L-LAH84 by Ohara

$$n(\lambda) = \sqrt{1 + \frac{1.83606\,\lambda^2}{\lambda^2 - 0.0098197} + \frac{0.34172\,\lambda^2}{\lambda^2 - 0.0385636} + \frac{1.3528\,\lambda^2}{\lambda^2 - 107.046}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[8.14 \times 10^{-6} + 2(9.68 \times 10^{-9})\,\Delta T + 3(-3.1 \times 10^{-12})\,\Delta T^2 + \frac{8.24 \times 10^{-7} + 2(8.84 \times 10^{-10})\,\Delta T}{\lambda - 0.23}\right] \Delta T = T - T_0, \quad T_0 = 25\,°\text{C}$$