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The **Wien approximation** is a formula that approximates
black-body radiation, converging to the true value
(the Planck function) at shorter wavelengths, i.e.,
a good approximation of one of the two tails of the spectrum.
The Wien approximation has the advantage of being a simpler function,
easier to manipulate algebraically and incorporate into
formulae describing astronomical phenomena. It consists of
the Planck function with the denominator's "subtraction of 1"
removed: at short wavelengths, this fraction
is quite small and such a modification to the denominator
affects the total only slightly.
The *Wien approximation* and the Rayleigh-Jeans law
are analogous, each approximating one end of a black body spectrum.
The *Wien approximation* formula (in the form based on frequency):

- B(ν,T) - spectral radiance (specific intensity).
- ν - wavelength.
- h - Planck constant.
- T - temperature.
- c - speed of light in a vacuum.
- k
_{B}- Boltzmann constant.

2hν^{3}1 B(ν,T) = ———— ———————— c^{2}e^{hν/(kBT)}

http://en.wikipedia.org/wiki/Wien_approximation

Rayleigh-Jeans law