N-LAK12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.17366\,\lambda^2}{\lambda^2 - 0.00577032} + \frac{0.588992\,\lambda^2}{\lambda^2 - 0.0200402} + \frac{0.978014\,\lambda^2}{\lambda^2 - 95.4873}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.67 \times 10^{-6} + 2(8.27 \times 10^{-9})\,\Delta T + 3(1.27 \times 10^{-12})\,\Delta T^2 + \frac{5.25 \times 10^{-7} + 2(6.3 \times 10^{-10})\,\Delta T}{\lambda - 0.162}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$