White Paper:
Colorimetric Technology
for Can Makers
Minimizing rejected and hold-forinspection product with easy-to-use
color measurement tools
Author: Amir Novini, President and CEO
Applied Vision Corporation
Keywords: beverage cans; color measurement; colorimetry;
consistency; accuracy; machine vision; camera-based systems;
innovative inspection technology
© 2011 Applied Vision Corporation; Applied Vision logo, Simply Smart Machines,
KromaKing and Global Colorimetric Standard (GCS) are trademarks or registered
trademarks of Applied Vision Corporation. Other names may be trademarks of their
respective owners. Printed in the USA. 1/11
White Paper: Colorimetric Technology for Can Makers
Contents
Background: Why measure color?......................................3
Methods of color measurement..........................................4
The human eye.................................................................4
Spectrophotometry..........................................................6
Delta-E values...................................................................7
Camera-based.................................................................8
Colorimetric technology......................................................8
KromaKing® tools for can makers.......................................9
About Applied Vision Corporation....................................11
White Paper: Colorimetric Technology for Can Makers
Background: Why measure color?
To the world’s leading beverage makers, the precise, trademarked colors
and patterns on their containers are their brand’s identity. Such are the
consistent graphics that consumers recognize and reach for. Moreover,
today’s cans are decorated with increasingly vibrant colors and dazzling
patterns to compete for the eye at retail.
Heightened standards
As can makers are aware, color and graphics variations violate brand
identity standards. At store level, anomalous-looking containers critically
impact shoppers’ purchasing decisions and brand perception. Accordingly,
top-brand beverage makers are trending to require 100% color consistency,
with the rest of the industry likely to follow.
Containers rejected and/or held for inspection (HFI) due to color nonconformity
and decoration defects are a significant cost concern for can makers. Highly
complex graphics and exacting requirements now heighten the risks.
“Containers rejected
and/or held for
inspection (HFI) due
to color nonconformity
are a significant cost
Global considerations
Achieving absolute color conformity across billions of cans is a difficult
prospect. Manufacturing them in multiple global locations adds to the
challenge. Although can decoration technology has never been more
sophisticated and precise, a host of conditions can adversely affect the
process at any time, in any plant location in the world.
concern for can makers.”
As marketers compete for the consumer’s attention, can colors and decorations
are becoming increasingly vivid and intricate.
To ensure 100% color consistency on beverage cans, precise color
measurement must be achieved by a method that is reliable and easy to
implement in manufacturing operations. This paper examines the methods
devised and used over recent decades to ensure color conformity, culminating
in colorimetric technology, its nature, advantages and limitations, and current
availability to can manufacturers.
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White Paper: Colorimetric Technology for Can Makers
Methods of color measurement
Ensuring color consistency requires a precise tool for measuring color.
In the case of beverage cans (including today’s aluminum bottles), that tool
must measure color on highly reflective, cylindrical-shaped surfaces, reliably
and accurately.
Ideally, that tool should also be able to register patterns, interpret process
colors and, based on a holistic view of the container, make simultaneous
measurements in multiple regions of interest. In real-world applications the
tool would also need to perform its functions on cans moving at line speed
and randomly oriented.
“The human eye is a poor
judge of color.”
After exploring and implementing colorimetric (color measurement)
inspection for 30 years, researchers at today’s Applied Vision Corporation
are keenly aware of the capacity and limitations of the human eye, and
expert in spectrophotometry and machine vision for container decoration
color measurement.
The human eye
Since their implementation decades ago, human inspectors have continually
proven unsuccessful at ensuring decoration consistency and color accuracy
in beverage can making operations. The reasons range from physiological
to practical.
Though capable of remarkable feats of color perception, the human eye is
a poor instrument for measuring color. The typical human eye responds to
wavelengths from about 390 to 750 nm and can distinguish unsaturated or
“process” colors created by mixes of multiple wavelengths, such as pinks
and purples.
Color perception differs substantially from individual to individual. The intensity,
color and spatial distribution of illumination govern how color is perceived.
Shape and shading are particularly influential, as demonstrated in the sets
of Color Perception Tests illustrated here. Moreover, production lines present
cans for inspection at rates ranging from hundreds to thousands per minute,
and in random orientation.
Not surprisingly, human inspectors – even those specially trained or said to
have “calibrated vision” – have been replaced with automatic inspection
systems by quality can makers worldwide. Yet while some of these systems
offer reliable decoration inspection and mixed-label detection, until recently
they have not been able to provide accurate color measurement.
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White Paper: Colorimetric Technology for Can Makers
The brown squares on the cube
are the exact same color. This
illusion occurs when the brain
judges quantities relative to their
surroundings rather than an
objective standard.
Color Perception Test 1
(Created by R. Ausbourne)
The illusion that B is lighter than A
is caused by the relative contrast of
the surrounding dark squares and
by the fact our vision compensates
for the shadow of the cylinder.
Color Perception Test 2
(Created by Edward H. Adelson, Professor of Vision Science at MIT)
Colors often appear brighter when
bordered by frames. Note how
the colors on the left appear more
vibrant and pure.
Color Perception Test 3
(Created by Rashid Bin Muhammad)
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White Paper: Colorimetric Technology for Can Makers
“A spectrophotometer,
because of practical
drawbacks, is not
ideal for can making
applications.”
Spectrophotometry
Light-measuring instruments, specifically spectrophotometer-based devices,
have been used to measure color for many years. A typical spectrophotometer
features a small (3 mm to 5 mm) aperture that allows it to measure color over
a tiny area of an object. As the illustrations show, when the light from the area
reflects into the device, a diffractive surface breaks it down into its primary
colors. A spectrophotometer can then provide very accurate measurements
of those colors. That accuracy is the key advantage of these devices, along
with their relatively low cost.
By design however, these devices have practical drawbacks for can making
applications. Because they only assess one tiny area at a time, they are
not suited to inspect complex colorful patterns. Nor are they designed to
measure process colors or halftones. In short, they do not perceive color
like the human eye, which, at point of sale, is the ultimate judge of container
color and pattern quality.
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White Paper: Colorimetric Technology for Can Makers
Delta-E values
Delta-E (dE) is a unit used to quantify the “distance” (difference) between a
reference color and a similarly colored sample, based on coordinates in the
so-called Lab color space. The printing, paint and ink industries, for example,
have traditionally relied on this metric, employing spectrophotometers to
obtain the measurements.
The Lab color model is intended to approximate human vision, in the sense
that it is perceptually uniform: the perceived difference between two colors
separated by one unit of distance (dE = 1) is independent of the region of
color space (red, green, etc.) in which the colors are located. A dE of one
or less represents a barely perceptible difference.
The adjoining figure illustrates the concept of perceptual uniformity and
non-uniformity. All colors that can be perceived by the human eye are
contained in the horseshoe-shaped region (this particular diagram represents
the so-called XYZ color space, which is not perceptually uniform). Colors
falling on the boundary of each ellipse are 10 dE units from the ellipse center.
Note how the sizes of the ellipses increase dramatically as one “moves” from
blue to green. Yet the perceptual “span” of the colors within each ellipse will
appear about the same to a typical observer. In the corresponding diagram
for Lab color space, each ellipse would be a circle and all the circles would
be the same size, with a radius of 10 dE.
Thus, human color perception acuity varies depending on the color, and the
dE metric attempts to compensate for this.
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White Paper: Colorimetric Technology for Can Makers
Camera-based
“To meet the demand for
100% color consistency,
practical tools for
Camera-based technology bridges the most useful capabilities of man and
machine for the purpose of practical, accurate color measurement. Spectrophotometers do not function the same way as the human eye, but cameras,
by design, “see” and classify color the way we do. Their matrix of RGB (red,
green, blue) sensors operates with sensitivity to light levels and color like the
photoreceptors (cones) in the eye, and the range of colors perceived by a
camera – its “gamut” – is similar to the gamut of human vision.
can makers exist in
inspection technology.”
Today’s digital cameras are reliable and affordable. With the advent of
bright and long-lived white LED illuminators, gigahertz-class computers and
“intelligent” algorithms for pattern recognition, camera-based color inspection
systems have, over the last decade, matured. They rival humans in qualitative
assessment of complex colorful patterns, and greatly exceed human capability
in quantitative color measurement, speed and dependability. To meet the
demand for 100% color consistency, practical inspection tools for can
makers now exist.
Colorimetric technology
Camera-based colorimetric technology has been a focus of machine vision’s
leading developers since 1979. This work converged and continued with the
establishment of Applied Vision Corporation in the 1990s.
In pursuit of accurate color measurement, fundamental challenges – such
as providing highly stable illumination and automatic self-calibration – have
been resolved. Based on advances in solid-state lighting, digital cameras,
computer processors and pattern-recognition algorithms, the KromaKing®
line of camera-based systems provides accurate, multiple-region color
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White Paper: Colorimetric Technology for Can Makers
measurement and inspection for products ranging from floor coverings,
to credit cards, to food and beverage containers.
To measure and inspect complex can decoration in a real-world setting
requires application-specific software, simple and intuitive user interfaces,
and robust algorithms. In some cases, dedicated part-handling mechanisms
are essential, such as indexing star wheels capable of spinning a can while it
is imaged with a line-scan camera, which generates an “unwrapped”
(complete outside) view of the container surface.
Applied Vision holds five patents
(with additional patents pending) for
color inspection.
To summarize the concepts simply and meaningfully, Applied Vision calls
this Global Colorimetric Standard (GCS) technology. GCS technology
provides consistent results worldwide, in a range of system designs for
varying inspection needs.
KromaKing® tools for can makers
For years, can makers worldwide have benefited from color decoration
inspection and mixed-label detection provided by intuitive, camera-based
KromaKing® systems. Applied Vision has invested tens of thousands of hours
of research and development in making color measurement and
decoration inspection reliable and easy for can makers. The goal is to
minimize rejected and HFI product, for increased operational productivity
and profitability.
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White Paper: Colorimetric Technology for Can Makers
Currently, a suite of KromaKing® colorimetric tools is available to address
various sizes and stages of two- and three-piece can manufacturing
operations. They include:
• High-speed online system (up to 3,000 cans per minute) for sampling
approximately 10% of every can for color drifts and process problems.
With randomly oriented cans traveling on a conveyor, the entire label is
sampled and inspected in just a few seconds of production.
• Low-speed online system (less than 300 cans per minute) for 100%
inspection of every can’s label.
Decorator blanket inspection covering
100% of production at line speeds.
• Decorator blanket inspection (up to 3,000 cans per minute) covering
100% of production at line speeds for process monitoring.
• Offline laboratory sampling system with 100% inspection of every
can’s label, including colorimetric (dE) measurements.
Each system measures multiple regions of color in complex patterns for
flaws, including color shifts, pattern defects and print registration errors.
Equally important, these systems feature Applied Vision’s field-proven
Touch-n-Go® user interface, a key to the successful deployment of machine
vision systems.
KromaKing® tools can make simultaneous NIST-traceable measurements in multiple
user-defined regions.
All of its colorimetric systems employ Applied Vision’s proprietary GCS
technology to ensure that cans conform, every run, wherever in the world
they are being made. The systems are supported and serviced by Applied
Vision’s global network of sales, service and support offices, OEMs and
full-service distributors in Asia, Australia, Europe, Latin America, the Middle
East, North America and the United Kingdom.
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White Paper: Colorimetric Technology for Can Makers
About Applied Vision Corporation
Color inspection has been a significant focus of Applied Vision since its
inception in 1997. The company currently holds five patents (with additional
patents pending) on applications related to color inspection.
Headquartered in Akron, Ohio, Applied Vision Corporation designs and
manufactures application specific machine vision inspection systems for
packaging, food and beverage containers and color printing. Its systems
are sold worldwide through original equipment manufacturers (OEMs),
value-added resellers (VARs), full service distributors and company-direct.
For more information on Applied Vision’s machine vision solutions,
contact the company at 330.926.2222 or visit www.appliedvision.com.
2020 Vision Lane
Akron, Ohio 44223 U.S.A.
Tel: +1.330.926.2222
Fax: +1.330.926.2250
Email: [email protected]
Web: www.appliedvision.com
Global sales, service and support:
Tel: +1.866.348.2383
Web: www.appliedvision.com/sales.html