Real-time identification of the singleness of a trapped bead in optical tweezers

Beads trapped in optical tweezers are aligned along the optical propagation direction, which makes it difficult to determine the number of beads with bright-field microscopy. This problem also dramatically influences the measurement of the optical trapping based single-molecule force spectroscopy. Here, we propose a video processing approach to count the number of trapped micro-objects in real time. The approach uses a normalized cross-correlation algorithm and image enhancement techniques to amplify a slight change of the image induced by the entry of an exotic object. As tested, this method introduces a ∼10% change per bead to the image similarity, and up to four beads, one-by-one falling into the trap, are identified. Moreover, the feasibility of the above analysis in a moving trap is investigated. A movement of the trap leads to a fluctuation of less than 2% for the similarity signal and can be ignored in most cases. The experimental results prove that image similarity measurement is a sensitive way to monitor the interruption, which is very useful, especially during experiments. In addition, the approach is easy to apply to an existing optical tweezers system.