LASF18A by Schott

$$n(\lambda) = \sqrt{1 + \frac{2.19337\,\lambda^2}{\lambda^2 - 0.0121931} + \frac{0.33259\,\lambda^2}{\lambda^2 - 0.053252} + \frac{1.6985\,\lambda^2}{\lambda^2 - 106.853}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.87 \times 10^{-6} + 2(1.15 \times 10^{-8})\,\Delta T + 3(-3.11 \times 10^{-11})\,\Delta T^2 + \frac{8.68 \times 10^{-7} + 2(1.23 \times 10^{-9})\,\Delta T}{\lambda - 0.254}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$