Morel A, Gentili B
Appl Opt. 1993 Nov 20;32(33):6864-79. doi: 10.1364/AO.32.006864.
For visible wavelengths and for most of the oceanic waters, the albedo for single scattering (?) is not high enough to generate within the upper layers of the ocean a completely diffuse regime, so that the upwelling radiances below the surface, as well as the water-leaving radiances, generally do not form an isotropic radiant field. The nonisotropic character and the resulting bidirectional reflectance are conveniently expressed by the Q factor, which relates a given upwelling radiance L(u) (θ',φ) to the upwelling irradiance E(u) (θ' is the nadir angle, φ is the azimuth angle, and Q = E(u)/L(u)); in addition the Q function is also dependent on the Sun's position. Another factor, denoted f, controls the magnitude of the global reflectance, R (= E(u) /E(d), where E(d) is the downwelling irradiance below the surface); f relates R to the backscattering and absorption coefficients of the water body (b(b) and a, respectively), according to R = f(b(b)/a). This f factor is also Sun angle dependent. By operating an azimuth-dependent Monte Carlo code, both these quantities, as well as their ratio (f/Q) have been studied as a function of the water optical characteristics, namely ? and η; η is the ratio of the molecular scattering to the total (molecular + particles) scattering. Realistic cases (including oceanic waters, with varying chlorophyll concentrations; several wavelengths involved in the remote sensing of ocean color and variable atmospheric turbidity) have been considered. Emphasis has been put on the geometrical conditions that would be typical of a satellite-based ocean color sensor, to derive and interpret the possible variations of the signal emerging from various oceanic waters, when seen from space under various angles and solar illumination conditions.
对于可见光波长以及大多数海水而言，单次散射反照率（?）不够高，无法在海洋上层形成完全漫射状态，因此海面以下的向上辐射以及离水辐射通常不会形成各向同性辐射场。非各向同性特征及由此产生的双向反射率可以用Q因子方便地表示，Q因子将给定的向上辐射L(u)(θ',φ)与向上辐照度E(u)联系起来（θ'是天底角，φ是方位角，且Q = E(u)/L(u)）；此外，Q函数还取决于太阳的位置。另一个因子，记为f，控制总反射率R的大小（R = E(u)/E(d)，其中E(d)是海面以下的向下辐照度）；f根据R = f(b(b)/a)将R与水体的后向散射系数和吸收系数（分别为b(b)和a）联系起来。这个f因子也与太阳角度有关。通过运行一个依赖方位角的蒙特卡罗代码，研究了这两个量以及它们的比值（f/Q）随水光学特性的变化，即?和η；η是分子散射与总散射（分子 + 粒子）的比值。考虑了实际情况（包括叶绿素浓度不同的海水、海洋颜色遥感涉及的几个波长以及可变的大气浑浊度）。重点关注了基于卫星的海洋颜色传感器典型的几何条件，以便推导和解释在不同角度和太阳光照条件下从太空观测时各种海水发出的信号可能的变化。
