Abstract

Accurate determination of the water-leaving radiance ($ L_w $) is key to correctly interpret in-water optical properties and to validate the atmospheric correction schemes in ocean color studies. Among the various approaches adopted to measure $ L_w $ in the field, the skylight-blocked approach (SBA) is the only scheme that can potentially measure $ L_w $ directly. However, the apparatus associated with an SBA system will introduce self-shading effects to the measured $ L_w $, which is required to be corrected for an accurate $ L_w $ determination. In this study, we experimentally evaluate several factors that could contribute to the self-shading effects of the SBA-measured $ L_w $, including solar zenith angle (${\sim}{18}^\circ {-} 64^\circ$), water’s optical properties, and cone size (radius of 22 mm and 45 mm). For waters with the total absorption coefficient at 440 nm as high as ${\sim}{6.0}\;{{\rm m}^{- 1}}$, the normalized root-mean-square difference between the SBA-measured $ L_w $ after shade correction and the “true” $ L_w $ is generally between ${\sim}{5}\%$ and ${\sim}{10}\%$ for wavelengths in the range of 400–750 nm. These results suggest that SBA can obtain highly accurate and precise $ L_w $ in nearly all natural aquatic environments.

© 2020 Optical Society of America

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