A Fiber-optic Hyperspectral Profiling System

Knowledge of optical properties, especially the hyperspectral properties, of Case-II waters is essential in remote sensing studies of Chinese coastal waters. Application of commercial spectroradiometers is often limited in these turbid waters for a number of reasons. In this study, based on fiber optics and a portable spectroradiometer that has been designed for use in the air, a system has been developed to measure hyperspectral light profiles in highly turbid waters. The system is mainly composed of:1) A portable spectroradiometer (GER 1500, 512 wave bands between 350 and 1050 nm);2) A 15-m PCS fiber (core size of 0.6 mm with SMA connectors at both ends) with a self-made adapter to couple the fiber-optic cable with the entrance slit of GER1500;3) A strong floating device to balance the profiling system and to provide the zero depth level;4) A diffuse reflection plaque with nearly 100% reflectivity in the air in the visible and near-infrared to enable the system to measure downwelling light;5) A laptop computer to display and obtain data in real-time.The system has been tested in three types of waters, namely in the nutrient-rich Furong Lake (Xiamen university), nearshore turbid waters near the Fujian coast, and offshore waters in the Taiwan strait, respectively.Reasonable results have been obtained from these experiments: both upwelling and downwelling light show exponential attenuation with increasing depth; subsurface remote-sensing reflectance (r_rs0-) is highly correlated to above-water remote-sensing reflectance (R_rs); diffuse attenuation coefficient at 490 nm (K₄90, m^-1) is also related to chlorophyll concentration. These results suggest that the system is stable and capable of characterizing light profiles in turbid coastal waters. In the offshore waters, however, because of the wavy sea, the short fiber-optic cable and small light attenuation, the system's performance is less satisfactory. The empirical coefficient between r_rs0- and R_rs has been found different from that reported in the literature; this demands further research.