NBFD25 by Hoya

$$n(\lambda) = \sqrt{3.28794 - 0.0158564\,\lambda^2 + 0.0456388\,\lambda^{-2} + 0.00330277\,\lambda^{-4} - 0.000216907\,\lambda^{-6} + 2.96259 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.95 \times 10^{-6} + 2(7.85 \times 10^{-9})\,\Delta T + 3(-1.91 \times 10^{-11})\,\Delta T^2 + \frac{8.43 \times 10^{-7} + 2(7.47 \times 10^{-10})\,\Delta T}{\lambda - 0.3}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$