Neutral lines and model-measurement closure in oceanic polarized light fields.

The polarization state of light in the maritime environment has untapped potential to reveal properties of the material constituents of the water column. Consequently, understanding the formation of polarization features is important for remote sensing and the fundamental science of ocean optics. In this study, the degree and angle of linear polarization (DoLP and AoLP) and the location of neutral points and neutral lines in the in-water upwelling and in-air downwelling oceanic light field were measured, modeled, and interpreted. The work is comprised of an optical closure experiment for polarized light fields utilizing data from PixPol, a polarized radiance fisheye camera system MASCOT, an in-water instrument for measuring Mueller matrix scattering elements; and Layertran, an ocean-atmosphere vectorial Monte Carlo model. Open questions from the literature regarding the occurrence of non-principal plane neutral points were revisited, and resolving instrument-limited ambiguities in measured Mueller matrices for backscatter angles greater than 150° was also of interest. Our results achieved good closure in the overall pattern of DoLP, AoLP, and neutral lines between the empirical measurements and modeling, especially in red wavelengths (670 nm). In blue and green wavelengths (444 nm and 500 nm), the peak measured DoLP in in-water upwelling radiances was systematically lower than the modeled DoLP, for reasons the study was unable to determine but may be a combination of factors. Results demonstrate that the formation of non-principal plane neutral points in the upwelling in-water radiance depends on the balance of direct and diffuse atmospheric light. While these neutral points can be present in natural conditions, the likelihood of their observation is increased by instrument shading hence they may be an observer effect. PixPol imaging confirmed the validity of smooth interpolation of the Mueller matrix for scattering angles greater than 150° despite the occurrence of features in this region in MASCOT measurements and Mie modeling. The results of this study provide a baseline for future work toward polarimetry methods, in particular, to exploit data from SPEXone and HARP2 on the NASA PACE platform.