N-BAF4 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.42056\,\lambda^2}{\lambda^2 - 0.00942015} + \frac{0.102721\,\lambda^2}{\lambda^2 - 0.0531087} + \frac{1.14381\,\lambda^2}{\lambda^2 - 110.279}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[9.39 \times 10^{-7} + 2(1.24 \times 10^{-8})\,\Delta T + 3(-9 \times 10^{-12})\,\Delta T^2 + \frac{6.17 \times 10^{-7} + 2(8.42 \times 10^{-10})\,\Delta T}{\lambda - 0.242}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$