水面原位多角度偏振反射率光谱特性分析与离水辐射提取

Analyzing Spectral Characteristics of Water Involving In-Situ Multiangle Polarized Reflectance and Extraction of Water-Leaving Radiance

偏振被定义为光的振动方向对于传播方向的不对称性。偏振信息是遥感数据空间、辐射、光谱信息之外的又一重要信息。对于光学遥感数据来说,偏振信息是对光谱信息的又一种补充。偏振遥感在水体应用中具有巨大潜力,其中一个非常重要的应用就是校正天空光水面镜面反射,从而得到包含水中物质信息的离水辐射信号。太阳入射光会在水面发生反射或者与水体中颗粒物发生散射作用,使得离开水面被遥感器接收的信号具有很强的偏振特性。目前开展的水体偏振遥感实验要么面向室外自然光条件下的清洁海洋水体,要么面向室内人造光源条件下的模拟水体,鲜有面向自然光条件下的浑浊内陆水体。本文通过组合地物光谱仪和汤姆森偏振棱镜,实现了自然光条件下内陆水体水面原位偏振反射率光谱测量。利用获取的典型内陆水体水面多角度偏振反射率光谱,定量分析了多角度观测条件下水体偏振光谱特性,以及从水面偏振信号中消除天空光水面镜面反射从而得到离水辐射信号的效果。当观测方位角为135°、观测天顶角为53°时,采用偏振测量剥离水表天空反射光的剥离效率较好,推荐采用该观测几何进行水面偏振光谱观测。相比于传统的非偏振水面光谱测量方法,水面偏振光谱测量方法受气象条件变化影响小,能够更精确的提取离水辐射。

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 of53°,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.