| 摘要 |
A method for accurately determining a center of target marks provided on a workpiece. The method involves imaging, at an imaging station, a circular target mark associated with the workpiece. A signal produced by the imaging operation is binary-encoding, in an image processor, and displayed on a monitor screen connected to said image processor. Starting points X1 and X2 of a different level of intensity are detected by scanning in one of either an X-axis direction or a Y-axis direction on a center of a cursor (shown on the monitor screen) when the center of the cursor is within the target mark. Other starting points Y1 and Y2 of a different level of intensity are detected by scanning in another of the X-axis direction or the Y-axis direction on a mid-point of coordinates of the starting points X1 and X2. Still other starting points X1' and X2' of a different level of intensity are detected by scanning, in parallel to the direction of the starting points X1 and X2, on a mid-point of coordinates of the starting points Y1 and Y2. A reference value of a diameter in the X-axis direction and the Y-axis direction of the target mark stored in a memory (ROM) is compared with a distance between Y1 and Y2 and a distance between X1' and X2', in order to determine an intersection point 0 of segments between points Y1 and Y2 and X1' and X2' as the center of the target mark when the comparison matches.
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