Functional Machine Vision Utilizing Full-Field Low-Coherence Interferometry

Abstract

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.