LAF2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.75302\,\lambda^2}{\lambda^2 - 0.00957243} + \frac{0.213979\,\lambda^2}{\lambda^2 - 0.0402787} + \frac{1.09939\,\lambda^2}{\lambda^2 - 102.533}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.93 \times 10^{-6} + 2(9.19 \times 10^{-9})\,\Delta T + 3(-1.65 \times 10^{-11})\,\Delta T^2 + \frac{6.2 \times 10^{-7} + 2(7.5 \times 10^{-10})\,\Delta T}{\lambda - 0.226}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$