LAFN21 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.88298\,\lambda^2}{\lambda^2 - 0.0093603} + \frac{0.240215\,\lambda^2}{\lambda^2 - 0.0354026} + \frac{1.23538\,\lambda^2}{\lambda^2 - 84.392}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.94 \times 10^{-6} + 2(1.01 \times 10^{-8})\,\Delta T + 3(-1.89 \times 10^{-11})\,\Delta T^2 + \frac{5.97 \times 10^{-7} + 2(7.69 \times 10^{-10})\,\Delta T}{\lambda - 0.193}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$