KF6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.17007\,\lambda^2}{\lambda^2 - 0.00814992} + \frac{0.095051\,\lambda^2}{\lambda^2 - 0.0388217} + \frac{0.907409\,\lambda^2}{\lambda^2 - 108.882}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.25 \times 10^{-6} + 2(1.75 \times 10^{-8})\,\Delta T + 3(-6.23 \times 10^{-11})\,\Delta T^2 + \frac{6 \times 10^{-7} + 2(8.44 \times 10^{-10})\,\Delta T}{\lambda - 0.195}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$