FK54 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.74532\,\lambda^2}{\lambda^2 - 0.00358172} + \frac{0.30233\,\lambda^2}{\lambda^2 - 0.0122885} + \frac{1.01885\,\lambda^2}{\lambda^2 - 213.017}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.98 \times 10^{-5} + 2(-1.04 \times 10^{-8})\,\Delta T + 3(2.37 \times 10^{-11})\,\Delta T^2 + \frac{4.17 \times 10^{-7} + 2(3.1 \times 10^{-10})\,\Delta T}{\lambda - 0.085}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$