H-K50 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{1.19525\,\lambda^2}{\lambda^2 - 0.00764077} + \frac{0.0895077\,\lambda^2}{\lambda^2 - 0.0318086} + \frac{0.920217\,\lambda^2}{\lambda^2 - 109.312}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.52 \times 10^{-6} + 2(1.8 \times 10^{-8})\,\Delta T + 3(-5.12 \times 10^{-11})\,\Delta T^2 + \frac{4.72 \times 10^{-7} + 2(6.41 \times 10^{-10})\,\Delta T}{\lambda - 0.193}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$