Wei Jianwei, Lee Zhongping, Lewis Marlon, Pahlevan Nima, Ondrusek Michael, Armstrong Roy
Opt Express. 2015 May 4;23(9):11826-37. doi: 10.1364/OE.23.011826.
The radiance transmittance (Tr) is the ratio of the water-leaving radiance (Lw(0+)) to the sub-surface upwelling radiance (Lu(0-)), which is an important optical parameter for ocean optics and ocean color remote sensing. Historically, a constant value (~0.54) based on theoretical presumptions has been adopted for Tr and is widely used. This optical parameter, however, has never been measured in the aquatic environments. With a robust setup to measure both Lu(0-) and Lw(0+) simultaneously in the field, this study presents Tr in the zenith direction between 350 and 700 nm measured in a wide range of oceanic waters. It is found that the measured Tr values are generally consistent with the long-standing theoretical value of 0.54, with mean relative difference less than 10%. In particular, the agreement within the spectral domain of 400-600 nm is found to be the best (with the averaged difference less than 5%). The largest difference is observed for wavelengths longer than 600 nm with the average difference less than 15%, which is related to the generally very small values in both Lu(0-) and Lw(0+) and rough environmental conditions. These results provide a validation of the setup for simultaneous measurements of upwelling radiance and water-leaving radiance and confidence in the theoretical Tr value used in ocean optics studies at least for oceanic waters.
辐射透过率（Tr）是离水辐射率（Lw(0+)）与次表层向上辐射率（Lu(0-)）的比值，它是海洋光学和海洋水色遥感的一个重要光学参数。从历史上看，基于理论假设采用了一个恒定值（约0.54）作为Tr，并被广泛使用。然而，这个光学参数从未在水生环境中进行过测量。本研究通过一个强大的野外同步测量Lu(0-)和Lw(0+)的装置，给出了在350至700纳米范围内，在多种海洋水体中测量得到的天顶方向的Tr。研究发现，测量得到的Tr值总体上与长期以来0.54的理论值一致，平均相对差异小于10%。特别是，发现在400 - 600纳米光谱范围内的一致性最好（平均差异小于5%）。在波长大于600纳米时观察到的最大差异，平均差异小于15%，这与Lu(0-)和Lw(0+)通常非常小的值以及恶劣的环境条件有关。这些结果验证了同步测量向上辐射率和离水辐射率的装置，并至少为海洋水体的海洋光学研究中使用的理论Tr值提供了信心。
