NBFD29 by Hoya

$$n(\lambda) = \sqrt{3.02348 - 0.0144463\,\lambda^2 + 0.0346733\,\lambda^{-2} + 0.00206374\,\lambda^{-4} - 0.000111262\,\lambda^{-6} + 1.4469 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.7 \times 10^{-7} + 2(6.66 \times 10^{-9})\,\Delta T + 3(-4.62 \times 10^{-11})\,\Delta T^2 + \frac{9.73 \times 10^{-7} + 2(1.1 \times 10^{-9})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$