${\displaystyle L_{\text{e}\text{,}\lambda }\text{(}\lambda ,T\text{)}=\frac{c_1}{\pi }{\lambda }^{-5}{\text{e}}^{\frac{c_2}{\lambda T}}}$

where L_e,λ is spectral radiance, λ is the wavelength in vacuum, T is thermodynamic temperature, $c_1 =2 \pi h c_0^2$, $c_2 =h c_0 / k$, 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 ${\displaystyle \frac{c_1}{\pi {\Omega }_0}}$ instead of ${\displaystyle \frac{c_1}{\pi }}$, 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 ${\displaystyle \frac{c_1}{\pi }}$.

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

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