By integrating B ν ν ( T ) cos ⁡ ⁡ ( θ θ ) over the frequency the radiance L (units: power / [area × solid angle]) is L = 2 π π 5 15 k 4 T 4 c 2 h 3 1 π π = σ σ T 4 cos ⁡ ⁡ ( θ θ ) π π by using ∫ ∫ 0 ∞ ∞ d x x 3 e x − − 1 = π π 4 15 with x ≡ ≡ h ν ν k T and with σ σ ≡ ≡ 2 π π 5 15 k 4 c 2 h 3 = 5.670373 × × 10 − − 8 W m 2 K 4 being the Stefan–Boltzmann constant.