A discussion of sources of error in laser-speckle based systems

Bernhard G. Zagar; Johannes Lettner

doi:10.1117/12.912325

11 October 2011 A discussion of sources of error in laser-speckle based systems

Bernhard G. Zagar, Johannes Lettner

Proceedings Volume 8306, Photonics, Devices, and Systems V; 830606 (2011) https://doi.org/10.1117/12.912325
Event: Photonics Prague 2011, 2011, Prague, Czech Republic

Abstract

Applying laser-speckle techniques in the material sciences as well as in methods to characterize surface conditions of specimen has become the method of choice especially if a non-contacting principle is sought. This is almost always the case for specimen that are small in at least one dimension as for example in the material testing of foils, fibres or micromaterials and certainly also if elevated test-temperatures are preventing standard gauges. In this paper some widely overlooked sources of errors that - if unavoidable - increase measurement uncertainty beyond the theoretical limit attainable are discussed and the magnitude of their influence is detailed. In particular the following effects are considered: The laser-source wavelength stability as well as its pointing stability, the effects caused by so-called schlieren occuring along the optical path as well as temperature effects causing changes in the systems geometry, thermally influencing the optical parameters of the imaging optics as well as the often overlooked and in most illumination systems unknown radius of curvature of the laser wavefronts used to illuminate the specimen. Small though as these influences seem, they might contribute significant uncertainties especially in material testing applications where the strain ε = ▵l/l is to be determined out of consecutive measurements of usually small changes in overall length l of the specimens geometry parameter. Typical values of ε are bounded by ±2000 ppm (the typical range of Hooke's law for steel). So values of ▵l on the order of tenth of micrometers for typical gauge lenghts around 50 mm yield ppm resolutions for ε. Analyzing the above mentioned error sources one can quickly see that all of them, if not taken care of appropriately, carry the potential to cause significantly larger deviations than the resolution sought after demands.

Citation Download Citation

Bernhard G. Zagar and Johannes Lettner "A discussion of sources of error in laser-speckle based systems", Proc. SPIE 8306, Photonics, Devices, and Systems V, 830606 (11 October 2011); https://doi.org/10.1117/12.912325

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