S-PHM52Q by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.26969\,\lambda^2}{\lambda^2 - 0.00621623} + \frac{0.308418\,\lambda^2}{\lambda^2 - 0.0207785} + \frac{1.04079\,\lambda^2}{\lambda^2 - 112.051}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.66 \times 10^{-6} + 2(8.1 \times 10^{-9})\,\Delta T + 3(2.34 \times 10^{-13})\,\Delta T^2 + \frac{4.31 \times 10^{-7} + 2(4.9 \times 10^{-10})\,\Delta T}{\lambda - 0.204}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$