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Plank's law describes the amount of electromagnetic energy with a certain wavelength radiated by a black body in thermal equilibrium.

spectral radiance has units of:

$ B _\nu (T) = \frac {[energy]}{[time][distance]^2[solidangle][Hz]} $

$ B _\lambda (T) = \frac {[energy]}{[time][distance]^2[solidangle][Angstrom]} $

The actual equations are of the form:

$ B _\nu (T) = \frac {2h {\nu}^3}{c^2} \frac {1}{e^{h\nu /(kT)} - 1} $

$ B _\lambda (T) = \frac {2hc^2}{\lambda^5} \frac {1}{e^{hc/(\lambda kT)} - 1} $