BAF13 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.57069\,\lambda^2}{\lambda^2 - 0.00939377} + \frac{0.151295\,\lambda^2}{\lambda^2 - 0.044948} + \frac{1.07638\,\lambda^2}{\lambda^2 - 113.934}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.12 \times 10^{-7} + 2(1.08 \times 10^{-8})\,\Delta T + 3(-1.98 \times 10^{-11})\,\Delta T^2 + \frac{6.82 \times 10^{-7} + 2(8.7 \times 10^{-10})\,\Delta T}{\lambda - 0.228}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$