N-SF1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.60865\,\lambda^2}{\lambda^2 - 0.0119655} + \frac{0.237726\,\lambda^2}{\lambda^2 - 0.059059} + \frac{1.51531\,\lambda^2}{\lambda^2 - 135.522}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.72 \times 10^{-6} + 2(8.05 \times 10^{-9})\,\Delta T + 3(-1.71 \times 10^{-11})\,\Delta T^2 + \frac{8.98 \times 10^{-7} + 2(1.34 \times 10^{-9})\,\Delta T}{\lambda - 0.276}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$