N-K5 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.08512\,\lambda^2}{\lambda^2 - 0.006611} + \frac{0.199562\,\lambda^2}{\lambda^2 - 0.0241109} + \frac{0.930512\,\lambda^2}{\lambda^2 - 111.983}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.13 \times 10^{-7} + 2(1.03 \times 10^{-8})\,\Delta T + 3(-3.4 \times 10^{-11})\,\Delta T^2 + \frac{4.73 \times 10^{-7} + 2(5.19 \times 10^{-10})\,\Delta T}{\lambda - 0.213}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$