S-FPL53 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.983532\,\lambda^2}{\lambda^2 - 0.00492235} + \frac{0.0695688\,\lambda^2}{\lambda^2 - 0.0193581} + \frac{1.11409\,\lambda^2}{\lambda^2 - 264.275}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.22 \times 10^{-5} + 2(-1.06 \times 10^{-8})\,\Delta T + 3(-4.91 \times 10^{-11})\,\Delta T^2 + \frac{2.77 \times 10^{-7} + 2(3.92 \times 10^{-10})\,\Delta T}{\lambda - 0.206}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$