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Laser speckle photometry - Contactless nondestructive testing technique

: Cikalova, Ulana; Nicolai, Jürgen; Bendjus, Beatrice; Schreiber, Jürgen


Doval, A.F. ; University of Vigo, Optical Metrology Research Group; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Speckle 2012, V International Conference on Speckle Metrology. Proceedings : 10 - 12 September 2012, Vigo, Spain
Bellingham, WA: SPIE, 2012 (SPIE Proceedings 8413)
ISBN: 978-0-8194-9090-2
ISSN: 0038-7355
Paper 84130X
International Conference on Speckle Metrology (Speckle) <5, 2012, Vigo>
Conference Paper
Fraunhofer IZFP, Institutsteil Dresden ( IKTS-MD) ()
Fraunhofer IKTS ()
lasers; nondestructive evaluation; photometry; speckle; Speckle pattern; CCD cameras; calibration; correlation function; diffusion

Laser Speckle Photometry (LSP) is a newly developed contactless, fast and completely optical nondestructive testing method based on the detection and analysis of thermally or mechanically activated characteristic speckle dynamics. The heat propagation or tension process causes locally different degrees of thermal/ mechanical expansion, which generates local and time-dependent strain fields, resulting in a local displacement of material surface. During this process, the normal surface slope and absolute height of the microscopic and mesoscopic surface segments, especially at rough sample surfaces, is transformed. These spatiotemporal changes include information about the material structure and conditions. Therefore, the proposed measurement technique includes a pulsed heating source for sample activation, a temperature detection of the sample at the measurement location in a distance from the heading point, a continuous wave laser for sample irradiation and activation of speckle patterns at the measurement point, and in addition, a fast CCD camera for the detection of the speckle movement during heat propagation at the measurement location. Laser Speckle Photometry can be used for evaluating material properties, such as hardness and porosity. The approach is based on the estimation of the "Speckle Thermal Diffusivity" parameter K, that can be determined using the thermal diffusion equation and the modified correlation function from the pixel intensity of the speckle image variations during thermal activation. After testing, the correlation between parameter K and hardness, and porosity respectively, was found. Furthermore, mechanical material stress changes, also at elevated operating temperatures, can be estimated by the presented technique using the calculated parameter K. In this case, the thermal excitation will be partially replaced by mechanical activation, such as the tension process. The technique of LSP and the results of calibration experiments are presented in this paper.