PBL26Y by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.29472\,\lambda^2}{\lambda^2 - 0.00986579} + \frac{0.108881\,\lambda^2}{\lambda^2 - 0.0477569} + \frac{0.220323\,\lambda^2}{\lambda^2 - 28.851}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.24 \times 10^{-6} + 2(1.52 \times 10^{-8})\,\Delta T + 3(-3.95 \times 10^{-11})\,\Delta T^2 + \frac{7.86 \times 10^{-7} + 2(7.36 \times 10^{-10})\,\Delta T}{\lambda - 0.228}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$