N-SF6HT by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.77932\,\lambda^2}{\lambda^2 - 0.0133714} + \frac{0.33815\,\lambda^2}{\lambda^2 - 0.0617534} + \frac{2.08734\,\lambda^2}{\lambda^2 - 174.018}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.93 \times 10^{-6} + 2(7.02 \times 10^{-9})\,\Delta T + 3(-2.4 \times 10^{-11})\,\Delta T^2 + \frac{9.84 \times 10^{-7} + 2(1.54 \times 10^{-9})\,\Delta T}{\lambda - 0.29}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$