ZKN7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.07715\,\lambda^2}{\lambda^2 - 0.00676602} + \frac{0.168079\,\lambda^2}{\lambda^2 - 0.0230643} + \frac{0.85189\,\lambda^2}{\lambda^2 - 89.0499}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.15 \times 10^{-5} + 2(1.73 \times 10^{-8})\,\Delta T + 3(-8.06 \times 10^{-11})\,\Delta T^2 + \frac{4.32 \times 10^{-7} + 2(7.05 \times 10^{-10})\,\Delta T}{\lambda - 0.179}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$