BASF2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.47886\,\lambda^2}{\lambda^2 - 0.0104304} + \frac{0.212805\,\lambda^2}{\lambda^2 - 0.0494154} + \frac{0.996611\,\lambda^2}{\lambda^2 - 120.571}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.69 \times 10^{-6} + 2(1.41 \times 10^{-8})\,\Delta T + 3(-3.18 \times 10^{-11})\,\Delta T^2 + \frac{9.24 \times 10^{-7} + 2(9.98 \times 10^{-10})\,\Delta T}{\lambda - 0.257}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$