A Discussion on Often Overlooked Sources of Error in Laser--Speckle Based Systems

Applying laser–speckle techniques in the material sci- ences as well as in methods to characterize surface con- ditions 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 stan- dard gauges. In this paper some widely overlooked sources of errors that — if unavoidable — increase measurement uncer- tainty beyond the theoretical limit attainable are dis- cussed and the magnitude of their influence is detailed. In particular the following effects are considered: The laser–source wavelength stability as well as its point- ing stability, the effects caused by so–called schlieren occuring along the optical path as well as temperature effects causing changes in the systems geometry, ther- mally influencing the optical parameters of the imaging optics as well as the often overlooked and in most il- lumination 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 mate- rial testing applications where the strain is to be determined out of consecutive measurements of usu- ally small changes in overall length l of the specimens geometry parameter. Typical values of e are bounded by ±2000 ppm (the typical range of Hooke’s law for steel). So values of delta_l on the order of tenth of mi- crometers for typical gauge lenghts around 50 mm yield ppm resolutions for e. 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.