N-LAK8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.33183\,\lambda^2}{\lambda^2 - 0.00620024} + \frac{0.546623\,\lambda^2}{\lambda^2 - 0.0216465} + \frac{1.19084\,\lambda^2}{\lambda^2 - 82.5828}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.1 \times 10^{-6} + 2(1.25 \times 10^{-8})\,\Delta T + 3(-1.6 \times 10^{-11})\,\Delta T^2 + \frac{4.3 \times 10^{-7} + 2(6.29 \times 10^{-10})\,\Delta T}{\lambda - 0.213}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$