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Colorimetric Technology for Can Makers
by Amir Novini, President - Applied Vision Corp. Posted 09/19/2011Minimizing rejected and hold-forinspection product with easy-to-use color measurement tools
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.
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.
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.
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.
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.
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
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. 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.
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.
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.
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.
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.
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 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.
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.
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 (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.
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.