BAK2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.01662\,\lambda^2}{\lambda^2 - 0.00592384} + \frac{0.319903\,\lambda^2}{\lambda^2 - 0.0203828} + \frac{0.937233\,\lambda^2}{\lambda^2 - 113.118}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.45 \times 10^{-6} + 2(1.1 \times 10^{-8})\,\Delta T + 3(4.89 \times 10^{-12})\,\Delta T^2 + \frac{5.16 \times 10^{-7} + 2(3.05 \times 10^{-10})\,\Delta T}{\lambda - 0.164}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$