S-BSM9 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.3702\,\lambda^2}{\lambda^2 - 0.00757631} + \frac{0.189397\,\lambda^2}{\lambda^2 - 0.0300788} + \frac{1.24202\,\lambda^2}{\lambda^2 - 131.35}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.78 \times 10^{-6} + 2(1.36 \times 10^{-8})\,\Delta T + 3(-3.52 \times 10^{-11})\,\Delta T^2 + \frac{4.7 \times 10^{-7} + 2(3.85 \times 10^{-10})\,\Delta T}{\lambda - 0.22}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$