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Proceedings Paper

Automatic focusing system for optical tweezers
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Paper Abstract

A design of a microscopy system tailored for optical tweezers with a capability of an automatic focusing is presented. In this design, we utilize lenses, motorized mechanical stage, lamp and a digital camera to magnify and see a micrometer sized spheres floating in a thin film of water. The system can automatically translate the stage that holds the specimen to obtain the best focused image. The best focused image is "sharp." Mathematically, the best focused image shows the maximum amount of high frequency terms from the images obtained by translating the stage. The metric that calculates how one image is focused is called the Focus Measure (FM). Unfortunately, low frequency components also increase this FM. And an optical imaging system is a low pass filtering system. Thus the primary concern is to lower the low frequency components in an image. The electric signals from each pixel of a CCD include noises that are inherent in each pixel. The result of this is an FM profile that has multiple local maxima. This is the most critical reason why an Automatic Focusing System (AFS) yields incorrect focusing results. Available techniques have been tested and from this experience, the most appropriate Focus Measure Filter (FMF) that has the sharpest FM despite the low frequency terms and noises has been selected. Furthermore, a maximum search algorithm that is immune to local maxima in an FM profile is discussed. Using this FMF and search algorithm, an Automatic focusing system (AFS) tailored for optical tweezers is presented. The system is implemented on personal computers equipped with Pentium 4 processors.

Paper Details

Date Published: 5 December 2005
PDF: 12 pages
Proc. SPIE 6048, Optomechatronic Actuators and Manipulation, 60480B (5 December 2005); doi: 10.1117/12.644461
Show Author Affiliations
Je-Hoon Song, Gwangju Institute of Science and Technology (South Korea)
Sun-Uk Hwang, Gwangju Institute of Science and Technology (South Korea)
In-Yong Park, Gwangju Institute of Science and Technology (South Korea)
Yong-Gu Lee, Gwangju Institute of Science and Technology (South Korea)


Published in SPIE Proceedings Vol. 6048:
Optomechatronic Actuators and Manipulation
Kee S. Moon, Editor(s)

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