LAFN8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.75346\,\lambda^2}{\lambda^2 - 0.0102515} + \frac{0.179854\,\lambda^2}{\lambda^2 - 0.0466225} + \frac{1.03073\,\lambda^2}{\lambda^2 - 86.101}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.51 \times 10^{-6} + 2(6.04 \times 10^{-9})\,\Delta T + 3(-1.33 \times 10^{-12})\,\Delta T^2 + \frac{6.38 \times 10^{-7} + 2(8.78 \times 10^{-10})\,\Delta T}{\lambda - 0.279}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$