SSKN8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.46998\,\lambda^2}{\lambda^2 - 0.00903675} + \frac{0.0963559\,\lambda^2}{\lambda^2 - 0.0450076} + \frac{1.02611\,\lambda^2}{\lambda^2 - 108.329}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.27 \times 10^{-7} + 2(1.14 \times 10^{-8})\,\Delta T + 3(-7.72 \times 10^{-12})\,\Delta T^2 + \frac{6.33 \times 10^{-7} + 2(8.35 \times 10^{-10})\,\Delta T}{\lambda - 0.185}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$