PATTERN INSPECTION SYSTEM FOR INSPECTING DEFECT OF LAND PATTERN FOR THROUGH-HOLE ON PRINTED BOARD

A pattern inspection system for inspecting a pattern of a land for a through-hole formed in a printed board. In the system there are included a first illumination device for optically illuminating the printed board, a second illumination device for illuminating the printed board with light modulated at a predetermined period and an image pickup responsive to reflection light and transmission light from the printed board due to the first and second illumination devices to photoelectrically convert the printed board into a gray level image. The gray level image is converted into a bi-level image by a bi-level conversion device and then converted into an edge image by an edge detecting device. The edge image is expanded and contracted by predetermined amounts so as to obtain a through-hole image corresponding to the through-hole. A defect detecting device detects a defect of the pattern of the printed board on the basis of the through-hole image and the bi-level image from the bi-level conversion device.