N-LAK34 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.26661\,\lambda^2}{\lambda^2 - 0.00589278} + \frac{0.665919\,\lambda^2}{\lambda^2 - 0.0197509} + \frac{1.12496\,\lambda^2}{\lambda^2 - 78.8894}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.96 \times 10^{-6} + 2(9.65 \times 10^{-9})\,\Delta T + 3(4.4 \times 10^{-12})\,\Delta T^2 + \frac{4.91 \times 10^{-7} + 2(5.28 \times 10^{-10})\,\Delta T}{\lambda - 0.161}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$