H-K6 by CDGM

$$n(\lambda) = \sqrt{2.25025 - 0.00758715\,\lambda^2 + 0.0126304\,\lambda^{-2} - 0.000229002\,\lambda^{-4} + 5.80226 \times 10^{-5}\,\lambda^{-6} - 2.84095 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.84 \times 10^{-6} + 2(1.68 \times 10^{-8})\,\Delta T + 3(-6.96 \times 10^{-11})\,\Delta T^2 + \frac{4.72 \times 10^{-7} + 2(-6.38 \times 10^{-11})\,\Delta T}{\lambda - 0.217}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$