BASF54 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.64804\,\lambda^2}{\lambda^2 - 0.011176} + \frac{0.26617\,\lambda^2}{\lambda^2 - 0.0533322} + \frac{1.1542\,\lambda^2}{\lambda^2 - 119.495}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.82 \times 10^{-6} + 2(1.34 \times 10^{-8})\,\Delta T + 3(-2.21 \times 10^{-11})\,\Delta T^2 + \frac{1.23 \times 10^{-6} + 2(1.34 \times 10^{-9})\,\Delta T}{\lambda - 0.242}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$