Figure 9.8 — Optically trapped particle tracked by interferometry Figure 9.8 — Optically trapped particle tracked by interferometry. Detected position along the (a) x, (b) y and (c) z-directions (solid lines) and average particle position (dashed lines) of an optically trapped polystyrene sphere (a = 1.03 μm) using forward interferometric detection with QPD. The fact that the average is not null is mainly due to the amplification stages in the QPD acquisition card. The lighter lines in (a) and (b) show the results of a simultaneous position detection using digital video microscopy. These are the same data shown in Fig. 9.3 rescaled and shifted so that their variance and average are equal to those of the interferometric data. The sampling rate is 2500 frames/s.