S-BSM2 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.867169\,\lambda^2}{\lambda^2 - 0.00369311} + \frac{0.672848\,\lambda^2}{\lambda^2 - 0.0181653} + \frac{1.18456\,\lambda^2}{\lambda^2 - 132.376}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.43 \times 10^{-6} + 2(1.37 \times 10^{-8})\,\Delta T + 3(-3.45 \times 10^{-11})\,\Delta T^2 + \frac{5.05 \times 10^{-7} + 2(5.49 \times 10^{-10})\,\Delta T}{\lambda - 0.203}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$