The definition of surface emissivity in thermal remote sensing

The conversion to surface temperature of radiometric observations of the Earth by spaceborne or airborne infrared sensors is necessary for numerous remote sensing applications. A common method of temperature retrieval relies upon inversion of an approximation to the equation of transfer which is linear in surface couplings, i.e., emissivities and reflectances. It has been recognized for years that the neglect of surface multiple scattering in this approach can cause significant errors. An adaptation of Cannons's method to this problem permits absorption of multiple scattering effects into "effective" couplings in the linearized equation of transfer. In the case of a homogeneous plane parallel atmosphere, the effective coupling constants can be expressed in terms of Chandrasekhar's X and Y functions. The resulting effective values can be used as a rough estimate of surface multiple scattering corrections for more realistic situations.