N-LAF35 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.51697\,\lambda^2}{\lambda^2 - 0.00750943} + \frac{0.455875\,\lambda^2}{\lambda^2 - 0.0260047} + \frac{1.07469\,\lambda^2}{\lambda^2 - 80.5945}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.98 \times 10^{-6} + 2(1.26 \times 10^{-8})\,\Delta T + 3(-1.23 \times 10^{-11})\,\Delta T^2 + \frac{6.24 \times 10^{-7} + 2(6.86 \times 10^{-10})\,\Delta T}{\lambda - 0.194}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$