KZFS1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.36618\,\lambda^2}{\lambda^2 - 0.00873165} + \frac{0.182049\,\lambda^2}{\lambda^2 - 0.0389834} + \frac{0.864315\,\lambda^2}{\lambda^2 - 62.9392}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.96 \times 10^{-6} + 2(1.39 \times 10^{-8})\,\Delta T + 3(-2.64 \times 10^{-11})\,\Delta T^2 + \frac{4.26 \times 10^{-7} + 2(5.19 \times 10^{-10})\,\Delta T}{\lambda - 0.242}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$