Chlorophyll concentration, Chl, is defined by the "OC4Me" Maximum Band Ratio (MBR) semi-analytical algorithm, developed by Morel et al  (cf. OReilly et al. , for a more general description of such algorithms). It is the latest version of the MERIS pigment index algorithm, which is fully described in the MERIS ATBD 2.9 and in Morel et al . A brief reminder is provided here. OC4Me is a polynomial based on the use of a semi-analytical model, itself based on the analysis of AOPs measured in situ over the past decades in various oceanic regions  .
|It is expressed as: log10 [Chl]=|
is the ratio of irradiance-reflectance of band i, among 443, 490 and 510 nm, over that of band j at 560 nm. The band for the numerator is selected so that the ratio is maximised. The above computation is embedded within an iterative loop including the computation of the irradiance-reflectance from the directional water leaving reflectance that requires a bi-directionality correction relying on the Chl estimate. Convergence is reached when the difference between two successive Chl estimates is below a threshold.
 Morel, A., Gentili, B., Claustre, H., Babin, M., Bricaud, A., Ras, J., et al. (2007). Optical properties of the "clearest" natural waters. Limnology and Oceanography, 52, 217-229.
 O'Reilly, J. E., Maritorena, S., Mitchell, B. G., Siegel, D. A., Carder, K. L., Garver, A., et al. (1998). Ocean color algorithms for SeaWiFS. Journal of Geophysical Research, 103, 24937-24953.
 Morel, A., Huot, Y., Gentili, B., Werdell, P. J., Hooker, S. B., & Franz, B. A. (2007). Examining the consistency of products derived from various ocean color sensors in open ocean (Case 1) waters in the perspective of a multi-sensor approach. Remote Sensing of Environment, 111, 69-88.
Bricaud, A., Morel, A., Babin, M., Allali, K., & Claustre, H. (1998). Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters: Analysis and implications for bio-optical models. Journal of Geophysical Research-Oceans, 103, 31033-31044.
Morel, A., & Maritorena, S. (2001). Bio-optical properties of oceanic waters: A reappraisal. Journal of Geophysical Research-Oceans, 106, 7163-7180