NBFD30 by Hoya

$$n(\lambda) = \sqrt{3.32413 - 0.0139718\,\lambda^2 + 0.0409323\,\lambda^{-2} + 0.00229156\,\lambda^{-4} - 0.000129065\,\lambda^{-6} + 1.84788 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.12 \times 10^{-6} + 2(9.35 \times 10^{-9})\,\Delta T + 3(-2.52 \times 10^{-11})\,\Delta T^2 + \frac{8.87 \times 10^{-7} + 2(9.43 \times 10^{-10})\,\Delta T}{\lambda - 0.271}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$