LASFN15 by Schott

$$n(\lambda) = \sqrt{1 + \frac{2.08895\,\lambda^2}{\lambda^2 - 0.0109013} + \frac{0.327776\,\lambda^2}{\lambda^2 - 0.0435008} + \frac{1.52129\,\lambda^2}{\lambda^2 - 101.184}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.09 \times 10^{-6} + 2(9.92 \times 10^{-9})\,\Delta T + 3(-1.53 \times 10^{-11})\,\Delta T^2 + \frac{7.58 \times 10^{-7} + 2(1.02 \times 10^{-9})\,\Delta T}{\lambda - 0.224}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$