PBH56 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.78553\,\lambda^2}{\lambda^2 - 0.0136046} + \frac{0.446685\,\lambda^2}{\lambda^2 - 0.0570875} + \frac{1.21749\,\lambda^2}{\lambda^2 - 129.968}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.53 \times 10^{-6} + 2(1.29 \times 10^{-8})\,\Delta T + 3(-3.4 \times 10^{-11})\,\Delta T^2 + \frac{1.93 \times 10^{-6} + 2(1.53 \times 10^{-9})\,\Delta T}{\lambda - 0.268}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$