H-ZK7 by CDGM

$$n(\lambda) = \sqrt{2.5623 - 0.0108845\,\lambda^2 + 0.0140998\,\lambda^{-2} + 0.000379257\,\lambda^{-4} - 2.41613 \times 10^{-5}\,\lambda^{-6} + 1.41616 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.31 \times 10^{-7} + 2(1.64 \times 10^{-8})\,\Delta T + 3(-6.04 \times 10^{-11})\,\Delta T^2 + \frac{3.53 \times 10^{-7} + 2(-1.4 \times 10^{-10})\,\Delta T}{\lambda - 0.23}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$