N-SF8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.55076\,\lambda^2}{\lambda^2 - 0.0114338} + \frac{0.209817\,\lambda^2}{\lambda^2 - 0.0582726} + \frac{1.46205\,\lambda^2}{\lambda^2 - 133.242}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.94 \times 10^{-6} + 2(9.7 \times 10^{-9})\,\Delta T + 3(-2.34 \times 10^{-11})\,\Delta T^2 + \frac{8.32 \times 10^{-7} + 2(1.15 \times 10^{-9})\,\Delta T}{\lambda - 0.276}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$