Schwenger Frédéric, Repasi Endre
Appl Opt. 2017 Feb 20;56(6):1662-1673. doi: 10.1364/AO.56.001662.
The knowledge of the spatial energy (or power) distribution of light beams reflected at the dynamic sea surface is of great practical interest in maritime environments. For the estimation of the light energy reflected into a specific spatial direction a lot of parameters need to be taken into account. Both whitecap coverage and its optical properties have a large impact upon the calculated value. In published literature, for applications considering vertical light propagation paths, such as bathymetric lidar, the reflectance of sea surface and whitecaps are approximated by constant values. For near-horizontal light propagation paths the optical properties of the sea surface and the whitecaps must be considered in greater detail. The calculated light energy reflected into a specific direction varies statistically and depends largely on the dynamics of the wavy sea surface and the dynamics of whitecaps. A 3D simulation of the dynamic sea surface populated with whitecaps is presented. The simulation considers the evolution of whitecaps depending on wind speed and fetch. The radiance calculation of the maritime scene (open sea/clear sky) populated with whitecaps is done in the short wavelength infrared spectral band. Wave hiding and shadowing, especially occurring at low viewing angles, are considered. The specular reflection of a light beam at the sea surface in the absence of whitecaps is modeled by an analytical statistical bidirectional reflectance distribution function (BRDF) of the sea surface. For whitecaps, a specific BRDF is used by taking into account their shadowing function. To ensure the credibility of the simulation, the whitecap coverage is determined from simulated image sequences for different wind speeds and compared to whitecap coverage functions from literature. The impact of whitecaps on the radiation balance for bistatic configuration of light source and receiver is calculated for a different incident (zenith/azimuth angles) of the light beam and is presented for two different wind speeds.
了解动态海面反射光束的空间能量（或功率）分布在海洋环境中具有重大的实际意义。为了估算反射到特定空间方向的光能，需要考虑许多参数。白帽覆盖率及其光学特性对计算值有很大影响。在已发表的文献中，对于考虑垂直光传播路径的应用，如测深激光雷达，海面和白帽的反射率用恒定值近似。对于近水平光传播路径，必须更详细地考虑海面和白帽的光学特性。计算得到的反射到特定方向的光能在统计上是变化的，并且在很大程度上取决于波浪海面的动力学和白帽的动力学。本文给出了一个带有白帽的动态海面的三维模拟。该模拟考虑了白帽随风速和 fetch 的演变。在短波红外光谱带中对白帽覆盖的海洋场景（开阔海域/晴空）进行辐射度计算。考虑了波浪遮挡和阴影，特别是在低视角下出现的情况。在没有白帽的情况下，光束在海面的镜面反射由海面的解析统计双向反射分布函数（BRDF）建模。对于白帽，通过考虑其阴影函数使用特定的 BRDF。为了确保模拟的可信度，从不同风速的模拟图像序列中确定白帽覆盖率，并与文献中的白帽覆盖率函数进行比较。针对光束的不同入射（天顶角/方位角），计算了白帽对光源和接收器双基地配置辐射平衡的影响，并给出了两种不同风速下的结果。
