PBL25Y by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.31961\,\lambda^2}{\lambda^2 - 0.0101863} + \frac{0.123753\,\lambda^2}{\lambda^2 - 0.0483594} + \frac{0.210055\,\lambda^2}{\lambda^2 - 27.3272}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[3.78 \times 10^{-7} + 2(1.35 \times 10^{-8})\,\Delta T + 3(-3.9 \times 10^{-11})\,\Delta T^2 + \frac{8.57 \times 10^{-7} + 2(7.83 \times 10^{-10})\,\Delta T}{\lambda - 0.228}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$