In terms of radiation intensity (I ph,ω = u ph f ph ħω ph D ph,ω /4 π, D ph,ω : photon density of states), this is called the equation of radiative transfer (ERT) ∂ ∂ I p h , ω ω ( ω ω p h , s ) u p h ∂ ∂ t + s ⋅ ⋅ ∇ ∇ I p h , ω ω ( ω ω p h , s ) = ∂ ∂ I p h , ω ω ( ω ω p h , s ) u p h ∂ ∂ t | s + σ σ p h , ω ω [ I p h , ω ω ( ω ω p h , T ) − − I p h ( ω ω p h , s ) ] + s ˙ ˙ p h , i . The net radiative heat flux vector is q r = q p h = ∫ ∫ 0 ∞ ∞ ∫ ∫ 4 π π s I p h , ω ω d Ω Ω d ω ω .