X

L_e, radiance
λ, wavelength in vacuum
T, thermodynamic temperature
c₁ = 2πhс²_o
c₂ =hс_o/k
h, Planck's constant
c_o, velocity (speed) of light in vacuum
k, the Boltzmann constant

NOTE 1 – The formula is sometimes written with X instead of X, 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.