SF6HT by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.72448\,\lambda^2}{\lambda^2 - 0.0134872} + \frac{0.390105\,\lambda^2}{\lambda^2 - 0.0569318} + \frac{1.04573\,\lambda^2}{\lambda^2 - 118.557}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[6.69 \times 10^{-6} + 2(1.78 \times 10^{-8})\,\Delta T + 3(-3.36 \times 10^{-11})\,\Delta T^2 + \frac{1.77 \times 10^{-6} + 2(1.7 \times 10^{-9})\,\Delta T}{\lambda - 0.269}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$