Metasurfaces are diffractive optical elements composed of subwavelength structures that, when leveraging birefringence, can serve as multifunctional freespace polarization optics. Metasurface gratings are flat optical elements designed to operate as full Stokes analyzers to enable compact polarimetric imaging. This work presents the calibration of a metagrating-based Stokes imaging system. A prototype metasurface full Stokes imaging polarimeter concept is compared to a benchtop instrument for 670, 532, and 460 nm metagratings. The metagrating Stokes imaging polarimeters are assessed for DOLP accuracy within ±0.5%. The 532 nm instrument is fully evaluated for polarization ellipse and degree of polarization accuracy.
The Mueller matrix dependence on an objects' albedo and surface texture are measured and these effects are used to inform modifications to a microfacet model for polarized light scattering. Four different textures are imparted on red LEGO bricks which are illuminated at 451 nm and 662 nm. These wavelengths yield measurements for both low and high albedo conditions respectively. Analysis of polarizance, depolarization index, polarization entropy, and matrix roots demonstrate that texture and albedo have distinct polarization and depolarization signatures. The root mean square deviations (RMSD) of the unmodified microfacet model from the measurements are about three times greater for high albedo compared to low albedo measurements. The surface texture trend is more subtle, but in all cases the RMSD decreases as roughness increases. A major contribution of this work is an adjustment to the microfacet model so that the polarized term is wavelength dependent. This adjusted model improves the RMSD more for the low albedo compared to the high albedo measurements. To improve model fits to high albedo measurements, a modified depolarization structure is introduced to reduce the RMSD of high albedo measurements by about a factor of two.
Short-wave infrared light is weakly absorbed by human epidermal and dermal tissue. Investigation of polarization characteristics of human skin taken in vivo distinguishes how depolarization index and the Mueller matrix are markers that can identify skin from a rough Lambertian surface material. A custom Near-Infrared imaging Mueller Matrix Polarimeter using an expanded, coherent 1550nm laser beam source is used to analyze the skin on the back of hands as a subject.
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