SF1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.55913\,\lambda^2}{\lambda^2 - 0.0121481} + \frac{0.284246\,\lambda^2}{\lambda^2 - 0.0534549} + \frac{0.968843\,\lambda^2}{\lambda^2 - 112.175}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.84 \times 10^{-6} + 2(1.7 \times 10^{-8})\,\Delta T + 3(-4.52 \times 10^{-11})\,\Delta T^2 + \frac{1.38 \times 10^{-6} + 2(1.26 \times 10^{-9})\,\Delta T}{\lambda - 0.259}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$