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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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