Errors in two particle tracking at close distances.

Tracking single and multiple particles is of great importance for many physical investigations in a variety of different areas. It is essential to find and eliminate sources of systematic errors in the particle position determination (PPD) and to determine the limits of its applicability to a given problem. Particularly when measuring the interactions between colloids at close distances, artifacts in the image taking process pose a great problem. By means of a simulation technique, we investigated the accuracy of the PPD using two-dimensional Gaussian and Gaussian-like fitting functions. For the distance between the two colloidal particles this revealed a systematic overestimation of the inter-particle distance of up to 1.9% of the particle diameter for the Gaussian fitting function. This deviation can be explained by the differences between the intensity distribution of the overlap of the simulated particles and the linear superposition of the Gaussian functions. Modifications of the fitting functions can reduce the systematic error significantly.