S-FPM2 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.761243\,\lambda^2}{\lambda^2 - 0.00321174} + \frac{0.747033\,\lambda^2}{\lambda^2 - 0.0140234} + \frac{0.938929\,\lambda^2}{\lambda^2 - 139.524}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.65 \times 10^{-5} + 2(2.94 \times 10^{-9})\,\Delta T + 3(-1.11 \times 10^{-12})\,\Delta T^2 + \frac{4.25 \times 10^{-7} + 2(3.23 \times 10^{-10})\,\Delta T}{\lambda - 0.176}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$