LAF13 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.8114\,\lambda^2}{\lambda^2 - 0.0106588} + \frac{0.248609\,\lambda^2}{\lambda^2 - 0.0467089} + \frac{1.10754\,\lambda^2}{\lambda^2 - 91.6858}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.11 \times 10^{-6} + 2(1.25 \times 10^{-8})\,\Delta T + 3(-1.31 \times 10^{-10})\,\Delta T^2 + \frac{7.86 \times 10^{-7} + 2(5.57 \times 10^{-10})\,\Delta T}{\lambda - 0.252}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$