H-LAK61 by CDGM

$$n(\lambda) = \sqrt{2.96884 - 0.0137037\,\lambda^2 + 0.0224807\,\lambda^{-2} + 0.000130227\,\lambda^{-4} + 3.52478 \times 10^{-5}\,\lambda^{-6} - 1.34827 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.47 \times 10^{-6} + 2(1.55 \times 10^{-8})\,\Delta T + 3(-6.52 \times 10^{-11})\,\Delta T^2 + \frac{4.66 \times 10^{-7} + 2(6.39 \times 10^{-10})\,\Delta T}{\lambda - 0.22}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$