The $\textit{Planck function}$ is used to compute the radiance emitted from objects that radiate like a perfect "Black Body". The inverse of the $\textit{Planck Function}$ is used to find the $\textit{Brightness Temperature}$ of an object. The precise formula for the Planck Function depends on whether the radiance is determined on a $\textit{per unit wavelength}$ or a $\textit{per unit frequency}$. In the ISO System of Quantities, $\textit{Planck Function}$ is defined by the formula: $Y = -G/T$, where $G$ is Gibbs Energy and $T$ is thermodynamic temperature.
http://pds-atmospheres.nmsu.edu/education_and_outreach/encyclopedia/planck_function.htm
http://www.star.nesdis.noaa.gov/smcd/spb/calibration/planck.html
The Planck function, $B_{\tilde{\nu}}(T)$, is given by:
$B_{\nu}(T) = \frac{2h c^2\tilde{\nu}^3}{e^{hc / k \tilde{\nu} T}-1}$
where, $\tilde{\nu}$ is wavelength, $h$ is Planck's Constant, $k$ is Boltzman's Constant, $c$ is the speed of light in a vacuum, $T$ is thermodynamic temperature.
http://www.iso.org/iso/catalogue_detail?csnumber=31890