N-FK58 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.738043\,\lambda^2}{\lambda^2 - 0.00339066} + \frac{0.363372\,\lambda^2}{\lambda^2 - 0.0117551} + \frac{0.989296\,\lambda^2}{\lambda^2 - 212.842}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.05 \times 10^{-5} + 2(-6.33 \times 10^{-9})\,\Delta T + 3(4.13 \times 10^{-11})\,\Delta T^2 + \frac{3.84 \times 10^{-7} + 2(1.63 \times 10^{-10})\,\Delta T}{\lambda - 0.073}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$