Novel coherent optical defect detecting methods, which can distinguish between opaque defects and transparent defects, are proposed. One of these novel methods makes use of a holographylike polarity reference signal and is suited to automatic detection systems. The other employs two independent light sources, and suitable for visual detection systems. Realizing these novel methods using omnidirectional spatial filters, simple but effective periodic pattern defect-detecting systems are formed, which can be easily accommodated with defect touching-up stages.
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