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Using Machine Vision Based System for Benchmarking Various Printing Plate Surfaces

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Traditionally a dot meter or an optical microscope fitted with a camera is used to analyze the dot position and dot area fraction of offset plates. Dot meters do not provide valuable information such as dot raggedness and dot positioning. These features are important when the plates are exposed digitally using laser diodes. The alignment of individual laser diodes is very important for controlling the position of the dots and in turn controlling the moire patterns appearing the final full color prints. Dot position verifies diode quality, alignment, and ensures output quality.

The optical microscope system that was used by Presstek in the past could capture optimal contrast images only of metallic plates. Also, the software that had been used for analysis was only capable of measuring dot percentage and a few dot quality parameters, and could not provide enough useful information about other plate and print quality features. To achieve accurate dot percentage results, the threshold values had to be changed manually for each nominal dot area fraction value. After each analysis, the measurement result of the actual dot area percentage had to be saved manually to a file from the software.

Presstek (a manufacturer of wide range of specialty offset printing plates) deals with plates of different materials, many of which are unique, so a new image inspection system was required to provide the quality assurance necessary across the various product lines. It was recognized that this system needed to be able to successfully measure on various plate media, and be able to adjust threshold values, store images, and save results to a file automatically. It was also necessary for the system to be able to perform various plate and print quality analyses including dot area fraction, dot position, feature location, etc.

An ImageXpert system with a 3-CCD color camera and high magnification optics is now in use. The combination of camera and ring light illumination source is capable of capturing images from various plate materials with excellent contrast. In addition, when co-axial illumination is necessary, images can be captured using our existing microscope and can be imported into ImageXpert for analysis. The software provided with the system is capable of measuring dot area percentage (even in the difficult 50% dot area range) with automatic thresholding, and it can measure dot positions, various dot quality attributes, and perform printability analysis on printed sheet as well as imaged plates.
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Document Type: Research Article

Publication date: January 1, 2003

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  • For more than 30 years, IS&T's series of digital printing conferences have been the leading forum for discussion of advances and new directions in 2D and 3D printing technologies. A comprehensive, industry-wide conference that brings together industry and academia, this meeting includes all aspects of the hardware, materials, software, images, and applications associated with digital printing systems?particularly those involved with additive manufacturing and fabrication?including bio-printing, printed electronics, page-wide, drop-on-demand, desktop and continuous ink jet, toner-based systems, and production digital printing, as well as the engineering capability, optimization, and science involved in these fields. In 2016, the conference changed its name formally to Printing for Fabrication to better reflect the content of the meeting and the evolving technology of printing.

    Please note: For purposes of its Digital Library content, IS&T defines Open Access as papers that will be downloadable in their entirety for free in perpetuity. Copyright restrictions on papers vary; see individual paper for details.

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