MP-NBFD10-20 by Hoya

$$n(\lambda) = \sqrt{3.27226 - 0.0188172\,\lambda^2 + 0.0272645\,\lambda^{-2} + 0.00327892\,\lambda^{-4} - 0.000328277\,\lambda^{-6} + 2.17434 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.22 \times 10^{-6} + 2(1.03 \times 10^{-8})\,\Delta T + 3(-3.04 \times 10^{-11})\,\Delta T^2 + \frac{9.23 \times 10^{-7} + 2(8.18 \times 10^{-10})\,\Delta T}{\lambda - 0.21}\right] \Delta T = T - T_0, \quad T_0 = 23\,°\text{C}$$