LF8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.26628\,\lambda^2}{\lambda^2 - 0.0091581} + \frac{0.130098\,\lambda^2}{\lambda^2 - 0.0447614} + \frac{0.867352\,\lambda^2}{\lambda^2 - 107.013}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.02 \times 10^{-6} + 2(1.8 \times 10^{-8})\,\Delta T + 3(-1.13 \times 10^{-10})\,\Delta T^2 + \frac{9.22 \times 10^{-7} + 2(7.6 \times 10^{-10})\,\Delta T}{\lambda - 0.183}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$