Drift Correction
Usually, the whole system drifts during the measuring. There are several methods to correct the drift, generally these can divided into 3 groups:
Frequency: The Brownian motion frequency is much larger than the drift frequency, so one can use high-pass filter in order to remove the drift. Similar effect can achieve by smoothing the data, and subtraction of the smoothed from the data (see figure).
Averaging over few beads: If few beads are shown in the frame, because every bead moving randomly, averaging over the position of them for every frame should give us the drift (it should subtracted from the data for having clean data).
Immobilized bead: If an immobilized bead is shown in the frame, we can take its position as a reference, and correct the data by the immobilized bead's position. (Another advantage of looking at immobilized bead, is the fact that the motion of it can tell us about the accuracy of the measure.)
Of course one can use more than one method.
Read more about this topic: Tethered Particle Motion, Data Analysis
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