FK52 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.01571\,\lambda^2}{\lambda^2 - 0.00515885} + \frac{0.169998\,\lambda^2}{\lambda^2 - 0.0173381} + \frac{0.805159\,\lambda^2}{\lambda^2 - 158.603}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.03 \times 10^{-5} + 2(-8.12 \times 10^{-9})\,\Delta T + 3(1.12 \times 10^{-11})\,\Delta T^2 + \frac{4.09 \times 10^{-7} + 2(4.22 \times 10^{-10})\,\Delta T}{\lambda - 0.139}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$