本論文提出一套新的自動對焦技術─「疊紋自動對焦技術」。利用螺旋疊紋，配合光學系統來分析反射光的光學特性，並藉由反射光之特性來開發自動對焦系統。
疊紋是藉由疊合兩間距相近的光柵，所構成的低頻圖形。本論文採用的研究方法敘述如下：一準直雷射光經由透鏡射向待測物，經待測物反射的光線仍由該透鏡接收。反射光的發散、收斂或準直的程度，將決定於待測物是否位於透鏡的焦平面上。我們將準直（收斂或發散）的反射光通過兩分離的螺旋光柵，將可觀察到輻射狀（順時針旋轉或逆時針旋轉）的螺旋疊紋。藉由疊紋形狀變化，搭配本研究開發的疊紋判讀技術，即可判斷待測物離焦程度。
根據實驗結果，本系統在長焦距聚焦透鏡下，工作距離最長可達200 um，其解析度為5 um。解析度在短焦距下可達0.05 um，其工作距離為2 um。由此可知，使用者可依據不同的使用需求，選擇需要的規格，達到對焦的目的。
本系統利用疊紋的方式，可以精密的對焦。搭配新式的演算法，可提升對焦速度。於光學檢測系統或雷射加工系統的應用上，將是一套有發展潛力的自動對焦技術。

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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