Polarization discrimination fluorescence retrieval from reflectance spectra of algae in seawater: comparison of multicomponent Mie scattering and polarized radiative transfer models with laboratory and field tests

Abstract

Previously, it was shown that it is possible to separate the elastic scattering from the chlorophyll fluorescence signal using a polarization discrimination technique. The separation procedure depends however fundamentally on the degree of polarization of the water leaving radiance. In this paper, we compare polarization effects and efficiency of the fluorescence retrieval by simulating the total and polarized reflectance of waterleaving radiances originating from elastic scattering in case 1 and case 2 waters using radiative transfer and multiple component Mie scattering programs. This is done by superimposing upon these reflectances the contribution of known fluorescence spectra, and by using our polarization discrimination procedure to invert the resulting data back into fluorescence spectra. It is shown that a Mie scattering code which does not take into account multiple scattering effects can strongly overestimate the degree of polarization for case 1 and case 2 waters as well as reflectances in the green part of the spectra. Making use of these results we propose and evaluate an improvement for the traditional height over baseline fluorescence extraction method which strongly overestimates fluorescence value for case 2 waters. For our approach we use inverse absorption spectra of water and chlorophyll a scaled up and fitted to the reflectance spectra in the red and near IR bands with the subsequent retrieval of the fluorescence spectrum and chlorophyll a concentration. Results and potential of this approach are discussed.