SupportGlossary Optical Processes & Properties 

Attenuation Coefficient, Diffuse (Kd)

More precisely known as the Irradiance Attenuation Coefficient, this quantifies the attenuation of plane irradiance under solar illumination.  This is often referred to as an apparent optical property because it depends not only on the water properties but also on the sun angle, sky conditions, depth, and shadowing by the measurement platform or the measuring instrument itself.  However it has been shown that at great enough depths, Kd reaches an asymptotic value that depends only on the properties of the water.  This asymptotic Kd can be considered an inherent optical property.
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