ZF50 by CDGM

$$n(\lambda) = \sqrt{2.92281 - 0.0130295\,\lambda^2 + 0.02954\,\lambda^{-2} + 0.00413084\,\lambda^{-4} - 0.000447215\,\lambda^{-6} + 3.71376 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.66 \times 10^{-6} + 2(2.91 \times 10^{-8})\,\Delta T + 3(-3.18 \times 10^{-10})\,\Delta T^2 + \frac{1.58 \times 10^{-6} + 2(6.74 \times 10^{-10})\,\Delta T}{\lambda - 0.244}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$