LAKL21 by Schott

$$n(\lambda) = \sqrt{1 + \frac{1.29854\,\lambda^2}{\lambda^2 - 0.00628044} + \frac{0.350522\,\lambda^2}{\lambda^2 - 0.0210843} + \frac{1.1709\,\lambda^2}{\lambda^2 - 85.4205}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[2.58 \times 10^{-6} + 2(1.19 \times 10^{-8})\,\Delta T + 3(-5.03 \times 10^{-11})\,\Delta T^2 + \frac{2.69 \times 10^{-7} + 2(2.47 \times 10^{-10})\,\Delta T}{\lambda - 0.265}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$