S-PHM53 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.09775\,\lambda^2}{\lambda^2 - 0.0123369} + \frac{0.434816\,\lambda^2}{\lambda^2 - -0.000372523} + \frac{1.13895\,\lambda^2}{\lambda^2 - 124.277}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-9.57 \times 10^{-6} + 2(1.38 \times 10^{-8})\,\Delta T + 3(-6.4 \times 10^{-12})\,\Delta T^2 + \frac{4.8 \times 10^{-7} + 2(8.06 \times 10^{-10})\,\Delta T}{\lambda - 0.131}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$