def _planck_deriv(self, lam, Teff):
        """
        Computes the derivative of the monochromatic blackbody intensity using
        the Planck function.

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

        Returns: the derivative of monochromatic blackbody intensity
        """

        expterm = np.exp(self.h*self.c/lam/self.k/Teff)
        return 2*self.h*self.c*self.c/self.k/Teff/lam**7 * (expterm-1)**-2 * (self.h*self.c*expterm-5*lam*self.k*Teff*(expterm-1))
Computes the derivative of the monochromatic blackbody intensity using
        the Planck function.

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

        Returns: the derivative of monochromatic blackbody intensity