BASF52 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.63872\,\lambda^2}{\lambda^2 - 0.00993239} + \frac{0.184589\,\lambda^2}{\lambda^2 - 0.0462893} + \frac{1.00747\,\lambda^2}{\lambda^2 - 81.6164}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.98 \times 10^{-6} + 2(1.17 \times 10^{-8})\,\Delta T + 3(6.65 \times 10^{-11})\,\Delta T^2 + \frac{6.5 \times 10^{-7} + 2(9.24 \times 10^{-10})\,\Delta T}{\lambda - 0.267}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$