LAFN7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.66843\,\lambda^2}{\lambda^2 - 0.010316} + \frac{0.298513\,\lambda^2}{\lambda^2 - 0.0469216} + \frac{1.07744\,\lambda^2}{\lambda^2 - 82.5079}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.27 \times 10^{-6} + 2(1.31 \times 10^{-8})\,\Delta T + 3(-3.32 \times 10^{-11})\,\Delta T^2 + \frac{8.88 \times 10^{-7} + 2(9.32 \times 10^{-10})\,\Delta T}{\lambda - 0.248}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$