NBFD32 by Hoya

$$n(\lambda) = \sqrt{2.89893 - 0.0133489\,\lambda^2 + 0.0301635\,\lambda^{-2} + 0.00162579\,\lambda^{-4} - 7.69665 \times 10^{-5}\,\lambda^{-6} + 1.01432 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.56 \times 10^{-7} + 2(5.75 \times 10^{-9})\,\Delta T + 3(-2.13 \times 10^{-11})\,\Delta T^2 + \frac{8.23 \times 10^{-7} + 2(9.24 \times 10^{-10})\,\Delta T}{\lambda - 0.26}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$