F5 by CDGM

$$n(\lambda) = \sqrt{2.56838 - 0.00914046\,\lambda^2 + 0.022598\,\lambda^{-2} + 0.000999853\,\lambda^{-4} - 3.66331 \times 10^{-5}\,\lambda^{-6} + 4.93325 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.39 \times 10^{-6} + 2(1.01 \times 10^{-8})\,\Delta T + 3(-5.9 \times 10^{-11})\,\Delta T^2 + \frac{1.05 \times 10^{-6} + 2(2.13 \times 10^{-9})\,\Delta T}{\lambda - 0.231}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$