FK3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{0.973347\,\lambda^2}{\lambda^2 - 0.00640795} + \frac{0.146642\,\lambda^2}{\lambda^2 - 0.0205653} + \frac{0.679304\,\lambda^2}{\lambda^2 - 80.4965}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.9 \times 10^{-6} + 2(1.23 \times 10^{-8})\,\Delta T + 3(-1.19 \times 10^{-10})\,\Delta T^2 + \frac{3.45 \times 10^{-7} + 2(7.72 \times 10^{-10})\,\Delta T}{\lambda - 0.18}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$