SK51 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.44113\,\lambda^2}{\lambda^2 - 0.00758547} + \frac{0.143968\,\lambda^2}{\lambda^2 - 0.0287396} + \frac{0.88199\,\lambda^2}{\lambda^2 - 94.6838}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.63 \times 10^{-6} + 2(7.38 \times 10^{-9})\,\Delta T + 3(-6.2 \times 10^{-11})\,\Delta T^2 + \frac{3.91 \times 10^{-7} + 2(2.64 \times 10^{-10})\,\Delta T}{\lambda - 0.23}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$