28 May 2014 Three dimensional imaging with multiple wavelength speckle interferometry
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We present the design, modeling, construction, and results of a three-dimensional imager based upon multiplewavelength speckle interferometry. Speckle imaging used in non-destructive evaluation is well-known but requires a precisely acquired reference image and can measure excursions only within the 2π ambiguity range determined by the illumination wavelength. Our approach is based upon earlier efforts pioneered by Takeda, but with updated illumination, imaging, and processing tools, in which a surface under test is illuminated with tunable laser light in a Michelson interferometer configuration. A speckled image is acquired at each laser frequency step creating a data hypercube. Interference between the reference wavefront and light from the object causes the amplitude of the speckles to cycle with laser tuning. Fourier transforming the hypercube in the laser frequency dimension reveals periods that map heights of surface features. Height resolution is determined by the maximum tuning range of the laser, which for our 16-nm tuning range provides approximately 18 micron resolution without any efforts at interpolation. The largest height without wraparound depends on the smallest tuning steps, which for our laser is 15 cm for 0.002 nm (1 GHz) tuning steps. In this way, objects with large discontinuous steps or holes can be imaged without confusion. Also, due to the illumination beam being normal to the surface under test, shadowing is eliminated. To inform our design and better understand our system’s limitations, we have developed extensive numerical models based upon Monte Carlo ray tracing in which speckle patterns are produced after scattering from model surfaces by coherent summing of rays at the detector plane. Data acquired by the system as well as modeling results will be shown.
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Bruce E. Bernacki, Bruce E. Bernacki, Bret D. Cannon, Bret D. Cannon, John T. Schiffern, John T. Schiffern, Albert Mendoza, Albert Mendoza, "Three dimensional imaging with multiple wavelength speckle interferometry", Proc. SPIE 9110, Dimensional Optical Metrology and Inspection for Practical Applications III, 91100A (28 May 2014); doi: 10.1117/12.2053834; https://doi.org/10.1117/12.2053834

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