LASF36A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.8687\,\lambda^2}{\lambda^2 - 0.011266} + \frac{0.257335\,\lambda^2}{\lambda^2 - 0.0509972} + \frac{1.74622\,\lambda^2}{\lambda^2 - 149.125}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.68 \times 10^{-6} + 2(1.04 \times 10^{-8})\,\Delta T + 3(-2.33 \times 10^{-11})\,\Delta T^2 + \frac{8.35 \times 10^{-7} + 2(1.19 \times 10^{-9})\,\Delta T}{\lambda - 0.251}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$