S-LAH97 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.0273\,\lambda^2}{\lambda^2 - 0.0183406} + \frac{0.989294\,\lambda^2}{\lambda^2 - 0.00371264} + \frac{1.25781\,\lambda^2}{\lambda^2 - 87.8511}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.65 \times 10^{-6} + 2(1.16 \times 10^{-8})\,\Delta T + 3(-7.77 \times 10^{-12})\,\Delta T^2 + \frac{4.9 \times 10^{-7} + 2(4.77 \times 10^{-10})\,\Delta T}{\lambda - 0.207}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$