FDS90-SG by Hoya

$$n(\lambda) = \sqrt{3.26063 - 0.0177804\,\lambda^2 + 0.0409029\,\lambda^{-2} + 0.00560769\,\lambda^{-4} - 0.00056434\,\lambda^{-6} + 5.47634 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.62 \times 10^{-6} + 2(1.29 \times 10^{-8})\,\Delta T + 3(-2.11 \times 10^{-11})\,\Delta T^2 + \frac{1.04 \times 10^{-6} + 2(1.27 \times 10^{-9})\,\Delta T}{\lambda - 0.296}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$