BASF64A by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.65554\,\lambda^2}{\lambda^2 - 0.0104486} + \frac{0.17132\,\lambda^2}{\lambda^2 - 0.0499395} + \frac{1.33664\,\lambda^2}{\lambda^2 - 118.961}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.6 \times 10^{-6} + 2(1.02 \times 10^{-8})\,\Delta T + 3(-2.68 \times 10^{-11})\,\Delta T^2 + \frac{7.87 \times 10^{-7} + 2(9.65 \times 10^{-10})\,\Delta T}{\lambda - 0.229}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$