code: def _planck_spi(self, lam, Teff):
          
  
          hclkt = self.h*self.c/lam/self.k/Teff
          expterm = np.exp(hclkt)
          return hclkt * expterm/(expterm-1)
docs: 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