The Stefan–Boltzmann law may be expressed as a formula for radiance as a function of temperature. Radiance is measured in watts per square metre per steradian (W⋅m ⋅sr). The Stefan–Boltzmann law for the radiance of a black body is: L Ω Ω ∘ ∘ = M ∘ ∘ π π = σ σ π π T 4 .