ZF6 by CDGM

$$n(\lambda) = \sqrt{2.96286 - 0.0104078\,\lambda^2 + 0.0356441\,\lambda^{-2} + 0.00245699\,\lambda^{-4} - 0.000149616\,\lambda^{-6} + 1.77828 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.04 \times 10^{-6} + 2(2.33 \times 10^{-8})\,\Delta T + 3(-5.02 \times 10^{-11})\,\Delta T^2 + \frac{1.58 \times 10^{-6} + 2(9.46 \times 10^{-10})\,\Delta T}{\lambda - 0.26}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$