SK3 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.30663\,\lambda^2}{\lambda^2 - 0.00708164} + \frac{0.239792\,\lambda^2}{\lambda^2 - 0.0238829} + \frac{0.984058\,\lambda^2}{\lambda^2 - 101.983}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.33 \times 10^{-6} + 2(9.01 \times 10^{-9})\,\Delta T + 3(-1.68 \times 10^{-11})\,\Delta T^2 + \frac{3.45 \times 10^{-7} + 2(8.42 \times 10^{-10})\,\Delta T}{\lambda - 0.255}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$