N-SF2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.47343\,\lambda^2}{\lambda^2 - 0.0109019} + \frac{0.163682\,\lambda^2}{\lambda^2 - 0.0585684} + \frac{1.36921\,\lambda^2}{\lambda^2 - 127.405}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.1 \times 10^{-6} + 2(1.75 \times 10^{-8})\,\Delta T + 3(6.62 \times 10^{-11})\,\Delta T^2 + \frac{7.51 \times 10^{-7} + 2(8.99 \times 10^{-10})\,\Delta T}{\lambda - 0.277}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$