S-BSM22 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.44306\,\lambda^2}{\lambda^2 - 0.00819209} + \frac{0.140786\,\lambda^2}{\lambda^2 - 0.0356911} + \frac{1.26094\,\lambda^2}{\lambda^2 - 131.959}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.82 \times 10^{-6} + 2(1.07 \times 10^{-8})\,\Delta T + 3(-3.24 \times 10^{-11})\,\Delta T^2 + \frac{4.7 \times 10^{-7} + 2(7.23 \times 10^{-10})\,\Delta T}{\lambda - 0.235}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$