def Planck_wavenumber(n, T):
<EOL>n = n*<NUM_LIT><EOL>return c_light * Planck_frequency(n*c_light, T)<EOL>
The Planck function (flux density for blackbody radition)
    in wavenumber space
    n is wavenumber in 1/cm
    T is temperature in Kelvin

    Formula from Raymond Pierrehumbert, "Principles of Planetary Climate", page 140.