BASF13 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.61152\,\lambda^2}{\lambda^2 - 0.0100755} + \frac{0.192653\,\lambda^2}{\lambda^2 - 0.0500719} + \frac{1.09087\,\lambda^2}{\lambda^2 - 110.403}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.82 \times 10^{-6} + 2(1.07 \times 10^{-8})\,\Delta T + 3(-9.18 \times 10^{-11})\,\Delta T^2 + \frac{7.01 \times 10^{-7} + 2(7.34 \times 10^{-10})\,\Delta T}{\lambda - 0.278}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$