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: [email protected].
Dr. Connolly is a consultant for Eltromat.