def _planck(self, lam, Teff):
        """
        Computes monochromatic blackbody intensity in W/m^3 using the
        Planck function.

        @lam: wavelength in m
        @Teff: effective temperature in K

        Returns: monochromatic blackbody intensity
        """

        return 2*self.h*self.c*self.c/lam**5 * 1./(np.exp(self.h*self.c/lam/self.k/Teff)-1)