LAF3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.73156\,\lambda^2}{\lambda^2 - 0.00953834} + \frac{0.150874\,\lambda^2}{\lambda^2 - 0.0407887} + \frac{1.06587\,\lambda^2}{\lambda^2 - 98.0759}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.35 \times 10^{-6} + 2(1.07 \times 10^{-8})\,\Delta T + 3(-9.38 \times 10^{-12})\,\Delta T^2 + \frac{5.72 \times 10^{-7} + 2(6.01 \times 10^{-10})\,\Delta T}{\lambda - 0.22}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$