Videos of the studied sample can be easily obtained with a camera attached to the microscope. The path of any particle moving within the sample can be obtained from video images using image processing techniques.
We use a particle tracking software programmed in LabView with IMAQ-Vision library to measure the XY-path of the particles from a video recording.The program makes the following image processing steps in order to locate the particle center of mass of a particle, with subpixel resolution.
In each recorded frame, a low pass filter is applied in order to reduce the camera noise. A shape detection nonlinear filter accentuates the contours of the objects. The obtained image is converted to a binary image using a threshold value either manually or automatically chosen. The obtained objects are dilated to ensure that all the contours define closed shapes, and finally holes are filled. The lasts steps consist of erode the objects to compensate the previous dilation, remove small particles (due to dust or noise), and finally, calculate the centroid of each particle.
This process can be done from a video recording but also in real-time by automatically processing each new frame registered by the camera. The tracking successfulness
strongly depends on the sample illumination, particle defocus or particle size. The image
processing parameters can be adjusted depending on the experimental conditions in order to optimize the efficiency of the
tracking process. Center of mass detection if 1um bead can be achieved with a resolution below 10 nm. Video detection is usually less accurate than back-focal-plane interferometry, but it has imaging capabilities.
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