N-FK56 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.911957\,\lambda^2}{\lambda^2 - 0.00450933} + \frac{0.12858\,\lambda^2}{\lambda^2 - 0.0153516} + \frac{0.983146\,\lambda^2}{\lambda^2 - 223.961}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.04 \times 10^{-5} + 2(-1.03 \times 10^{-8})\,\Delta T + 3(2.43 \times 10^{-12})\,\Delta T^2 + \frac{3.41 \times 10^{-7} + 2(4.37 \times 10^{-10})\,\Delta T}{\lambda - 0.138}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$