N-LAF2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.80984\,\lambda^2}{\lambda^2 - 0.0101712} + \frac{0.157296\,\lambda^2}{\lambda^2 - 0.0442432} + \frac{1.093\,\lambda^2}{\lambda^2 - 100.688}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.64 \times 10^{-6} + 2(9.2 \times 10^{-9})\,\Delta T + 3(-6 \times 10^{-12})\,\Delta T^2 + \frac{6.43 \times 10^{-7} + 2(6.11 \times 10^{-10})\,\Delta T}{\lambda - 0.22}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$