K5G20 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.14094\,\lambda^2}{\lambda^2 - 0.00694945} + \frac{0.145001\,\lambda^2}{\lambda^2 - 0.0310574} + \frac{37.4706\,\lambda^2}{\lambda^2 - 4536.26}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.22 \times 10^{-6} + 2(8.45 \times 10^{-9})\,\Delta T + 3(-3.31 \times 10^{-11})\,\Delta T^2 + \frac{5.44 \times 10^{-7} + 2(4.95 \times 10^{-10})\,\Delta T}{\lambda - 0.214}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$