BASF57 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.47161\,\lambda^2}{\lambda^2 - 0.00915436} + \frac{0.190746\,\lambda^2}{\lambda^2 - 0.0444771} + \frac{1.04759\,\lambda^2}{\lambda^2 - 110.563}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.79 \times 10^{-6} + 2(1.69 \times 10^{-8})\,\Delta T + 3(7.21 \times 10^{-12})\,\Delta T^2 + \frac{6.81 \times 10^{-7} + 2(2.55 \times 10^{-10})\,\Delta T}{\lambda - 0.259}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$