If the radiation field is in equilibrium with the material medium, then the radiation will be homogeneous (independent of position) so that dI ν = 0 and: κ κ ν ν B ν ν = j ν ν which is another statement of Kirchhoff's law, relating two material properties of the medium, and which yields the radiative transfer equation at a point around which the medium is in thermodynamic equilibrium: d I ν ν d s = κ κ ν ν ρ ρ ( B ν ν − − I ν ν ) .