N-SF64 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.59164\,\lambda^2}{\lambda^2 - 0.0118623} + \frac{0.219908\,\lambda^2}{\lambda^2 - 0.0594585} + \frac{1.46929\,\lambda^2}{\lambda^2 - 133.311}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.06 \times 10^{-6} + 2(9.78 \times 10^{-9})\,\Delta T + 3(-1.67 \times 10^{-11})\,\Delta T^2 + \frac{8.67 \times 10^{-7} + 2(1.23 \times 10^{-9})\,\Delta T}{\lambda - 0.279}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$