SF5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.46142\,\lambda^2}{\lambda^2 - 0.0111826} + \frac{0.247713\,\lambda^2}{\lambda^2 - 0.0508595} + \frac{0.949996\,\lambda^2}{\lambda^2 - 112.042}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.59 \times 10^{-6} + 2(1.76 \times 10^{-8})\,\Delta T + 3(-2.03 \times 10^{-11})\,\Delta T^2 + \frac{1.17 \times 10^{-6} + 2(1.09 \times 10^{-9})\,\Delta T}{\lambda - 0.255}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$