Planck's law

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

where

*L*_{e} is the radiance;

*λ* is the wavelength in vacuum;

*T* is the thermodynamic temperature;

*h* is the Planck constant;

*c*_{0} is the velocity (speed) of light in vacuum;

*k* is the Boltzmann constant

NOTE 1 The formula is sometimes written with _{} instead of _{}, where *Ω*_{0} is the solid angle of magnitude 1 sr.

NOTE 2 For a detector in a medium of refractive index *n*, the measured radiance is: *n*^{-2}*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 is then *c*_{1} instead of _{}.

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

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