Estimating the Diffuse Downwelling Attenuation Coefficient from in Situ Photographs

The rate at which water absorbs and scatters light in the vertical direction is governed by the diffuse downwelling attenuation coefficient. Despite having some dependence on environmental conditions, the diffuse downwelling attenuation coefficient is strongly influenced by water constituents, making it useful for optically characterizing water bodies, assessing water quality, or quantifying visibility. Here, we present a simple method with which the spectral diffuse downwelling attenuation coefficient can be estimated from in situ photographs of a color target, taken by an ordinary camera. This estimation is achieved through optimization contingent upon knowing the following: 1) the reflectance spectra of the color target patches, 2) the spectral response of the camera, and 3) the depths at which the photos are taken. We present results from nine locations in the northern Red Sea, which reasonably match ground truth values obtained using a commercial underwater radiometer. Our method utilizes low-cost parts that can be set up to be lowered from a boat, or carried by a diver. Data processing needs little computational resources, and in principle, the system can be configured as a real-time tool for monitoring pollution, blooms, climate change effects, etc., especially in coastal areas where ocean color data from satellites may be unavailable or operating profiling instruments may be logistically challenging. The resulting data can also be useful for validating satellite-derived estimates, where available, or supporting underwater computer vision applications.