Experimental evaluation of the self-shadow and its correction for on-water measurements of water-leaving radiance.

ABSTRACT

Accurate determination of the water-leaving radiance (Lw) 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 Lw in the field, the skylight-blocked approach (SBA) is the only scheme that can potentially measure Lw directly. However, the apparatus associated with an SBA system will introduce self-shading effects to the measured Lw, which is required to be corrected for an accurate Lw determination. In this study, we experimentally evaluate several factors that could contribute to the self-shading effects of the SBA-measured Lw, including solar zenith angle (∼18∘-64∘), 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 ∼6.0m-1, the normalized root-mean-square difference between the SBA-measured Lw after shade correction and the "true" Lw is generally between ∼5% and ∼10% for wavelengths in the range of 400-750 nm. These results suggest that SBA can obtain highly accurate and precise Lw in nearly all natural aquatic environments.