Sequential detection of web defects

A system for detecting defects on a moving web having a sequential series of identical frames comprising:

• imaging means for forming a real-time image of a frame;
• comparison means for comparing elements of an exemplar frame with corresponding elements of a real-time image from said imaging means;
• said comparison means using sequential detection on each pair of elements to provide:
• an acceptable indication if the pair of elements are determined to be statistically identical;
• and
• a defective indication if the pair of elements are determined to be statistically not identical;
• and
• a look-again indication if the pair of elements is neither acceptable nor defective, said comparison means recursively comparing said element of said exemplar frame with corresponding elements of other frames on said web until one of said acceptable or defective indications occur.