def _planck_spi(self, lam, Teff):
        """
        Computes the spectral index of the monochromatic blackbody intensity
        using the Planck function. The spectral index is defined as:

            B(lambda) = 5 + d(log I)/d(log lambda),

        where I is the Planck function.

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

        Returns: the spectral index of monochromatic blackbody intensity
        """

        hclkt = self.h*self.c/lam/self.k/Teff
        expterm = np.exp(hclkt)
        return hclkt * expterm/(expterm-1)