LASFN9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.97888\,\lambda^2}{\lambda^2 - 0.0118537} + \frac{0.320435\,\lambda^2}{\lambda^2 - 0.0527382} + \frac{1.92901\,\lambda^2}{\lambda^2 - 166.257}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[9.44 \times 10^{-7} + 2(1.14 \times 10^{-8})\,\Delta T + 3(-1.87 \times 10^{-11})\,\Delta T^2 + \frac{9.22 \times 10^{-7} + 2(1.22 \times 10^{-9})\,\Delta T}{\lambda - 0.255}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$