ZBAF51 by CDGM

$$n(\lambda) = \sqrt{2.75902 - 0.00581805\,\lambda^2 + 0.0266099\,\lambda^{-2} - 0.00075486\,\lambda^{-4} + 0.000176405\,\lambda^{-6} - 6.72712 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.51 \times 10^{-6} + 2(1.79 \times 10^{-8})\,\Delta T + 3(-4.04 \times 10^{-11})\,\Delta T^2 + \frac{7.85 \times 10^{-7} + 2(6.9 \times 10^{-11})\,\Delta T}{\lambda - 0.221}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$