光源偏振对水体颗粒后向散射系数测量的影响

Impact of Light Polarization on the Measurement of Water Particulate Backscattering Coefficient

颗粒后向散射系数是水体主要的固有光学特性,也是海洋水色的决定因子和海洋水色卫星遥感反演的基础参数。当前,利用光学仪器现场原位测量是获取水体颗粒后向散射系数的主要方法。由于光源自身和仪器光路中镜面反射和折射,其出射光源可能存在一定的偏振,进而会影响水体后向散射系数测量的精度。目前,关于水体后向散射系数测量仪器的光源偏振性及其对颗粒后散射系数测量精度影响的研究尚为空白。针对该问题,以应用广泛的水体后向散射测量仪HydroScat6（HS-6）为例,对其出射光源的偏振特性进行了系统测量,并进一步开展了光源偏振对水体颗粒后向散射系数测量精度影响的实验研究。结果表明,HS-6六个光学通道除590nm通道出射光源偏振度略低外（~15%）,其他通道出射光源中心波长偏振度均在20%~30%。因此,HS-6出射光源具有显著的偏振特性。光源偏振对颗粒后向散射系数测量具有不可忽略的影响,且影响程度随着波段,线偏振角度及悬浮泥沙浓度而变化。不同悬浮泥沙浓度下,光源偏振引起的420,442,470,510,590和670nm六个波段的平均偏差可达15.49%,11.27%,12.79%,14.43%,13.76%,12.46%。因此,利用光学仪器现场测量颗粒后向散射系数需要考虑光源偏振的影响,并需尽可能降低出射光源的偏振度。

Particulate backscattering coefficient is a main inherent optical properties（IOPs）of water,which is also a determining factor of ocean color and a basic parameter for inversion of satellite ocean color remote sensing.In-situ measurement with optical instruments is currently the main method for obtaining the particulate backscattering coefficient of water.Due to reflection and refraction by the mirrors in the instrument optical path,the emergent light source from the instrument may be partly polarized,thus to impact the measurement accuracy of water backscattering coefficient.At present,the light polarization of measuring instruments and its impact on the measurement accuracy of particulate backscattering coefficient are still poorly known.For this reason,taking a widely used backscattering coefficient measuring instrument HydroScat6（HS-6）as an example in this paper,the polarization characteristic of the emergent light from the instrument was systematically measured,and further experimental study on the impact of the light polarization on the measurement accuracy of the particulate backscattering coefficient of water was carried out.The results show that the degree of polarization（DOP）of the central wavelength of emergent light ranges from20%to 30%for all of the six channels of the HS-6,except the 590 nm channel from which the DOP of the emergent light is slightly low（~15%）.Therefore,the emergent light from the HS-6has significant polarization.Light polarization has non-neglectable impact on the measurement of particulate backscattering coefficient,and the impact degree varies with the wave band,linear polarization angle and suspended particulate matter（SPM）concentration.At different SPM concentrations,the mean difference caused by light polarization can reach 15.49%,11.27%,12.79%,14.43%,13.76%,and 12.46% in six bands,420,442,470,510,590,and 670 nm,respectively.Consequently,the impact of light polarization on the measurement of particulate backscattering coefficient with an optical instrument should be taken into account,and the DOP of the emergent light should be reduced as much as possible.