S-PHM52 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.09967\,\lambda^2}{\lambda^2 - 0.0132719} + \frac{0.478125\,\lambda^2}{\lambda^2 - -0.00060165} + \frac{1.13214\,\lambda^2}{\lambda^2 - 130.595}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.15 \times 10^{-5} + 2(5.4 \times 10^{-9})\,\Delta T + 3(-3.69 \times 10^{-11})\,\Delta T^2 + \frac{4.44 \times 10^{-7} + 2(9.12 \times 10^{-10})\,\Delta T}{\lambda - 0.166}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$