BASF6 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.54539\,\lambda^2}{\lambda^2 - 0.00971809} + \frac{0.167516\,\lambda^2}{\lambda^2 - 0.046635} + \frac{1.06396\,\lambda^2}{\lambda^2 - 118.421}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.8 \times 10^{-6} + 2(1.23 \times 10^{-8})\,\Delta T + 3(6.86 \times 10^{-11})\,\Delta T^2 + \frac{7.32 \times 10^{-7} + 2(1.15 \times 10^{-9})\,\Delta T}{\lambda - 0.242}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$