ZF7LTT by CDGM

$$n(\lambda) = \sqrt{3.11725 - 0.00977959\,\lambda^2 + 0.0431753\,\lambda^{-2} + 0.0025228\,\lambda^{-4} - 0.0001174\,\lambda^{-6} + 2.07881 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.35 \times 10^{-6} + 2(1.94 \times 10^{-8})\,\Delta T + 3(-4.68 \times 10^{-11})\,\Delta T^2 + \frac{1.77 \times 10^{-6} + 2(1.3 \times 10^{-9})\,\Delta T}{\lambda - 0.27}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$