S-NPH2 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{2.0387\,\lambda^2}{\lambda^2 - 0.0170796} + \frac{0.43727\,\lambda^2}{\lambda^2 - 0.0749255} + \frac{2.96711\,\lambda^2}{\lambda^2 - 174.155}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-4.13 \times 10^{-6} + 2(1.29 \times 10^{-8})\,\Delta T + 3(-2.65 \times 10^{-11})\,\Delta T^2 + \frac{1.02 \times 10^{-6} + 2(1.58 \times 10^{-9})\,\Delta T}{\lambda - 0.337}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$