Spectral Invariance Hypothesis Study of Polarized Reflectance with the Ground-Based Multiangle SpectroPolarimetric Imager

Many models used to represent the boundary condition for the separation of atmospheric scattering from the surface reflection in polarized remote sensing measurements, assuming that the polarized surface reflectance is spectrally neutral. To test this hypothesis, referred to the spectral invariance hypothesis, JPL's Ground-based Multiangle SpectroPolarimetric Imager (GroundMSPI) was used to measure the polarized bidirectional reflectance factors (pBRFs) of different types of outdoor surfaces. GroundMSPI measures the linear polarization Stokes parameters ($I$, $Q$, and $U$) at three wavebands centered at 470, 660, and 865 nm. Images of pBRF were acquired at the three wavelengths and the best-fitting slopes of pairwise spectral regressions were determined. The spectral invariance hypothesis predicts that these slopes should be unity. All region types, excluding grass, meet this criterion within 8% for all three wavelengths. Grass measurements show a large mean deviation of 31.1% from the expected slope when regressing 865-nm data against other wavelengths due to the differences in the spectral absorbance of chlorophyll. Angle of linear polarization (AoLP) analysis of cotton crops is presented as a method to isolate cases where the scattered light is dominated by single reflection within the plants from cases where multiple scattering plays a larger role. Spectral invariance is observed in those cases where specular reflection from the top surfaces of the leaves is primarily responsible for the measured polarization.