ZF12 by CDGM

$$n(\lambda) = \sqrt{2.99507 - 0.0165673\,\lambda^2 + 0.0256701\,\lambda^{-2} + 0.00670445\,\lambda^{-4} - 0.000837477\,\lambda^{-6} + 6.26403 \times 10^{-5}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.33 \times 10^{-6} + 2(2.64 \times 10^{-8})\,\Delta T + 3(-2.01 \times 10^{-11})\,\Delta T^2 + \frac{1.43 \times 10^{-6} + 2(1.62 \times 10^{-9})\,\Delta T}{\lambda - 0.284}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$