BAF51 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.5323\,\lambda^2}{\lambda^2 - 0.0096827} + \frac{0.136199\,\lambda^2}{\lambda^2 - 0.0448276} + \frac{1.12287\,\lambda^2}{\lambda^2 - 123.776}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.3 \times 10^{-7} + 2(8.91 \times 10^{-9})\,\Delta T + 3(-2.9 \times 10^{-12})\,\Delta T^2 + \frac{7.94 \times 10^{-7} + 2(1.11 \times 10^{-9})\,\Delta T}{\lambda - 0.196}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$