| dc.description.abstract | In this paper, a novel active auto-focus system based on moiré effect is presented.
The moiré pattern refers to a geometrical interference fringes, and is formed by two gratings that lie in contact, with a small angle between the grating lines. As a result, we see a fringe pattern with much lower frequency than the individual gratings. The moiré effect can be applied to topographic mapping, analysis of strain, and measurement of focal length of lenses. Our auto-focus method is described below. First, a collimated laser beam passes through a focus lens and incident on the sample. If the sample locates on the focus of the lens, the light which reflects to the lens will be collimated. On the contrary, the reflected light will be divergent or convergent when the sample is out of focus. Then the reflective light passes through two spiral gratings with Talbot distance apart, and three types of the moiré patterns are observed. That is, when the sample moves along the optical axis, the divergent (or convergent) reflected light will produce a clockwise (or counterclockwise) spiral moiré pattern, and collimated one produces radial moiré pattern. We can determine whether the sample locates on focus or not by the radial or spiral moiré pattern.
We used a linear CCD to capture a part of the moiré pattern, and determine the spiral magnitude by a new defined focus value (FV). In principle, FV is proportional to the defocus distance of the specimen. Therefore, FV is used as a feedback signal to the auto-focus system. The experimental results show that our auto-focus system has been successfully applied to polished material with a resolution of 0.05μm. In addition, the dynamic range can be 200μm with long-focus lens.
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