H-K1 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.841283\,\lambda^2}{\lambda^2 - 103.642} + \frac{1.01521\,\lambda^2}{\lambda^2 - 0.00623225} + \frac{0.204021\,\lambda^2}{\lambda^2 - 0.022057}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.14 \times 10^{-6} + 2(1.96 \times 10^{-8})\,\Delta T + 3(-3.87 \times 10^{-11})\,\Delta T^2 + \frac{5.49 \times 10^{-7} + 2(-2.63 \times 10^{-10})\,\Delta T}{\lambda - 0.161}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$