N-KZFS2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.23698\,\lambda^2}{\lambda^2 - 0.00747171} + \frac{0.153569\,\lambda^2}{\lambda^2 - 0.0308054} + \frac{0.903976\,\lambda^2}{\lambda^2 - 70.1731}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.77 \times 10^{-6} + 2(1.31 \times 10^{-8})\,\Delta T + 3(-1.23 \times 10^{-11})\,\Delta T^2 + \frac{3.84 \times 10^{-7} + 2(5.51 \times 10^{-10})\,\Delta T}{\lambda - 0.196}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$