Because apparent/TOA reflectance (which is dimensionless) and remote sensing reflectance from ocean color product (inverse sterdian).
At the sea Level, apparent reflectance, also called, diffuse reflectance, is the ratio of the upwelling irradiance to the downwelling irradiance below the water surface, R(0-).
Remote sensing reflectance, is directionnal, and represent the upwelling radiance to the downwelling irradiance above water surface, Rrs=Lw(0+)/Ed(0+).
Lu(0-)/Eu(0-) is 1/Q where Q is the directionnal factor (see Loisel and MOrel, 1998).
So, Rrs = [R(0-)/Q] (t(w,a) t(aw)/n2) where n is the refractive index, and the t are the transmitance at the sea surface.
and by calculating you get Lw=0.544 Lu(0-).
You can have all these details in Mobley( Mobley, C.D., 1994. Light and Water: Radiative Transfer in Natural Waters.Academic Press, San Diego, CA.
Loisel and MOrel (2001) (you can get on my page)
hope it helps
Irrespective of the sensor used to collect data, apparent reflectance is the ratio of upwelling irradiance (Eu) to the downwelling irradiance (Ed). The units cancel out, which is why it is dimensionless. It is calculated using the measured TOA upwelling radiance (Lu), the solar flux (Eo), and the solar zenith angle (zo) as follows:
Apparent reflectance = (pi * Lu)/(cos(zo) * Eo)
Since the apparent reflectance is a TOA product, it includes atmospheric effects.
The remote sensing reflectance (Rrs) is the ratio of the upwelling radiance to the downwelling irradiance, and is calculated as follows:
Rrs = Lu/(cos(zo)*Eo)
The L2 Rrs product is a surface-level product, derived after some sort of atmospheric correction.
At the sea Level, apparent reflectance, also called, diffuse reflectance, is the ratio of the upwelling irradiance to the downwelling irradiance below the water surface, R(0-).
Remote sensing reflectance, is directionnal, and represent the upwelling radiance to the downwelling irradiance above water surface, Rrs=Lw(0+)/Ed(0+).
Lu(0-)/Eu(0-) is 1/Q where Q is the directionnal factor (see Loisel and MOrel, 1998).
So, Rrs = [R(0-)/Q] (t(w,a) t(aw)/n2) where n is the refractive index, and the t are the transmitance at the sea surface.
and by calculating you get Lw=0.544 Lu(0-).
You can have all these details in Mobley( Mobley, C.D., 1994. Light and Water: Radiative Transfer in Natural Waters.Academic Press, San Diego, CA.
Loisel and MOrel (2001) (you can get on my page)
hope it helps
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