Influence of detector type on object position measurement in optical tweezers.

Determining the optical potential is one of the crucial aspects of the analysis and control of optical tweezers. It requires finding the scale factor of displacements in an optical tweezer recording, which typically is hard to obtain directly through a photodetector, and instead is usually done by fitting the statistical features of the data to the Langevin dynamics predictions. We argue that this procedure is harder for measurements made using a Segmented Quadrant Detector, for which these scale factors can vary along different axes due to image distortions. Subsequently, it can lead to a false recognition of the potential's anisotropy. We quantify this phenomenon by comparing simultaneous measurements from a Segmented Quadrant Detector and a camera, as well as from the 2D Lateral Effect Detector and a camera. We then discuss which experimental and data analysis techniques should be used to properly determine the geometry of the optical potential.