Multi-spatial-frequency and phase-shifting profilometry using a liquid crystal phase modulator
2012
Optical profilometry is widely applied for measuring the morphology of objects
by projecting predetermined patterns on them. In this technique, the compact size is
one of the interesting issues for practical applications. The generation of pattern
by the interference of coherent light sources has a potential to reduce the
dimension of the illumination part. Moreover, this method can make fine patterns
without projection optics, and the illumination part is free of restriction from the
numerical aperture of the projection optics. In this paper, a phase-shifting
profilometry is implemented by using a single liquid crystal (LC) cell. The LC phase
modulator is designed to generate the interference patterns with several different
spatial frequencies by changing selection of the spacing between the micro-pinholes.
We manufactured the LC phase modulator and calibrated it by measuring the phase
modulation amount depending on an applied voltage. Our optical profilometry using
the single LC cell can generate multi-spatial frequency patterns as well as four
steps of the phase-shifted patterns. This method can be implemented compactly, and
the reconstructed depth profile is obtained without a phase-unwrapping
algorithm.
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