ZF13 by CDGM

$$n(\lambda) = \sqrt{3.06161 - 0.0145895\,\lambda^2 + 0.0328147\,\lambda^{-2} + 0.0052399\,\lambda^{-4} - 0.000560505\,\lambda^{-6} + 4.64234 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.7 \times 10^{-6} + 2(2.35 \times 10^{-8})\,\Delta T + 3(-1.53 \times 10^{-10})\,\Delta T^2 + \frac{1.59 \times 10^{-6} + 2(6.42 \times 10^{-10})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$