N-KF9 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.19287\,\lambda^2}{\lambda^2 - 0.00839155} + \frac{0.0893347\,\lambda^2}{\lambda^2 - 0.0404011} + \frac{0.92082\,\lambda^2}{\lambda^2 - 112.572}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.66 \times 10^{-6} + 2(8.44 \times 10^{-9})\,\Delta T + 3(-1.01 \times 10^{-11})\,\Delta T^2 + \frac{6.1 \times 10^{-7} + 2(6.96 \times 10^{-10})\,\Delta T}{\lambda - 0.217}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$