Functional Machine Vision Utilizing Full-Field Low-Coherence Interferometry
Presented is an investigation of noncontact micrometer profilometry through fullfield
low-coherence interferometry, focusing on a functional machine vision approach.
Thus, the goals of the research were: simplicity yet functionality, robustness
yet versatility, and precision yet cost-effectiveness.
Most modern low-coherence interferometry techniques use analog electromechanical
scanning in either lateral or axial dimensions to perform three-dimensional
measurement-this is burdensome and inefficient if approximate knowledge of the
sample exists before measurement. This work innovates with the achievement of
true random access full-field measurement without analog electromechanical scanning.
Lateral scanning was implemented by electronically accessing small random
regions of interest on a complementary metal-oxide semiconductor camera at very
fast frame rates. By decoupling optical carrier generation from axial scanning,
two previously unreported random depth access techniques are demonstrated that
feature a digital stepper motor. One method utilized a beat frequency between
two acousto-optic modulators and a complementary metal-oxide semiconductor
camera; the other employed a charge-coupled device camera and linear piezoelectric
transducer scanning over 4 periods of the optical carrier. A significant
benefit of the these techniques is the possibility of full-field phase retrieval of the
interferometric signal inside a 14 µm coherence envelope without electromechanical
stepping. This is highlighted with the nanometer surface profile of a standard
engineering gauge block by a novel heterodyne phase retrieval algorithm.
Specific application of the work is to inexpensive and functional full-field dimensional
metrology demanding micrometer resolution over millimeter ranges.
Previously unreported application of random depth access full-field low-coherence
interferometry to a small punch test is reported. Rapid, noncontact, threedimensional
profiling of a mild steel disk deformed with an applied force is illustrated.
The benefits of this approach over conventional maximum displacement
measurements are noncontact full-field measurement, averaged cross-sectional
profiling, uniform deformation analysis, and applied force calibration.
This Thesis presents experimental work, signal processing techniques, and theoretical
considerations, from a single point low-coherence interferometer through
to the development of a three-dimensional random access functional machine vision
system.
EGAN Patrick;
2007-02-13
JRC36265
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