KZFS12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.55625\,\lambda^2}{\lambda^2 - 0.0102013} + \frac{0.239769\,\lambda^2}{\lambda^2 - 0.0469278} + \frac{0.947888\,\lambda^2}{\lambda^2 - 69.8371}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.36 \times 10^{-6} + 2(1.32 \times 10^{-8})\,\Delta T + 3(-1.81 \times 10^{-11})\,\Delta T^2 + \frac{6.86 \times 10^{-7} + 2(6.81 \times 10^{-10})\,\Delta T}{\lambda - 0.253}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$