BASF12 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.54702\,\lambda^2}{\lambda^2 - 0.0102838} + \frac{0.168942\,\lambda^2}{\lambda^2 - 0.049695} + \frac{0.974756\,\lambda^2}{\lambda^2 - 111.173}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.77 \times 10^{-6} + 2(1.43 \times 10^{-8})\,\Delta T + 3(-7.21 \times 10^{-11})\,\Delta T^2 + \frac{6.93 \times 10^{-7} + 2(3.51 \times 10^{-10})\,\Delta T}{\lambda - 0.282}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$