S-BSM93 by Ohara

$$n(\lambda) = \sqrt{2.64813 - 0.0115535\,\lambda^2 + 0.0155175\,\lambda^{-2} + 0.000532117\,\lambda^{-4} - 3.39069 \times 10^{-5}\,\lambda^{-6} + 2.09935 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-8.24 \times 10^{-7} + 2(1.36 \times 10^{-8})\,\Delta T + 3(1.73 \times 10^{-12})\,\Delta T^2 + \frac{4.66 \times 10^{-7} + 2(8.6 \times 10^{-10})\,\Delta T}{\lambda - 0.197}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$