LF5HTi by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.28553\,\lambda^2}{\lambda^2 - 0.00939886} + \frac{0.158358\,\lambda^2}{\lambda^2 - 0.0452567} + \frac{0.892175\,\lambda^2}{\lambda^2 - 110.545}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.26 \times 10^{-6} + 2(1.17 \times 10^{-8})\,\Delta T + 3(-4.14 \times 10^{-11})\,\Delta T^2 + \frac{8.24 \times 10^{-7} + 2(7.78 \times 10^{-10})\,\Delta T}{\lambda - 0.232}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$