NBFD38 by Hoya

$$n(\lambda) = \sqrt{2.66466 - 0.0134073\,\lambda^2 + 0.0222652\,\lambda^{-2} + 0.000781843\,\lambda^{-4} - 2.18931 \times 10^{-5}\,\lambda^{-6} + 4.16162 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.15 \times 10^{-6} + 2(1.08 \times 10^{-8})\,\Delta T + 3(-2.94 \times 10^{-12})\,\Delta T^2 + \frac{6.99 \times 10^{-7} + 2(2.66 \times 10^{-10})\,\Delta T}{\lambda - 0.216}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$