N-FK51A by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.971248\,\lambda^2}{\lambda^2 - 0.00472302} + \frac{0.216901\,\lambda^2}{\lambda^2 - 0.0153576} + \frac{0.904652\,\lambda^2}{\lambda^2 - 168.681}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.83 \times 10^{-5} + 2(-7.89 \times 10^{-9})\,\Delta T + 3(-1.63 \times 10^{-12})\,\Delta T^2 + \frac{3.74 \times 10^{-7} + 2(3.46 \times 10^{-10})\,\Delta T}{\lambda - 0.15}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$