D-K9GT-25 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{1.08234\,\lambda^2}{\lambda^2 - 107.908} + \frac{0.548439\,\lambda^2}{\lambda^2 - 0.00219093} + \frac{0.72\,\lambda^2}{\lambda^2 - 0.013407}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[4.87 \times 10^{-6} + 2(1.55 \times 10^{-8})\,\Delta T + 3(-1.35 \times 10^{-11})\,\Delta T^2 + \frac{4.17 \times 10^{-7} + 2(5.2 \times 10^{-10})\,\Delta T}{\lambda - 0.177}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$