Appl Opt. 2022 May 20;61(15):4527-4534. doi: 10.1364/AO.458737.
Salinity is an important environmental factor regulating the aquatic system structure of lakes and other water bodies. Changes in salinity, which can be caused by human activities, can adversely impact the life of water organisms. The refractive index, which can be directly related to water salinity, also controls the polarimetric properties of light reflected from the water surface. In this study, polarimetric measurements of smooth water surfaces with different salinity content were performed at different viewing zenith angles in the wavelength range of 450-1000 nm in the specular reflection directions. The results show that the light reflected from the water surface (defined as reflectance factor) in one measurement direction can be replaced by the reflectance factor derived from polarimetric measurements, and if the polarizer absorptance is considered, the average relative difference is less than 3%. The degree of linear polarization (DOLP) was used to retrieve the refractive indices of water with different salinities based on the Fresnel reflection coefficient. The inverted refractive indices not only have high accuracy (uncertainty from 0.9% to 1.8%) but also have a very strong relationship with the water salinity content. Our study shows the possibility of estimating the variation in water salinity using multi-angular polarimetric measurements.
盐度是调节湖泊和其他水体水系统结构的一个重要环境因素。盐度的变化（可能是人为活动引起的）会对水生物的生命产生不利影响。折射率与水的盐度直接相关，它也控制着从水面反射的光的偏振特性。在这项研究中，在镜面反射方向，在 450-1000nm 的波长范围内，对具有不同盐度含量的光滑水面在不同视场天顶角进行了偏振测量。结果表明，在一个测量方向上从水面反射的光（定义为反射率因子）可以用偏振测量得到的反射率因子来代替，如果考虑起偏器吸收率，则平均相对差异小于 3%。基于菲涅耳反射系数，使用线偏振度（DOLP）来反演具有不同盐度的水的折射率。反演的折射率不仅具有很高的精度（不确定性为 0.9%至 1.8%），而且与水的盐度含量有很强的关系。我们的研究表明，利用多角度偏振测量来估计水盐度变化是可能的。
