N-SF66 by Schott

$$n(\lambda) = \sqrt{1 + \frac{2.0246\,\lambda^2}{\lambda^2 - 0.0147053} + \frac{0.470187\,\lambda^2}{\lambda^2 - 0.0692998} + \frac{2.5997\,\lambda^2}{\lambda^2 - 161.818}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.3 \times 10^{-6} + 2(1.15 \times 10^{-8})\,\Delta T + 3(4.31 \times 10^{-11})\,\Delta T^2 + \frac{9.62 \times 10^{-7} + 2(1.62 \times 10^{-9})\,\Delta T}{\lambda - 0.322}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$