N-BASF2 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.53652\,\lambda^2}{\lambda^2 - 0.0108436} + \frac{0.156971\,\lambda^2}{\lambda^2 - 0.0562279} + \frac{1.30197\,\lambda^2}{\lambda^2 - 131.34}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.89 \times 10^{-6} + 2(1.22 \times 10^{-8})\,\Delta T + 3(-1.61 \times 10^{-11})\,\Delta T^2 + \frac{7.77 \times 10^{-7} + 2(9.96 \times 10^{-10})\,\Delta T}{\lambda - 0.256}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$