S-FPM4 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.685585\,\lambda^2}{\lambda^2 - 0.00288172} + \frac{0.62338\,\lambda^2}{\lambda^2 - 0.0124702} + \frac{0.914178\,\lambda^2}{\lambda^2 - 153.577}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.77 \times 10^{-5} + 2(-4.38 \times 10^{-10})\,\Delta T + 3(1.28 \times 10^{-11})\,\Delta T^2 + \frac{3.47 \times 10^{-7} + 2(2.37 \times 10^{-10})\,\Delta T}{\lambda - 0.212}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$