SF16 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.40807\,\lambda^2}{\lambda^2 - 0.010674} + \frac{0.222238\,\lambda^2}{\lambda^2 - 0.0496647} + \frac{0.927122\,\lambda^2}{\lambda^2 - 110.9}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.2 \times 10^{-6} + 2(1.65 \times 10^{-8})\,\Delta T + 3(-9.77 \times 10^{-12})\,\Delta T^2 + \frac{1.07 \times 10^{-6} + 2(1.09 \times 10^{-9})\,\Delta T}{\lambda - 0.251}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$