ZF52 by CDGM

$$n(\lambda) = \sqrt{3.24264 - 0.00779546\,\lambda^2 + 0.0533809\,\lambda^{-2} + 0.00151154\,\lambda^{-4} + 7.22195 \times 10^{-5}\,\lambda^{-6} + 1.66027 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.94 \times 10^{-6} + 2(2.59 \times 10^{-8})\,\Delta T + 3(-2.29 \times 10^{-10})\,\Delta T^2 + \frac{1.88 \times 10^{-6} + 2(1.39 \times 10^{-9})\,\Delta T}{\lambda - 0.284}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$