SF2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.40302\,\lambda^2}{\lambda^2 - 0.0105795} + \frac{0.231768\,\lambda^2}{\lambda^2 - 0.0493227} + \frac{0.939057\,\lambda^2}{\lambda^2 - 112.406}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.1 \times 10^{-6} + 2(1.75 \times 10^{-8})\,\Delta T + 3(-1.29 \times 10^{-11})\,\Delta T^2 + \frac{1.08 \times 10^{-6} + 2(1.03 \times 10^{-9})\,\Delta T}{\lambda - 0.249}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$