SF3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.57231\,\lambda^2}{\lambda^2 - 0.0120382} + \frac{0.339661\,\lambda^2}{\lambda^2 - 0.0531604} + \frac{1.03594\,\lambda^2}{\lambda^2 - 120.005}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.72 \times 10^{-6} + 2(1.74 \times 10^{-8})\,\Delta T + 3(-3.21 \times 10^{-11})\,\Delta T^2 + \frac{1.49 \times 10^{-6} + 2(1.41 \times 10^{-9})\,\Delta T}{\lambda - 0.26}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$