LAFN28 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.75883\,\lambda^2}{\lambda^2 - 0.0087281} + \frac{0.315198\,\lambda^2}{\lambda^2 - 0.0293021} + \frac{1.1903\,\lambda^2}{\lambda^2 - 85.1781}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.89 \times 10^{-6} + 2(1.02 \times 10^{-8})\,\Delta T + 3(-1.91 \times 10^{-11})\,\Delta T^2 + \frac{5.88 \times 10^{-7} + 2(7.57 \times 10^{-10})\,\Delta T}{\lambda - 0.181}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$