where h is the Planck constant, ν is the frequency, λ is the wavelength, k is the Boltzmann constant, c is the speed of light in the medium, T is the temperature of that surface. For a real surface, the spectral exitance is equal to: M e , ν ν = ε ε M e , ν ν ∘ ∘ = 2 π π h ε ε ν ν 3 c 2 1 e h ν ν k T − − 1 , M e , λ λ = ε ε M e , λ λ ∘ ∘ = 2 π π h ε ε c 2 λ λ 5 1 e h c λ λ k T − − 1 .