H-F1 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.132376\,\lambda^2}{\lambda^2 - 0.0545903} + \frac{1.15565\,\lambda^2}{\lambda^2 - 116.786} + \frac{1.37548\,\lambda^2}{\lambda^2 - 0.0101495}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.94 \times 10^{-7} + 2(1.57 \times 10^{-8})\,\Delta T + 3(-4.33 \times 10^{-11})\,\Delta T^2 + \frac{7.95 \times 10^{-7} + 2(7.41 \times 10^{-10})\,\Delta T}{\lambda - 0.247}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$