LAK11 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.05088\,\lambda^2}{\lambda^2 - 0.00482822} + \frac{0.651491\,\lambda^2}{\lambda^2 - 0.0183962} + \frac{1.03286\,\lambda^2}{\lambda^2 - 89.8416}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.8 \times 10^{-6} + 2(1 \times 10^{-8})\,\Delta T + 3(-5.32 \times 10^{-12})\,\Delta T^2 + \frac{4.33 \times 10^{-7} + 2(6.62 \times 10^{-10})\,\Delta T}{\lambda - 0.206}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$