S-BSM16 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.1449\,\lambda^2}{\lambda^2 - 0.0137035} + \frac{0.439564\,\lambda^2}{\lambda^2 - -0.00186514} + \frac{1.27688\,\lambda^2}{\lambda^2 - 119.536}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.59 \times 10^{-7} + 2(1.35 \times 10^{-8})\,\Delta T + 3(-3.47 \times 10^{-11})\,\Delta T^2 + \frac{4 \times 10^{-7} + 2(5.04 \times 10^{-10})\,\Delta T}{\lambda - 0.201}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$