N-F2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.39757\,\lambda^2}{\lambda^2 - 0.00995906} + \frac{0.159201\,\lambda^2}{\lambda^2 - 0.0546932} + \frac{1.26865\,\lambda^2}{\lambda^2 - 119.248}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.62 \times 10^{-7} + 2(1.17 \times 10^{-8})\,\Delta T + 3(-2.35 \times 10^{-11})\,\Delta T^2 + \frac{7.47 \times 10^{-7} + 2(9.81 \times 10^{-10})\,\Delta T}{\lambda - 0.263}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$