Analyzing spectral characteristics of water involving in-situ multiangle polarized reflectance and extraction of water-leaving radiance
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2016, Peking University Press. All right reserved. Polarization is defined as an asymmetry in the direction of vibration with respect to the direction of light propagation. Polarization information is an important component to remote sensed data, which comprises spatial, spectral, and radiation information. In optical remote sensing, polarization information supplements spectral information. Polarization-based remote sensing has a significant application potential for analyzing the spectral characteristics of water bodies, wherein a very important technique is eliminating the mirror reflection caused by skylight on the water surface and extracting water-leaving radiance that carry the constituent information. The incident sunlight on the surface of water either reflects or scatters owing to the existence of particles in water, which results in water-leaving signals with strong polarization characteristics. The ongoing experiments on remote sensing involve water polarization cover either clean ocean waters under natural light or indoor simulations of water under artificial light; however, turbid inland waters under natural light have rarely been investigated. Through the combination of a field spectroradiometer and a Thompson polarizing prism, this study obtained in-situ measurements of the spectral polarization reflectance over inland waters under natural light conditions. Using the obtained multiangle polarized reflectance spectra, the polarization spectral characteristics of water under multiangle viewing conditions were quantitatively analyzed, and the water-leaving radiance was achieved by eliminating skylight reflection. When observing water bodies at an azimuth of 135 and a zenith of 53, the measurement of polarization to eliminate skylight reflection had better elimination efficiency than at other viewing angles, and this observation angle was recommended for conducting spectral polarized above-water observations. Compared with the traditional methods including non-polarized above-water measurements, the proposed method is less prone to being affected by changes in weather conditions, and it can extract water-leaving radiance more accurately.