H-K2 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.951358\,\lambda^2}{\lambda^2 - 0.0112895} + \frac{0.969615\,\lambda^2}{\lambda^2 - 99.1744} + \frac{0.273207\,\lambda^2}{\lambda^2 - -0.00245666}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.23 \times 10^{-6} + 2(2.28 \times 10^{-8})\,\Delta T + 3(-4.09 \times 10^{-11})\,\Delta T^2 + \frac{4.76 \times 10^{-7} + 2(3 \times 10^{-10})\,\Delta T}{\lambda - 0.032}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$