Proceedings of Topical Meeting on Optoinformatics (pdf-format, 1.21 ...

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14 OPTOINFORMATICS’05 DETERMINATION OF SPECKLE DISPLACEMENT BY HYBRID OPTICAL-DIGITAL SPECKLE CORRELATOR Sakharuk O. M., Fityo N. V., Muravsky L. I., Yezhov P. V.* Karpenko Physico-Mechanical Institute <strong>of</strong> NAS <strong>of</strong> Ukraine, Lviv, Ukraine *Institute <strong>of</strong> Physics <strong>of</strong> NAS <strong>of</strong> Ukraine, Kiev, Ukraine The comparative analysis <strong>of</strong> the hybrid optical-digital speckle correlation technique and digital speckle correlation technique is carried out. Description <strong>of</strong> the hybrid optical-digital speckle correlator (ODSC) with first digital stage and second optical stage is represented. The systematic and random errors <strong>of</strong> speckle pattern’s displacements obtained by ODSC are analyzed. Recently, methods <strong>of</strong> digital speckle correlation have been used for variety <strong>of</strong> nondestructive testing problem solutions [1] . However, the traditional digital speckle correlation (DSC) techniques doesn’t cover advantages <strong>of</strong> nonlinear transformation and spatial filtration <strong>of</strong> image spectrum for improvement <strong>of</strong> correlator performance, namely decrease noise and narrow correlation peak, which lead to exact definition <strong>of</strong> peak position and thus displacement <strong>of</strong> an image. Only Chen et al. [2] have used the Kumar-Hasselbrook filter [3] to improve DSC performance. Later, Muravsky et al. [4] have proposed optical speckledisplacement correlation technique based on joint transform correlator architecture for study <strong>of</strong> in-plane speckle displacements, where nonlinear transformation (median and subset median binarization) and filtration (fringe adjusted filter) <strong>of</strong> a joint power spectrum (JPS) <strong>of</strong> two input images were used. We have created hybrid optical-digital speckle correlator with digital first stage and optical second stage that possess advantages <strong>of</strong> nonlinear spatial transformation and filtration. This correlator is combined with a tensile-testing machine, which apply the external loading to studied specimen <strong>of</strong> structural material. A strainless surface S 1 and a strained surface S 2 <strong>of</strong> the specimen are illuminated by a light source. The CMOS-camera captures the speckle patterns <strong>of</strong> these surfaces and enters these patterns into the PC (ODSC first stage). To study systematic and random errors, we have used computer-generated speckle patterns r and g. The PC divide speckle patterns r and g into the equal quantity <strong>of</strong> identical subimages r mn , and g m , n and produces the joint spectrum (JS) <strong>of</strong> each corresponding pair <strong>of</strong> these subimages R * m,n S m,n . Then PC performs JS interpolation and nonlinear transformation to raise accuracy definition <strong>of</strong> correlation peak position at output <strong>of</strong> ODSC. Further, the transformed JS is inserted into an electrically addressed spatial light modulator (EASLM), which can be treated as the second stage's input. The second stage is an optical Fourier processor OFP containing a laser diode and Fourier lens. The transformed JS is recorded on the EASLM and is read by a laser beam. A correlation response is produced on the ODSC output. The correlation peak is detected by a sensor array <strong>of</strong> a camera and its coordinates are defined by using PC. Usage <strong>of</strong> time-consuming algorithm <strong>of</strong> subpixel resolution for peak determination can be omitted in given system by recording <strong>of</strong> correlation peak with the whole array <strong>of</strong> camera.
SAINT-PETERSBURG, October 17 – 20, 2005 15 So, the elaborated ODSC allows accelerating the time-consuming processes <strong>of</strong> subimage cross-correlation and high-precision determination <strong>of</strong> correlation peak location. The performance <strong>of</strong> this system was studied by analysing the computer generated speckle patterns introduced at the correlator’s input. The systematic and random errors <strong>of</strong> speckle pattern’s displacements obtained by ODSC were analyzed. Presentation <strong>of</strong> this paper was partially supported by ICO travel-grant program. 1. Digital speckle pattern interferometry and related techniques / Ed. by P.K.Rastogi. – Chichester: John Wiley and Sons, 2001. 2. Chen D.J., Chiang F.P., Tan F.P., Don H.S. Digital speckle-displacement measurement using a complex spectrum method // Appl. Opt., 32, 1839-1949, (1993). 3. B. V. K. Vijaya Kumar, L. Hassebrook Performance measures for correlation filters // Appl. Opt., 29, 2997-3006, (1990). 4. L.I. Muravsky, O.P. Maksymenko, O.M. Sakharuk, “Use <strong>of</strong> a joint transform correlator architecture for study <strong>of</strong> speckle displacements,” Optics Communication, 240, №4-6, 275-291, (2004).
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