N-SF5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.52482\,\lambda^2}{\lambda^2 - 0.0112548} + \frac{0.187086\,\lambda^2}{\lambda^2 - 0.0588995} + \frac{1.42729\,\lambda^2}{\lambda^2 - 129.142}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.51 \times 10^{-7} + 2(1.07 \times 10^{-8})\,\Delta T + 3(-2.4 \times 10^{-11})\,\Delta T^2 + \frac{7.85 \times 10^{-7} + 2(1.15 \times 10^{-9})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$