ZF51 by CDGM

$$n(\lambda) = \sqrt{3.06187 - 0.0138734\,\lambda^2 + 0.0329852\,\lambda^{-2} + 0.00507142\,\lambda^{-4} - 0.000538607\,\lambda^{-6} + 4.36089 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.8 \times 10^{-6} + 2(2.76 \times 10^{-8})\,\Delta T + 3(-6.03 \times 10^{-11})\,\Delta T^2 + \frac{1.6 \times 10^{-6} + 2(1.02 \times 10^{-9})\,\Delta T}{\lambda - 0.279}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$