LAF9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.75304\,\lambda^2}{\lambda^2 - 0.012525} + \frac{0.342146\,\lambda^2}{\lambda^2 - 0.0547228} + \frac{1.04243\,\lambda^2}{\lambda^2 - 104.326}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.3 \times 10^{-6} + 2(1.42 \times 10^{-8})\,\Delta T + 3(-1.45 \times 10^{-10})\,\Delta T^2 + \frac{1.48 \times 10^{-6} + 2(1.34 \times 10^{-9})\,\Delta T}{\lambda - 0.262}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$