BASF56 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.46892\,\lambda^2}{\lambda^2 - 0.0103259} + \frac{0.201528\,\lambda^2}{\lambda^2 - 0.0489114} + \frac{0.960506\,\lambda^2}{\lambda^2 - 116.457}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.55 \times 10^{-6} + 2(1.3 \times 10^{-8})\,\Delta T + 3(-1.03 \times 10^{-11})\,\Delta T^2 + \frac{9.64 \times 10^{-7} + 2(1.01 \times 10^{-9})\,\Delta T}{\lambda - 0.245}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$