S-BSM71 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.50848\,\lambda^2}{\lambda^2 - 0.00812769} + \frac{0.1581\,\lambda^2}{\lambda^2 - 0.0354201} + \frac{1.36815\,\lambda^2}{\lambda^2 - 136.11}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-8.87 \times 10^{-7} + 2(1.45 \times 10^{-8})\,\Delta T + 3(2.75 \times 10^{-12})\,\Delta T^2 + \frac{6.65 \times 10^{-7} + 2(1.72 \times 10^{-9})\,\Delta T}{\lambda - 0.146}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$