In the fractional bandwidth formulation, x = h ν ν k B T = h c λ λ k B T , and the integration is with respect to d ( ln ⁡ ⁡ x ) = d ( ln ⁡ ⁡ ν ν ) = d ν ν ν ν = − − d λ λ λ λ = − − d ( ln ⁡ ⁡ λ λ ) .