S-LAH99 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{2.39141\,\lambda^2}{\lambda^2 - 0.0131468} + \frac{0.439219\,\lambda^2}{\lambda^2 - 0.0553226} + \frac{2.38358\,\lambda^2}{\lambda^2 - 161.26}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.87 \times 10^{-7} + 2(9.24 \times 10^{-9})\,\Delta T + 3(-2.01 \times 10^{-11})\,\Delta T^2 + \frac{1.05 \times 10^{-6} + 2(1.26 \times 10^{-9})\,\Delta T}{\lambda - 0.266}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$