KF3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.15615\,\lambda^2}{\lambda^2 - 0.00789265} + \frac{0.102117\,\lambda^2}{\lambda^2 - 0.0345076} + \frac{0.927165\,\lambda^2}{\lambda^2 - 113.531}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.56 \times 10^{-6} + 2(1.47 \times 10^{-8})\,\Delta T + 3(-2.93 \times 10^{-11})\,\Delta T^2 + \frac{5.79 \times 10^{-7} + 2(-4.07 \times 10^{-11})\,\Delta T}{\lambda - 0.194}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$