S-NPH7 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.68237\,\lambda^2}{\lambda^2 - 0.0131432} + \frac{0.33965\,\lambda^2}{\lambda^2 - 0.064504} + \frac{2.25049\,\lambda^2}{\lambda^2 - 181.386}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.34 \times 10^{-5} + 2(4.63 \times 10^{-9})\,\Delta T + 3(-1.77 \times 10^{-11})\,\Delta T^2 + \frac{1.02 \times 10^{-6} + 2(1.36 \times 10^{-9})\,\Delta T}{\lambda - 0.305}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$