Single-particle tracking PALM

Sparse photoactivation allows successive positions of individual labelled molecules to be linked into trajectories; activation of fresh subsets yields many tracks in living cells. This extends photoactivated localization microscopy from accumulated positions to mobility. For free two-dimensional diffusion with independent per-coordinate fluorescence localization precision $\sigma$, instantaneous frames give mean squared displacement $4D\Delta t+4{\sigma }^2$. Motion blur, tracking errors, confinement and photobleaching must be accounted for before assigning mobility classes.^[ref]