BAF8 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.44967\,\lambda^2}{\lambda^2 - 0.00919053} + \frac{0.131601\,\lambda^2}{\lambda^2 - 0.042573} + \frac{0.939225\,\lambda^2}{\lambda^2 - 112.154}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.14 \times 10^{-6} + 2(1.35 \times 10^{-8})\,\Delta T + 3(6.17 \times 10^{-11})\,\Delta T^2 + \frac{4.81 \times 10^{-7} + 2(3.31 \times 10^{-10})\,\Delta T}{\lambda - 0.293}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$