PBL1Y by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.24773\,\lambda^2}{\lambda^2 - 0.00926607} + \frac{0.101955\,\lambda^2}{\lambda^2 - 0.0451754} + \frac{0.35048\,\lambda^2}{\lambda^2 - 45.0187}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.01 \times 10^{-6} + 2(1.18 \times 10^{-8})\,\Delta T + 3(-4.19 \times 10^{-11})\,\Delta T^2 + \frac{6.98 \times 10^{-7} + 2(6.69 \times 10^{-10})\,\Delta T}{\lambda - 0.226}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$