KEYWORDS: Linear filtering, Image filtering, Gaussian filters, Image fusion, Data fusion, Time metrology, Digital filtering, Nonlinear filtering, Wavefronts, Optical testing
Electronic speckle pattern interferometry (ESPI) can provide
accurate contour measurement in the micron range and short
measurement times far below one second. An advantage of this
method is that illumination axis and observation axis can be
identical in contrast to e.g. triangulation. Therefore ESPI
represents an interesting alternative to other optical measurement
principles used for surface profiling. Typical surfaces in
industrial applications often show discontinuities, like steps or
holes. An unambiguous measurement of such surfaces is only
possible if the synthetic wavelength is chosen larger than the
largest surface step. Since the noise level introduced to the
measurement increases proportional to the synthetic wavelength,
unambiguous measurements suffer from a loss of accuracy. The
solution for this problem is the combination of two or more
synthetic wavelengths. In contrast to other publications
(hierarchical, pixel-wise approach or temporal phase unwrapping)
our novel area-based approach uses only two synthetic wavelengths
minimizing measurement time and device complexity. The use of
areas instead of pixels allows a lower signal to noise ratio and a
smaller number of synthetic wavelengths (in our case only two)
respectively, compared to the hierarchical pixel based approach.
In this paper we present the steps required during pre-processing
(laser tilt and wave front compensation) and the opportunities and
drawbacks of different algorithms used for the fusion of the two
images gained from different synthetic wavelengths.
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