N-BAF3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.3486\,\lambda^2}{\lambda^2 - 0.00871493} + \frac{0.107644\,\lambda^2}{\lambda^2 - 0.0478406} + \frac{1.13207\,\lambda^2}{\lambda^2 - 112.936}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.4 \times 10^{-6} + 2(1.24 \times 10^{-8})\,\Delta T + 3(-9.39 \times 10^{-12})\,\Delta T^2 + \frac{5.91 \times 10^{-7} + 2(7.44 \times 10^{-10})\,\Delta T}{\lambda - 0.235}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$