Proceedings Article | 11 September 2012
KEYWORDS: Speckle, Temperature metrology, Calibration, Photometry, Nondestructive evaluation, CCD cameras, Correlation function, Diffusion, Laser development, Speckle pattern
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.
