F6 by CDGM

$$n(\lambda) = \sqrt{2.5685 - 0.00836232\,\lambda^2 + 0.0234448\,\lambda^{-2} + 0.000828924\,\lambda^{-4} - 1.18355 \times 10^{-5}\,\lambda^{-6} + 4.09562 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.05 \times 10^{-6} + 2(1.51 \times 10^{-8})\,\Delta T + 3(-6.65 \times 10^{-11})\,\Delta T^2 + \frac{1 \times 10^{-6} + 2(9.43 \times 10^{-10})\,\Delta T}{\lambda - 0.24}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$