KZFS6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.35164\,\lambda^2}{\lambda^2 - 0.00849653} + \frac{0.13576\,\lambda^2}{\lambda^2 - 0.0388357} + \frac{0.84449\,\lambda^2}{\lambda^2 - 67.2773}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.02 \times 10^{-6} + 2(1.05 \times 10^{-8})\,\Delta T + 3(-1.91 \times 10^{-11})\,\Delta T^2 + \frac{5.58 \times 10^{-7} + 2(4.26 \times 10^{-10})\,\Delta T}{\lambda - 0.175}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$