TIF3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.22928\,\lambda^2}{\lambda^2 - 0.00852951} + \frac{0.116803\,\lambda^2}{\lambda^2 - 0.0528175} + \frac{1.05307\,\lambda^2}{\lambda^2 - 117.616}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-9.46 \times 10^{-6} + 2(1.16 \times 10^{-8})\,\Delta T + 3(-4.91 \times 10^{-11})\,\Delta T^2 + \frac{6.2 \times 10^{-7} + 2(7.22 \times 10^{-10})\,\Delta T}{\lambda - 0.27}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$