techniques are compared. The robustness of the digital speckle-displacement measurement technique and four JTC versions to input noise of speckle patterns and output noise of a correlation response is studied.
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