The University of Maine Maine In-situ Sound & Color Lab
School of Marine Sciences
Anisotropy in the Backscattering of Marine-like Particles: Indications of Size and Shape

By modelling the single scattering in the exact backward direction (180°) and 5° around, the field of view of an instrument measuring backscattering is simulated. Calculations of the volume scattering function (VSF) are made for spheroidal particles with sizes, shapes and refractive indices similar to organic marine particles found in the natural environment. Results show that information about size and shape can be gathered from the intensity patterns in the backscattering. Comparison between the calculated "effective" Mueller matrices of these non-spheres and volume-equivalent spheres provides insight into the information available from backscattering polarimetry on the effects of size and shape in light scattering by different particles. Incorporating the azimuthal variation inherent in such measurements, only the phase function (the (1,1)-element) and the circular polarisation component of the scattering matrix (the (4,4)-element) remain anisotropic as particle size and shape change. A ringed pattern in both these elements is observed for spheres with sizes in the order, and greater, of the incident wavelength of light. This is due to the enhanced structure of the backscattering function for spheres, which in comparison, is dampened for non-spheres. Furthermore, the directions of the peaks of the (2,2)- and (3,3)-elements for spheres are rotated 45° for non-spheres. It is convenient to measure only the parallel and perpendicular polarised components of the resulting scattering (observations using a backscattering LISST developed by Sequoia Scientific, Inc. are compared with theoretical calculations). From the Stokes vectors resulting from the Mueller matrices above, linearly polarised light introduces anisotropy in the scattered perpendicular field component that persists regardless of size, the structure of which is quite different for spheres and non-spheres. As size increases for spheres, peaks appear along the polarisation direction of the parallel component and are enhanced. These along with the angular width around the peak in the degree of polarisation provides an idea of the extent of non-sphericity and relative sizes of the particles in question.

Clavano, W.R., E. Boss, and Agrawal, Y.C., 2006. Anisotropy in the Backscattering of Marine-like Particles: Indications of Size and Shape. 2006 Spring Ocean Sciences Meeting.

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