S-BSM25 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.34814\,\lambda^2}{\lambda^2 - 0.00695364} + \frac{0.34753\,\lambda^2}{\lambda^2 - 0.0277863} + \frac{1.38798\,\lambda^2}{\lambda^2 - 142.138}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.67 \times 10^{-6} + 2(1.31 \times 10^{-8})\,\Delta T + 3(-3.66 \times 10^{-11})\,\Delta T^2 + \frac{4.35 \times 10^{-7} + 2(3.4 \times 10^{-10})\,\Delta T}{\lambda - 0.271}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$