The total power emitted per unit area at the surface of a black body (P) may be found by integrating the black body spectral flux found from Lambert's law over all frequencies, and over the solid angles corresponding to a hemisphere (h) above the surface. P = ∫ ∫ 0 ∞ ∞ d ν ν ∫ ∫ h d Ω Ω B ν ν cos ⁡ ⁡ ( θ θ )