S-NPH1 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.75157\,\lambda^2}{\lambda^2 - 0.0135005} + \frac{0.364006\,\lambda^2}{\lambda^2 - 0.0668245} + \frac{2.47874\,\lambda^2}{\lambda^2 - 170.756}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-6.7 \times 10^{-6} + 2(1.01 \times 10^{-8})\,\Delta T + 3(-3.04 \times 10^{-11})\,\Delta T^2 + \frac{8.91 \times 10^{-7} + 2(1.32 \times 10^{-9})\,\Delta T}{\lambda - 0.324}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$