Another way of characterizing the radiance distribution is via the mean photon energy ⟨ ⟨ E phot ⟩ ⟩ = π π 4 30 ζ ζ ( 3 ) k T ≈ ≈ ( 3.7294 × × 10 − − 23 J / K ) ⋅ ⋅ T , where ζ ζ is the Riemann zeta function. The wavelength corresponding to the mean photon energy is given by λ λ ⟨ ⟨ E ⟩ ⟩ ≈ ≈ ( 0.532 65 c m ⋅ ⋅ K ) / T .