BASF1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.40046\,\lambda^2}{\lambda^2 - 0.009899} + \frac{0.176869\,\lambda^2}{\lambda^2 - 0.0474295} + \frac{0.924677\,\lambda^2}{\lambda^2 - 115.946}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.32 \times 10^{-8} + 2(1.65 \times 10^{-8})\,\Delta T + 3(-1.18 \times 10^{-11})\,\Delta T^2 + \frac{9.23 \times 10^{-7} + 2(5.25 \times 10^{-10})\,\Delta T}{\lambda - 0.237}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$