S-NPH3 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{2.09835\,\lambda^2}{\lambda^2 - 0.0179124} + \frac{0.489088\,\lambda^2}{\lambda^2 - 0.0776653} + \frac{2.94009\,\lambda^2}{\lambda^2 - 160.93}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.98 \times 10^{-6} + 2(1.36 \times 10^{-8})\,\Delta T + 3(-3.16 \times 10^{-11})\,\Delta T^2 + \frac{1.15 \times 10^{-6} + 2(1.85 \times 10^{-9})\,\Delta T}{\lambda - 0.34}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$