LAK31 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.41825\,\lambda^2}{\lambda^2 - 0.00679115} + \frac{0.409404\,\lambda^2}{\lambda^2 - 0.0219285} + \frac{1.15432\,\lambda^2}{\lambda^2 - 80.3981}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.3 \times 10^{-6} + 2(1.43 \times 10^{-8})\,\Delta T + 3(-8.71 \times 10^{-12})\,\Delta T^2 + \frac{5.25 \times 10^{-7} + 2(3.06 \times 10^{-10})\,\Delta T}{\lambda - 0.11}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$