BAF5 by CDGM

$$n(\lambda) = \sqrt{2.52216 - 0.00832443\,\lambda^2 + 0.0186334\,\lambda^{-2} + 0.000582653\,\lambda^{-4} - 1.12806 \times 10^{-5}\,\lambda^{-6} + 2.08956 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.8 \times 10^{-8} + 2(1.12 \times 10^{-8})\,\Delta T + 3(-6.6 \times 10^{-11})\,\Delta T^2 + \frac{7.53 \times 10^{-7} + 2(3.18 \times 10^{-9})\,\Delta T}{\lambda - 0.217}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$