In studies of aquatic environments, an optical property, KPAR, which represents the diffuse attenuation coefficient of Photosynthetic Available Radiation (PAR), is used frequently. Because water's diffuse attenuation coefficient is highly spectral dependent and PAR is spectrally narrowing to wavelengths with less attenuation coefficients with increasing depth, this KPAR is significantly depth dependent in the upper water column even for well-mixed waters. In this article, with an aim for more reliable attenuation products and more accurate description of PAR profiles, the ambiguity associated with KPAR is highlighted, and the proper representation of the vertical variation of KPAR is advocated.