The Design of a Polarimeter and its Use for the Study of the Variation of Downwelling Polarized Radiance Distribution with Depth in the Ocean

The spectral polarized radiance distribution provides the most complete description of the light field that can be measured. However, this is a very difficult parameter to measure near the surface because of its large dynamic range, dependence on incoming sky conditions, and waves at the air-sea interface. The measurement of the Stokes vector of the downwelling polarized light field requires the combination of at least four images, all of which must be obtained simultaneously. To achieve this, a new polarimeter (which we call DPOL) has been designed, characterized, calibrated and deployed. The description of the DPOL, its calibrations and characterizations are discussed. The uncertainties in the retrieval of Stokes vector and other derived parameters are also discussed. This instrument is equipped with four fish-eye lenses (180° field of view) with polarizers behind each lens in a different orientation, a coherent optical fiber bundle with 4 arms, a spectral filter changer assembly and a charged coupled-device (CCD) imaging camera. With this system, a single image contains 4 separate fisheye images, each a whole hemisphere of the same scene, each with different polarization information. Using these 4 images and applying appropriate calibration parameters allows us to calculate the four-element Stokes vector and then the total degree of polarization and the angle of plane of polarization of the incoming light field in a hemisphere of desired directions. Under the Office of Naval Research RaDyO (Radiance under a Dynamic Ocean) program, DPOL has been used in the Santa Barbara Channel and Hawaii field experiments. In most cases, data on sky polarization were collected with a separate camera (Sky-Cam) simultaneously with the DPOL. The data and results with these two camera systems in these experiments are presented and are compared. Data on the inherent optical properties of water from the same field experiments collected by collaborators will be shown. Our measurements show that very near the surface, for clear sky conditions, the dominant source of polarization is the refracted sky light. As one progresses in the water column, the polarization due to light scattering by the water increases and polarization due to the water becomes dominant. The dependence of the in-water light field polarization on the sky and surface wave conditions, solar zenith and azimuth angles, the depth of the instrument, the viewing angle, the wavelength of light, the inherent optical properties (IOP’s) of water are discussed