About 10% of the total light detected by a satellite aimed at the ocean is water-leaving radiance, while the other 90% of the light is due to atmospheric effects.
Corrections must be applied to the data to remove this atmospheric radiance (See Flowchart of MODIS atmospheric correction). Once the radiance signal has been corrected for atmospheric light scattering, the signal is then corrected for the solar zenith angle. This gives us the normalized water leaving radiance, which is the radiance that would be measured exiting the flat surface of the ocean with the Sun at zenith (directly overhead) and the atmosphere absent. The normalized water-leaving radiances are then used in algorithms to produce geophysical values, such as cholorphyll concentration.
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Further reading
Validation results
REFERENCE: Gordon, H. R., and Wang, M. (1994), Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with
SeaWiFS: a preliminary algorithm. Appl. Opt. 33:443-452.
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