N-SSK8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.44858\,\lambda^2}{\lambda^2 - 0.0086931} + \frac{0.117966\,\lambda^2}{\lambda^2 - 0.0421567} + \frac{1.06938\,\lambda^2}{\lambda^2 - 111.301}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.34 \times 10^{-7} + 2(1.27 \times 10^{-8})\,\Delta T + 3(-1.75 \times 10^{-11})\,\Delta T^2 + \frac{5.4 \times 10^{-7} + 2(7.05 \times 10^{-10})\,\Delta T}{\lambda - 0.224}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$