LAK21 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.22718\,\lambda^2}{\lambda^2 - 0.00602076} + \frac{0.420784\,\lambda^2}{\lambda^2 - 0.0196863} + \frac{1.01285\,\lambda^2}{\lambda^2 - 88.437}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.36 \times 10^{-6} + 2(1.15 \times 10^{-8})\,\Delta T + 3(1.11 \times 10^{-11})\,\Delta T^2 + \frac{3.1 \times 10^{-7} + 2(2.78 \times 10^{-10})\,\Delta T}{\lambda - 0.234}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$