H-ZK9A by CDGM

$$n(\lambda) = \sqrt{2.58408 - 0.0111281\,\lambda^2 + 0.0154282\,\lambda^{-2} + 2.64034 \times 10^{-5}\,\lambda^{-4} + 2.77466 \times 10^{-5}\,\lambda^{-6} - 1.24494 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.4 \times 10^{-6} + 2(1.73 \times 10^{-8})\,\Delta T + 3(1.08 \times 10^{-10})\,\Delta T^2 + \frac{5.4 \times 10^{-7} + 2(7.75 \times 10^{-10})\,\Delta T}{\lambda - 0.097}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$