S-BSM28 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.43823\,\lambda^2}{\lambda^2 - 0.0085978} + \frac{0.1281\,\lambda^2}{\lambda^2 - 0.0408618} + \frac{1.34356\,\lambda^2}{\lambda^2 - 143.71}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-2.01 \times 10^{-6} + 2(1.24 \times 10^{-8})\,\Delta T + 3(-3.72 \times 10^{-11})\,\Delta T^2 + \frac{4.79 \times 10^{-7} + 2(4.64 \times 10^{-10})\,\Delta T}{\lambda - 0.264}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$