ZF2 by CDGM

$$n(\lambda) = \sqrt{2.7082 - 0.00794171\,\lambda^2 + 0.0292225\,\lambda^{-2} + 0.000884932\,\lambda^{-4} + 6.68621 \times 10^{-7}\,\lambda^{-6} + 5.65903 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.92 \times 10^{-7} + 2(2.05 \times 10^{-8})\,\Delta T + 3(2.6 \times 10^{-11})\,\Delta T^2 + \frac{1.28 \times 10^{-6} + 2(3.74 \times 10^{-10})\,\Delta T}{\lambda - 0.232}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$