BASF51 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.67507\,\lambda^2}{\lambda^2 - 0.0105667} + \frac{0.212822\,\lambda^2}{\lambda^2 - 0.0476665} + \frac{0.966557\,\lambda^2}{\lambda^2 - 92.0389}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.09 \times 10^{-5} + 2(1.34 \times 10^{-8})\,\Delta T + 3(-3.68 \times 10^{-11})\,\Delta T^2 + \frac{9.17 \times 10^{-7} + 2(9.32 \times 10^{-10})\,\Delta T}{\lambda - 0.238}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$