NBFD15 by Hoya

$$n(\lambda) = \sqrt{3.154 - 0.0132568\,\lambda^2 + 0.0341033\,\lambda^{-2} + 0.00184722\,\lambda^{-4} - 0.000108073\,\lambda^{-6} + 1.29759 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.02 \times 10^{-6} + 2(7.39 \times 10^{-9})\,\Delta T + 3(-1.85 \times 10^{-11})\,\Delta T^2 + \frac{7.78 \times 10^{-7} + 2(6.81 \times 10^{-10})\,\Delta T}{\lambda - 0.268}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$