ZF4 by CDGM

$$n(\lambda) = \sqrt{2.89235 - 0.0179339\,\lambda^2 + 0.0230095\,\lambda^{-2} + 0.00563906\,\lambda^{-4} - 0.000698155\,\lambda^{-6} + 5.2529 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.94 \times 10^{-6} + 2(1.2 \times 10^{-8})\,\Delta T + 3(-6.11 \times 10^{-11})\,\Delta T^2 + \frac{8.57 \times 10^{-7} + 2(1.51 \times 10^{-9})\,\Delta T}{\lambda - 0.293}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$