S-BSM81 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{0.996357\,\lambda^2}{\lambda^2 - 0.0144822} + \frac{0.651393\,\lambda^2}{\lambda^2 - 0.00154826} + \frac{1.22433\,\lambda^2}{\lambda^2 - 89.9819}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.81 \times 10^{-6} + 2(1.37 \times 10^{-8})\,\Delta T + 3(-4.03 \times 10^{-11})\,\Delta T^2 + \frac{3.96 \times 10^{-7} + 2(7.56 \times 10^{-10})\,\Delta T}{\lambda - 0.182}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$