code: def clearness_index_zenith_independent(clearness_index, airmass,
                                         max_clearness_index=2.0):
      
      # Perez eqn 1
      kt_prime = clearness_index / _kt_kt_prime_factor(airmass)
      kt_prime = np.maximum(kt_prime, 0)
      kt_prime = np.minimum(kt_prime, max_clearness_index)
      return kt_prime
docs: Calculate the zenith angle independent clearness index.
  
      Parameters
      ----------
      clearness_index : numeric
          Ratio of global to extraterrestrial irradiance on a horizontal
          plane
  
      airmass : numeric
          Airmass
  
      max_clearness_index : numeric, default 2.0
          Maximum value of the clearness index. The default, 2.0, allows
          for over-irradiance events typically seen in sub-hourly data.
          NREL's SRRL Fortran code used 0.82 for hourly data.
  
      Returns
      -------
      kt_prime : numeric
          Zenith independent clearness index
  
      References
      ----------
      .. [1] Perez, R., P. Ineichen, E. Maxwell, R. Seals and A. Zelenka,
             (1992). "Dynamic Global-to-Direct Irradiance Conversion Models".
             ASHRAE Transactions-Research Series, pp. 354-369