Photographic film is usually manufactured on a wide, very long continuous roll, known as a web. Producers of this film product face a unique problem whereby the film can become embossed by particles stuck to the rollers that route the film through the process. Dimples form at regular intervals along the travel direction of the film making that cross-web strip of the film unusable. This kind of defect in the product is known as a repeat defect.

The repeat defect detection system was conceived and designed for a provider of laser scanning inspection equipment to be used for quality control by major film manufacturers. The repeat defect detector employed a microprocessor and signal processing methods that would locate and identify repeating marks at regular intervals. This process was carried out simultaneously in 100 cross-web locations. The precise spacing of the repeating marks was determined, along with the cross-web position, allowing the operator to take corrective action by cleaning the particle from the roller with the appropriate diameter, at the indicated cross-web location.

A complete description of the detection method can be found in patent #4134684.pdf