${\displaystyle L_{\text{e},\lambda }(\lambda ,T)=\frac{\partial L_{\text{e}}(\lambda ,T)}{\partial \lambda }=\frac{c_1}{\pi }{\lambda }^{-5}{({\text{e}}^{\frac{c_2}{\lambda T}}-1)}^{-1}}$

where L_e,λ is the spectral radiance, λ is the wavelength in vacuum, T is the 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 n²L_e,λ(λ, T).

Note 3 to entry: Planck's law can also be expressed to give the 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: Both quantities (radiance and radiant exitance) apply to the unpolarized radiation as emitted.

Note 5 to entry: For the up-to-date values of the Planck constant, h, the speed of light in vacuum, c₀, and the Boltzmann constant, k, see CODATA.

Note 6 to entry: This entry was numbered 845-04-05 in IEC 60050-845:1987.