Abstract
Current material identification techniques rely on estimating reflectivity or emissivity which vary with viewing angle. As off-nadir remote sensing platforms become increasingly prevalent, techniques robust to changing viewing geometries are desired. A technique leveraging polarimetric hyperspectral imaging (P-HSI), to estimate complex index of refraction, N̂(ν̃), an inherent material property, is presented. The imaginary component of N̂(ν̃) is modeled using a small number of "knot" points and interpolation at in-between frequencies ν̃. The real component is derived via the Kramers-Kronig relationship. P-HSI measurements of blackbody radiation scattered off of a smooth quartz window show that N̂(ν̃) can be retrieved to within 0.08 RMS error between 875 cm-1 ≤ ν̃ ≤ 1250 cm-1. P-HSI emission measurements of a heated smooth Pyrex beaker also enable successful N̂(ν̃) estimates, which are also invariant to object temperature.
Highlights
Many material classification and identification methods have been developed using hyperspectral imagery, see [1] for a good overview
Estimating the complex index of refraction may be advantageous for material identification because, in a vast majority of operational scenarios, it is invariant to viewing geometry — birefringent materials and metamaterials being potential exceptions — and insensitive to object temperature over typical terrestrial variations
N (ν) when emitted light was the primary signature. Both experiments presented demonstrate the feasibility of refractive index estimation using polarimetric hyperspectral imaging (P-HSI)
Summary
Many material classification and identification methods have been developed using hyperspectral imagery, see [1] for a good overview. When working in the long-wave infrared (LWIR), emissivity is often the estimated quantity for material identification This type of problem is commonly known as temperature-emissivity separation (TES) because it is necessary to determine both the temperature and spectral emissivity of a material to fully describe its emitted radiance. This is typically an underdetermined problem because, in addition to the unknown emissivity values in each spectral band, the temperature is unknown. Estimating the complex index of refraction may be advantageous for material identification because, in a vast majority of operational scenarios, it is invariant to viewing geometry — birefringent materials and metamaterials being potential exceptions — and insensitive to object temperature over typical terrestrial variations
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