LAK9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.46232\,\lambda^2}{\lambda^2 - 0.0072427} + \frac{0.3444\,\lambda^2}{\lambda^2 - 0.0243353} + \frac{1.15508\,\lambda^2}{\lambda^2 - 85.4687}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.11 \times 10^{-6} + 2(1.11 \times 10^{-8})\,\Delta T + 3(1.82 \times 10^{-12})\,\Delta T^2 + \frac{4.74 \times 10^{-7} + 2(-3.47 \times 10^{-10})\,\Delta T}{\lambda - 0.146}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$