F2G12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.34702\,\lambda^2}{\lambda^2 - 0.00980851} + \frac{0.210038\,\lambda^2}{\lambda^2 - 0.0471788} + \frac{19.5351\,\lambda^2}{\lambda^2 - 2279.15}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.19 \times 10^{-6} + 2(1.62 \times 10^{-8})\,\Delta T + 3(-2.2 \times 10^{-11})\,\Delta T^2 + \frac{9.55 \times 10^{-7} + 2(8.12 \times 10^{-10})\,\Delta T}{\lambda - 0.25}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$