H-K7 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.854802\,\lambda^2}{\lambda^2 - 103.204} + \frac{0.442227\,\lambda^2}{\lambda^2 - 0.0179643} + \frac{0.820484\,\lambda^2}{\lambda^2 - 0.00433832}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.57 \times 10^{-6} + 2(1.18 \times 10^{-8})\,\Delta T + 3(-3.26 \times 10^{-11})\,\Delta T^2 + \frac{4.6 \times 10^{-7} + 2(7.2 \times 10^{-10})\,\Delta T}{\lambda - 0.213}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$