KF9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.16422\,\lambda^2}{\lambda^2 - 0.00787954} + \frac{0.118007\,\lambda^2}{\lambda^2 - 0.037252} + \frac{0.923055\,\lambda^2}{\lambda^2 - 109.925}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.13 \times 10^{-6} + 2(1.4 \times 10^{-8})\,\Delta T + 3(-2.12 \times 10^{-11})\,\Delta T^2 + \frac{6.25 \times 10^{-7} + 2(6.96 \times 10^{-10})\,\Delta T}{\lambda - 0.2}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$