Precise Position Detection of a Mesh-Backed Printing Screen Based on the Smallest Enclosing Circle Detection

In this paper we describe a new method of position detection for a mesh-backed printing screen with an alignment mark that is overlapped by mesh patterns. A circular hole is selected as an alignment mark so that at least a portion of its contour can be observed through the windows of the mesh wires. To recognize the true contour, the smallest circle enclosing binary patterns acquired with a transmitted illumination is detected. Then the mark position is calculated by approximating the recognized contour to a circle with the least square fitting. To speed up the procedure, a convex hull and the farthest-point Voronoi diagram is generated before the smallest enclosing circle detection. According to the experimental results, an accuracy of 0.2 pixels and a processing time of 1.0 seconds have been achieved.