From WeoGeo Support

Remote Sensing Reflectance

Remote Sensing Reflectance (R_rs) is a remote sensing term most often used in marine and oceanic remote sensing. It is the ratio of Water Leaving Radiance (L_w) and downwelling Irradiance (E_d).

Atmospheric corrections algorithms, like TAFFKA, often employ a land mask so to not remove a non existing surface effect from terrestrial targets. Thus over land, R_rs would turn out to the ratio of upwelling Radiance (L_u) and downwelling Irradiance (E_d).

R_rs is often multiplied by a factor of π to convert it to the more widely accepted remote sensing term Reflectance. However, this assumption fails for targets that do not approximate a Lambertian surface.