L-LAH81 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.89927\,\lambda^2}{\lambda^2 - 0.0102902} + \frac{0.270979\,\lambda^2}{\lambda^2 - 0.0424227} + \frac{1.33164\,\lambda^2}{\lambda^2 - 100.968}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[7.78 \times 10^{-6} + 2(1.14 \times 10^{-8})\,\Delta T + 3(-1.69 \times 10^{-11})\,\Delta T^2 + \frac{7.56 \times 10^{-7} + 2(7.92 \times 10^{-10})\,\Delta T}{\lambda - 0.241}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$