Calculates light absorption as a function of depth in clear water, using the mean absorption coefficient for pure water derived from Pope & Fry 1997 Absorption spectrum ~380–700 nm of pure water. II. Integrating cavity measurements. Effectively, this assumes that attenuation is solely a function of absorption from pure water. In other words, the irradiance attenuation coefficient (Kd) will be equal to just the absorption coefficient for pure water.