N-SSK20 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.18392\,\lambda^2}{\lambda^2 - 0.00687637} + \frac{0.406467\,\lambda^2}{\lambda^2 - 0.0246713} + \frac{1.10391\,\lambda^2}{\lambda^2 - 132.063}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.76 \times 10^{-6} + 2(1.26 \times 10^{-8})\,\Delta T + 3(-3.08 \times 10^{-12})\,\Delta T^2 + \frac{8.59 \times 10^{-7} + 2(9.53 \times 10^{-10})\,\Delta T}{\lambda - 0.151}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$