S-BSM15 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.953128\,\lambda^2}{\lambda^2 - 0.00387639} + \frac{0.637614\,\lambda^2}{\lambda^2 - 0.0185095} + \frac{1.65246\,\lambda^2}{\lambda^2 - 159.442}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-3.24 \times 10^{-7} + 2(1.31 \times 10^{-8})\,\Delta T + 3(-3.71 \times 10^{-11})\,\Delta T^2 + \frac{4.21 \times 10^{-7} + 2(6.87 \times 10^{-10})\,\Delta T}{\lambda - 0.207}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$