S-FPL51 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.17011\,\lambda^2}{\lambda^2 - 0.00616204} + \frac{0.0475711\,\lambda^2}{\lambda^2 - 0.0263373} + \frac{0.763832\,\lambda^2}{\lambda^2 - 141.883}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.91 \times 10^{-5} + 2(-4.44 \times 10^{-9})\,\Delta T + 3(-4.62 \times 10^{-11})\,\Delta T^2 + \frac{3.74 \times 10^{-7} + 2(4.39 \times 10^{-10})\,\Delta T}{\lambda - 0.163}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$