H-BAK7A by CDGM

$$n(\lambda) = \sqrt{2.42067 - 0.00907063\,\lambda^2 + 0.014848\,\lambda^{-2} - 1.63555 \times 10^{-5}\,\lambda^{-4} + 3.76516 \times 10^{-5}\,\lambda^{-6} - 1.56034 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.77 \times 10^{-6} + 2(1.48 \times 10^{-8})\,\Delta T + 3(2.37 \times 10^{-11})\,\Delta T^2 + \frac{6.1 \times 10^{-7} + 2(7.99 \times 10^{-10})\,\Delta T}{\lambda - 0.144}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$