Taking into account the independence of direction of the spectral radiance of radiation from the surface of a black body in thermodynamic equilibrium, one has L (dA, dν) = B ν (T) and so d Φ Φ ( d A , θ θ , d Ω Ω , d ν ν ) d Ω Ω = B ν ν ( T ) d A d ν ν cos ⁡ ⁡ θ θ .