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The Planck function is a function of wavelength (or frequency) and temperature that describes a distribution of the intensity (aka radiance) at each wavelength or frequency (i.e., the specific intensity aka spectral radiance) in black-body radiation. The form for frequency:
2hν3 1 B(ν,T) = ———— ————————————— c2 ehν/(kBT) - 1
Note that the formula for the Plank function for the distribution over wavelength does not simply plug in c/wavelength into the above formula: the relation of their differentials must be taken into account.
Sometimes the equation is cited including π as an additional factor: the form with π produces the total energy emitted per unit area of the black body per wavelength or frequency, i.e., the spectral flux density a unit area produces.
Much of astrophysics uses the term intensity for the quantity of radiation emitted through a surface into a given solid angle, specific intensity being the radiance at a given wavelength or frequency. More common terms are radiance and spectral radiance.