The objective of this process is to derive several ocean colour products characterising the transparency from the spectrum of normalised water-leaving reflectances.
Within the Instrument Processing Facility (IPF), the only product is the diffuse attenuation coefficient at 490 nm, expressed in units of m-1.
The algorithm applied here is OK2-560 as described in Morel et al, 2007. It is derived as the sum of the (hypothetical) value of the diffuse attenuation coefficient for pure seawater Kw(490) and a contribution linked to the biological state of the water. The latter is expressed, in log-transform, as a polynomial of the ratio of irradiance-reflectance between 490 and 560 nm, in a very similar way to the OC4Me Chlorophyll retrieval algorithm. Since it is derived using the same bio-geophysical approach, it has the same validity domain: Case 1 waters.

K_d(490) = K_w(490) + K_bio(490) with log₁₀(K_bio(490)) = 

Reference:Morel et al., 2007
Morel A, Huot Y, Gentili B, Werdell PJ, Hooker SB, et al. (2007). Examining the consistency of products derived from various ocean sensors in open ocean (Case 1) waters in the perspective of a multi-sensor approach. Remote Sens. Environ. 111:69–88