KZFSN2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.2617\,\lambda^2}{\lambda^2 - 0.0076742} + \frac{0.128881\,\lambda^2}{\lambda^2 - 0.033013} + \frac{0.898521\,\lambda^2}{\lambda^2 - 70.8271}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.07 \times 10^{-6} + 2(1.41 \times 10^{-8})\,\Delta T + 3(-1.91 \times 10^{-11})\,\Delta T^2 + \frac{3.1 \times 10^{-7} + 2(4.28 \times 10^{-10})\,\Delta T}{\lambda - 0.251}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$