H-K12 by CDGM

$$n(\lambda) = \sqrt{2.31506 - 0.00827156\,\lambda^2 + 0.0129536\,\lambda^{-2} + 0.000268505\,\lambda^{-4} - 5.46028 \times 10^{-6}\,\lambda^{-6} + 6.24326 \times 10^{-7}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.57 \times 10^{-6} + 2(1.35 \times 10^{-8})\,\Delta T + 3(-5.16 \times 10^{-11})\,\Delta T^2 + \frac{7.91 \times 10^{-7} + 2(9.19 \times 10^{-10})\,\Delta T}{\lambda - 0.086}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$