H-K9L by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.614555\,\lambda^2}{\lambda^2 - 0.0145988} + \frac{0.656775\,\lambda^2}{\lambda^2 - 0.0028777} + \frac{1.02699\,\lambda^2}{\lambda^2 - 107.653}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.04 \times 10^{-6} + 2(1.48 \times 10^{-8})\,\Delta T + 3(-5.17 \times 10^{-11})\,\Delta T^2 + \frac{3.85 \times 10^{-7} + 2(4.55 \times 10^{-10})\,\Delta T}{\lambda - 0.211}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$