N-SSK2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.4306\,\lambda^2}{\lambda^2 - 0.00823983} + \frac{0.153151\,\lambda^2}{\lambda^2 - 0.0333737} + \frac{1.01391\,\lambda^2}{\lambda^2 - 106.871}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[5.21 \times 10^{-6} + 2(1.34 \times 10^{-8})\,\Delta T + 3(-1.01 \times 10^{-11})\,\Delta T^2 + \frac{5.21 \times 10^{-7} + 2(5.87 \times 10^{-10})\,\Delta T}{\lambda - 0.199}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$