BAK1 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.12366\,\lambda^2}{\lambda^2 - 0.00644743} + \frac{0.309277\,\lambda^2}{\lambda^2 - 0.0222284} + \frac{0.881512\,\lambda^2}{\lambda^2 - 107.298}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.43 \times 10^{-7} + 2(1.26 \times 10^{-8})\,\Delta T + 3(-1.05 \times 10^{-11})\,\Delta T^2 + \frac{5.78 \times 10^{-7} + 2(7.13 \times 10^{-10})\,\Delta T}{\lambda - 0.157}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$