LAF20 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.65107\,\lambda^2}{\lambda^2 - 0.00970502} + \frac{0.118475\,\lambda^2}{\lambda^2 - 0.0428927} + \frac{1.11549\,\lambda^2}{\lambda^2 - 114.058}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-7.01 \times 10^{-7} + 2(8.04 \times 10^{-9})\,\Delta T + 3(-2.56 \times 10^{-11})\,\Delta T^2 + \frac{7.6 \times 10^{-7} + 2(7.56 \times 10^{-10})\,\Delta T}{\lambda - 0.129}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$