Light scattered from a single particle suspended in a quadrupole trap is focused on the detector array in a charged coupled diode (CCD) camera. The video signals obtained from two successive rows of sensors framing the center of the image are compared and the difference used to control a superposed vertical field, holding the particle at trap center. The system is effective for liquid and solid particles in the size range from 1 to 10 μm trapped in vacuum or vapor.

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© 1990 American Institute of Physics.
1990
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