FCD1 by Hoya

$$n(\lambda) = \sqrt{2.21811 - 0.00579943\,\lambda^2 + 0.00834707\,\lambda^{-2} + 6.50465 \times 10^{-5}\,\lambda^{-4} + 8.51422 \times 10^{-6}\,\lambda^{-6} - 5.88523 \times 10^{-7}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.92 \times 10^{-5} + 2(-8.39 \times 10^{-9})\,\Delta T + 3(5.31 \times 10^{-12})\,\Delta T^2 + \frac{3.68 \times 10^{-7} + 2(4.15 \times 10^{-10})\,\Delta T}{\lambda - -0.143}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$