Planck's law

law giving the spectral distribution of radiance of a Planckian radiator as a function of wavelength and temperature

where L_e,λ is the spectral radiance, λ is the wavelength in vacuum, T is the 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:
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 .

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.

Note 7 to entry: This entry was numbered 17-958 in CIE S 017:2011.