LAKN6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.15477\,\lambda^2}{\lambda^2 - 0.00583258} + \frac{0.497049\,\lambda^2}{\lambda^2 - 0.0194614} + \frac{1.00554\,\lambda^2}{\lambda^2 - 97.8668}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.03 \times 10^{-6} + 2(1.02 \times 10^{-8})\,\Delta T + 3(-7.8 \times 10^{-12})\,\Delta T^2 + \frac{3.68 \times 10^{-7} + 2(5.89 \times 10^{-10})\,\Delta T}{\lambda - 0.229}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$