SF12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.39867\,\lambda^2}{\lambda^2 - 0.0103416} + \frac{0.238404\,\lambda^2}{\lambda^2 - 0.0492697} + \frac{1.03604\,\lambda^2}{\lambda^2 - 118.328}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.51 \times 10^{-6} + 2(1.64 \times 10^{-8})\,\Delta T + 3(-4.23 \times 10^{-11})\,\Delta T^2 + \frac{1.03 \times 10^{-6} + 2(1.02 \times 10^{-9})\,\Delta T}{\lambda - 0.254}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$