The radiant exitance of a black surface, according to the Stefan–Boltzmann law, is equal to: M e ∘ ∘ = σ σ T 4 , where σ is the Stefan–Boltzmann constant, and T is the temperature of that surface. For a real surface, the radiant exitance is equal to: M e = ε ε M e ∘ ∘ = ε ε σ σ T 4 , where ε is the emissivity of that surface.