In our Optical Metrology laboratories, we deal with the problem of demodulating temporal sequences
of interferograms. These sequences of interferograms are obtained by means of optical
testing of transient events using an Electronic Speckle Pattern Interferometry system (ESPI). It is
well known that using Phase Stepping Interferometry techniques (PSI), one can obtain the modulated
interferogram phase with at least three equally temporal phase shifted interferograms. To
obtain these three (or more than three) phase shifted interferograms with a conventional ESPI array,
it is necessary to have a static object under test. On the other hand, if the object under test is not
static, we can make the analysis using dual-pulse subtraction ESPI, introducing a spatial frequency
carrier. However, in our case, we will use a conventional ESPI technique, with a continuous laser
and without a frequency carrier. Thus, as we pretend to analyze transient deformations or events
without frequency carrier, we can not use the demodulation methods used in dual-pulse subtraction
ESPI, nor PSI techniques because it results almost impossible to take the least amount of interferograms
with the required linear phase shift (or temporal carrier) among them. To accomplish
this, it will be necessary look for alternatives to demodulate temporal sequences of interferograms
without a frequency carrier and without linear phase-shifting. Here, we present the groundwork
aimed at demodulating sequences of interferograms without a frequency carrier, where traditional
PSI techniques are unable to detect the phase correctly.
In this work, a study for the measurement of a nonspecular object shape by using the fringe pattern projection technique
is presented. This study uses only one modulating grating, unlike the moire projection method, which uses two
modulating gratings. The proposed method was tested with several objects, and the results will be shown at the end of
the presented work. To obtain the modulating phase of the generated fringe patterns, we used step phase shifting
algorithms and the Fourier transform method. The obtained results show that the proposed method has an acceptable
precision with errors around 0.5%.
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