D-FK61-25 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.894432\,\lambda^2}{\lambda^2 - 0.00436679} + \frac{0.321652\,\lambda^2}{\lambda^2 - 0.014191} + \frac{0.748704\,\lambda^2}{\lambda^2 - 149.771}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.45 \times 10^{-5} + 2(-2.09 \times 10^{-9})\,\Delta T + 3(-6.72 \times 10^{-13})\,\Delta T^2 + \frac{3.69 \times 10^{-7} + 2(2.37 \times 10^{-10})\,\Delta T}{\lambda - 0.173}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$