Angle of linear polarization images of outdoor scenes

Abstract

Observations from the Ground-based Multiangle SpectroPolarimetric Imager (GroundMSPI) are used to relate angle of linear polarization (AoLP) measurements to material properties and illumination conditions in sunlit outdoor environments. GroundMSPI is a push-broom spectropolarimetric camera with an uncertainty in degree of linear polarization (DoLP) of AE0.005. This polarimetric accuracy yields useful AoLP images even when the DoLP is less than 0.02. AoLP images are reported with respect to dependency on surface texture, surface orientation, albedo, and illumination conditions. Agreement with well-known principles of polarized light scattering is illustrated, and several special cases are described. Expected observations of AoLP tangential to surface orientation and AoLP perpendicular to the scattering plane are reported. Significant changes in the AoLP are observed from common variations in outdoor illumination conditions. Also, simple variants in material properties change the dominant polarized light scattering process and thus the AoLP. Measurement examples that isolate a 90 deg AoLP flip are shown for a sunny and cloudy day as well as an object of high and low albedo. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Parameter-based imaging from passive multispectral polarimetric measurements

“…However, if the illumination polarization is known, the extension of the analysis to this case is relatively straightforward. We note that the fourth Stokes component, corresponding to the circular polarization state, is not measured by the Ground-MSPI due to its insignificant contribution in most natural scenes [1][2]4,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”

“…For clear sky conditions, the direct illumination generally dominates unless the Sun is low in the sky. Thus, in this work, as well as other passive remote sensing studies [1,[15][16][17][18][19][20][21][22], the scene illumination is considered to be unpolarized. This assumption may not be accurate for certain conditions such as a partly cloudy sky or for relatively smooth surfaces that produce significant specular reflections of the partially polarized sky irradiance.…”

“…We assume the diffuse scattering is completely unpolarized and Lambertian where the light encounters enough multiple scattering events within the bulk of the material that the scattering directions and polarization state are uniformly random. In general, the diffuse scattering may exhibit some polarization and directional preference due to the re-emergence processes [23], but the assumptions of unpolarized and Lambertian scattering are reasonable especially for remote sensing scenarios where the target's surface roughness is typically large compared to the incident wavelength [9][10][11][12][13][14][15][16][17][18][19][20][21][22]. In contrast, the surface scattering that develops based on a microfacet model, can produce a directional lobe and polarization preference.…”

“…describes the depolarization phenomena of the diffuse scattering, which is defined with no polarization preference in this case [9][10][11][12][13][14][15][16][17][18][19][20][21][22]. A convenient reflectance parameter ρ d , based on Kubelka-Munk (KM) theory [27] and the work by Le Hors et al [26], has been developed to describe the intensity ratio of the diffuse scattering to the scattering from a perfect Lambertian surface.…”

“…To our knowledge, this is the first time that images of surface parameters have been generated from polarimetric field data. The polarimetric data for this work was collected with the Ground Multiangle SpectroPolarimetric Imager (Ground-MSPI) [15][16], which is a camera system developed to provide Stokes parameter images at narrowband wavelengths of 470 nm, 660 nm and 865 nm [17][18][19][20][21][22].…”