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Guaranteeing Food-Packaging Appeal

Photonics Spectra
Oct 2002
Dr. Christine Connolly Contributing Editor

What you see on a package of potato chips or cookies may not be what you find inside. One of the biggest problems with today’s high-speed presses used in food-package printing involves inspection of print color. Food manufacturers’ brand colors make the product quickly recognizable, so color accuracy is critical. Equally important, food pictured on the packaging should appear appetizing, a perception easily upset by subtle color changes.

One traditionally measures color with colorimeters, densitometers and spectrophotometers that must be brought into contact with the surface of the printed material. This is obviously not possible with presses running at a speed of 400 m/min or more. With reels up to 30 km long, sampling at every reel change provides too little in the way of quality control. Some spectrophotometers can measure color at a distance of a few centimeters, and these are useful where material of constant color is being produced, but they cannot measure the different colors printed in a design, such as in potato chip packets, for example.

Camera-based vision technology offers an alternative. Applications based on the AccuColour system from Eltromat GmbH, for instance, use a xenon flash to freeze the motion of the web and a triggered video camera to capture images of the moving product. Image processing and color transformation techniques extract pixel data from different colored regions of the design and present it in a form directly related to human color perception. This enables press operators to determine immediately not only whether a color is going out of tolerance, but also how to change the inks to bring the color back.

Underlying the vision technology is a camera control system that keeps the measurements stable and that ensures that any reported changes in measurement come from variations in the color of printing and not from changes in camera response or illumination variability.

This system uses the same twin-beam principle that underpins the accuracy of spectrophotometers: a calibrated white tile within the camera’s field of view measured at every image capture that is used to correct any variations in the image before processing color data. Camera aperture and channel gain controls also adjust automatically.

At the start of a print run, the operators adjust the inks until all areas of the print are visually acceptable. Then they capture a master image on the system and mark rectangles over the areas to monitor. The camera-based system captures an image every five seconds and logs the color of every area. A traffic light system immediately warns of any color going out of tolerance. An on-screen display shows the direction of movement of each color so that the press crew can adjust the inks and avoid producing faulty print.

The five-second-interval color log provides a valuable quality assurance record, with a frequency of measurement unobtainable by other color measuring approaches.

Contact: Eddie Gaskell, Eltromat GmbH, Bradford, UK; +44 1274 307888; e-mail: egaskell@eltromat.de.

Dr. Connolly is a consultant for Eltromat.


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