S-BSM18 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.927886\,\lambda^2}{\lambda^2 - 0.00417549} + \frac{0.708859\,\lambda^2}{\lambda^2 - 0.0184692} + \frac{1.18611\,\lambda^2}{\lambda^2 - 122.21}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.5 \times 10^{-7} + 2(1.22 \times 10^{-8})\,\Delta T + 3(-3.26 \times 10^{-11})\,\Delta T^2 + \frac{5.94 \times 10^{-7} + 2(6.22 \times 10^{-10})\,\Delta T}{\lambda - 0.141}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$