Wien's law,

approximate form of Planck's law, valid with an approximation better than one part in 1 000 when the product λT is smaller than 0,002 m·K

where L_e,λ is spectral radiance, λ is the wavelength in vacuum, T is thermodynamic temperature, , c₂ = h c₀ / k, h is the Planck constant, c₀ is the speed of light in vacuum, and k is the Boltzmann constant

Note 1 to entry: The formula is sometimes written with instead of , where Ω₀ is the solid angle of magnitude 1 sr.

Note 2 to entry: For a detector in a medium of refractive index n, the measured radiance is:
n^-2L_e,λ(λ, T).

Note 3 to entry: Wien's law can also be expressed to give the approximate spectral distribution of radiant exitance, M_e,λ(λ, T); the first factor in the formula is then c₁ instead of .

Note 4 to entry: The quantities radiance, as referred to in Note 2, and radiant exitance, as referred to in Note 3, apply to the unpolarized radiation as emitted.

Note 5 to entry: This entry was numbered 845-04-06 in IEC 60050-845:1987.

Note 6 to entry: This entry was numbered 17-1435 in CIE S 017:2011.