H-BAK2 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.618389\,\lambda^2}{\lambda^2 - 0.00174654} + \frac{0.7184\,\lambda^2}{\lambda^2 - 0.0158113} + \frac{1.07451\,\lambda^2}{\lambda^2 - 126.585}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.82 \times 10^{-6} + 2(1.82 \times 10^{-8})\,\Delta T + 3(-4.21 \times 10^{-11})\,\Delta T^2 + \frac{4.7 \times 10^{-7} + 2(1.02 \times 10^{-10})\,\Delta T}{\lambda - 0.209}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$