Formally, the wavelength version of Wien's displacement law states that the spectral radiance of black-body radiation per unit wavelength, peaks at the wavelength λ λ peak given by: λ λ peak = b T where T is the absolute temperature and b is a constant of proportionality called Wien's displacement constant, equal to 2.897 771 955... × 10 m⋅K, ‍ or b ≈ 2898 μm ⋅K.