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

L_e, radiance
λ, wavelength in vacuum
T, thermodynamic temperature
c₁ = 2πhс²₀
c₂ =hс₀/k
h, Planck's constant
c₀, velocity (speed) of light in vacuum
k, the Boltzmann constant

NOTE 1 – 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 steradian.

NOTE 2 – For a detector in a medium of refractive index n, the measured radiance is n² L_{e λ}(λ, T)

NOTE 3 – Planck's law can be also expressed to give the spectral concentration of radiant exitance M_{e, λ}(λ, T); the first factor in the formula (1), is then c₁ instead of c₁/π.

NOTE 4 – Both quantities (radiance and exitance) apply to the unpolarized radiation as emitted.