SK6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.21206\,\lambda^2}{\lambda^2 - 0.00665603} + \frac{0.347221\,\lambda^2}{\lambda^2 - 0.0224385} + \frac{0.972025\,\lambda^2}{\lambda^2 - 110.082}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.41 \times 10^{-6} + 2(1.35 \times 10^{-8})\,\Delta T + 3(-2.77 \times 10^{-11})\,\Delta T^2 + \frac{4.91 \times 10^{-7} + 2(4.18 \times 10^{-10})\,\Delta T}{\lambda - 0.2}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$