H-K3 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.296005\,\lambda^2}{\lambda^2 - 0.018599} + \frac{0.94042\,\lambda^2}{\lambda^2 - 0.00513633} + \frac{1.00567\,\lambda^2}{\lambda^2 - 97.5554}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.31 \times 10^{-6} + 2(2.11 \times 10^{-8})\,\Delta T + 3(-2.04 \times 10^{-11})\,\Delta T^2 + \frac{4.15 \times 10^{-7} + 2(4.8 \times 10^{-10})\,\Delta T}{\lambda - 0.136}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$