BAF2 by CDGM

$$n(\lambda) = \sqrt{2.41667 - 0.00746726\,\lambda^2 + 0.0168668\,\lambda^{-2} - 1.29697 \times 10^{-5}\,\lambda^{-4} + 5.15993 \times 10^{-5}\,\lambda^{-6} - 1.81804 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.08 \times 10^{-7} + 2(2.09 \times 10^{-8})\,\Delta T + 3(-1.93 \times 10^{-10})\,\Delta T^2 + \frac{8.16 \times 10^{-7} + 2(6.26 \times 10^{-11})\,\Delta T}{\lambda - 0.159}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$