S-BSM4 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.962443\,\lambda^2}{\lambda^2 - 0.00468062} + \frac{0.595939\,\lambda^2}{\lambda^2 - 0.0178772} + \frac{1.10558\,\lambda^2}{\lambda^2 - 115.896}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.56 \times 10^{-7} + 2(1.32 \times 10^{-8})\,\Delta T + 3(-4.04 \times 10^{-11})\,\Delta T^2 + \frac{3.16 \times 10^{-7} + 2(5.36 \times 10^{-10})\,\Delta T}{\lambda - 0.263}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$