LLF7 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.22925\,\lambda^2}{\lambda^2 - 0.0088607} + \frac{0.122566\,\lambda^2}{\lambda^2 - 0.0433084} + \frac{0.888851\,\lambda^2}{\lambda^2 - 106.216}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.66 \times 10^{-6} + 2(1.42 \times 10^{-8})\,\Delta T + 3(-1.18 \times 10^{-10})\,\Delta T^2 + \frac{5.96 \times 10^{-7} + 2(6.93 \times 10^{-10})\,\Delta T}{\lambda - 0.257}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$