H-BAK1 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.685985\,\lambda^2}{\lambda^2 - 0.0154934} + \frac{0.901677\,\lambda^2}{\lambda^2 - 106.589} + \frac{0.622429\,\lambda^2}{\lambda^2 - 0.0022267}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-8.49 \times 10^{-7} + 2(1.79 \times 10^{-8})\,\Delta T + 3(-4.72 \times 10^{-11})\,\Delta T^2 + \frac{4.45 \times 10^{-7} + 2(6.36 \times 10^{-10})\,\Delta T}{\lambda - 0.203}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$