Bernhard Zagar, Johannes Lettner,
"A Discussion on Often Overlooked Sources of Error in Laser--Speckle Based Systems"
: Proc. SPIE 8306, 830606 (2011), 8-2011

Original Titel:

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

Sprache des Titels:

Englisch

Original Buchtitel:

Proc. SPIE 8306, 830606 (2011)

Original Kurzfassung:

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.