LAK23 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.12743\,\lambda^2}{\lambda^2 - 0.00525835} + \frac{0.608745\,\lambda^2}{\lambda^2 - 0.0189139} + \frac{0.998635\,\lambda^2}{\lambda^2 - 87.5468}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.77 \times 10^{-6} + 2(9.52 \times 10^{-9})\,\Delta T + 3(-2.61 \times 10^{-12})\,\Delta T^2 + \frac{4.66 \times 10^{-7} + 2(5.63 \times 10^{-10})\,\Delta T}{\lambda - 0.165}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$