TF3 by CDGM

$$n(\lambda) = \sqrt{2.54142 - 0.0106608\,\lambda^2 + 0.0217167\,\lambda^{-2} - 0.000401848\,\lambda^{-4} + 0.000122476\,\lambda^{-6} - 4.77764 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.03 \times 10^{-6} + 2(2.69 \times 10^{-8})\,\Delta T + 3(-1.67 \times 10^{-10})\,\Delta T^2 + \frac{5.52 \times 10^{-7} + 2(-6.81 \times 10^{-10})\,\Delta T}{\lambda - 0.246}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$