LAKN13 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.25792\,\lambda^2}{\lambda^2 - 0.00621495} + \frac{0.553403\,\lambda^2}{\lambda^2 - 0.021681} + \frac{1.06336\,\lambda^2}{\lambda^2 - 104.494}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.73 \times 10^{-6} + 2(1.17 \times 10^{-8})\,\Delta T + 3(-2.39 \times 10^{-11})\,\Delta T^2 + \frac{3.95 \times 10^{-7} + 2(4.43 \times 10^{-10})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$