code: def _get_cct(x, y, z):
          
          x_e = 0.3320
          y_e = 0.1858
  
          n = ((x / (x + z + z)) - x_e) / ((y / (x + z + z)) - y_e)
  
          a_0 = -949.86315
          a_1 = 6253.80338
          a_2 = 28.70599
          a_3 = 0.00004
  
          t_1 = 0.92159
          t_2 = 0.20039
          t_3 = 0.07125
  
          cct = a_0 + a_1 * numpy.exp(-n / t_1) + a_2 * numpy.exp(-n / t_2) + a_3 * numpy.exp(-n / t_3)
          return cct
docs: Reference
          Hernandez-Andres, J., Lee, R. L., & Romero, J. (1999).
          Calculating correlated color temperatures across the entire gamut of daylight and skylight chromaticities.
          Applied Optics, 38(27), 5703-5709.