The Equation of Radiative Transfer

Let j_ν [erg g^-1 s^-1 Hz^-1] be the mass emission coefficient.  It represents the radiant energy emitted, per unit mass of emitting material, per second, per unit frequency interval.  In most cases of interest, the emission is isotropic.

Let κ_ν [cm² g^-1] be the mass absorption coefficient.  It represents the absorption cross section, per unit mass of emitting material.  In most cases of interest κ_ν is also isotropic.