Measuring water clarity with a black disk

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Abstract
Black targets have a major theoretical advantage over reflective targets such as the Secchi disk for assessing the visual clarity of water because their sighting ranges depend only on two optical properties of the water: the beam attenuation coefficient and the diffuse attenuation coefficient. The theory of visibility of black objects applies to observations made with a black disk in 11 rivers and eight lakes ranging 60‐fold in water clarity. The horizontal sighting range of the black disk is similar to visual ranges of practical importance in water, for aquatic animals as well as man, and yields an immediate estimate of the beam attenuation coefficient. The black disk is expected to be of particular value for assessing the clarity of river waters that are too shallow for deployment of the Secchi disk.

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