BAK50 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.950105\,\lambda^2}{\lambda^2 - 0.00520357} + \frac{0.469379\,\lambda^2}{\lambda^2 - 0.0187889} + \frac{0.921416\,\lambda^2}{\lambda^2 - 98.408}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.32 \times 10^{-5} + 2(1.61 \times 10^{-8})\,\Delta T + 3(-2.85 \times 10^{-11})\,\Delta T^2 + \frac{5.65 \times 10^{-7} + 2(6.63 \times 10^{-10})\,\Delta T}{\lambda - 0.155}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$