The BTE for photon particle momentum p ph = ħω ph s / u ph along direction s experiencing absorption/emission s ˙ ˙ f , p h − − e (= u ph σ ph,ω [ f ph (ω ph, T) - f ph (s)], σ ph,ω : spectral absorption coefficient), and generation/removal s ˙ ˙ f , p h , i , is ∂ ∂ f p h ∂ ∂ t + u p h s ⋅ ⋅ ∇ ∇ f p h = ∂ ∂ f p h ∂ ∂ t | s + u p h σ σ p h , ω ω [ f p h ( ω ω p h , T ) − − f p h ( s ) ] + s ˙ ˙ f , p h , i .