by John Kaverman, Innovative Marking Systems
Technology Feature
July-August2008
I believe we can all agree that color is the single most misunderstood property of the products that we decorate. If you don’t believe me, ask me about the customer who faxed me a color chip.
For someone to perceive any color you have to have three things: a light source, an observer and an object. These three components of color perception are commonly referred to as the “color triplet.” Each component plays an important role in how color is perceived. This article will identify the variables inherent to each component and explain how minimizing their influence on the perception of color is the key to successful color representation.
Light Source
Varying lighting conditions will change a color’s appearance. For example, the color of an object may be perceived differently under fluorescent lighting than under natural sunlight. Therefore, when evaluating the appearance of color on a pad printed part, it is imperative that you do so under the same lighting conditions as your customer does.
Short of having a customer representative on hand to evaluate color with you each day, the best way to ensure that youre both on the same page with regard to lighting conditions is to have your customer specify the lighting conditions to be used for evaluating color. The most obvious method for accomplishing this is to use standardized light booths.
Observer
Unless youre using a color measurement device such as a spectrophotometer for color evaluation, the process will always be subjective. No two human observers will evaluate a given color the same, regardless.
In all cases, the proportion of long-wavelength-sensitive cones to medium-wavelength-sensitive cones in the retina as well as the profile of light sensitivity in each type of cone and the amount of yellowing in the lens and macular pigment of the eye differs from one person to the next. Second, a given person’s perception of color can vary depending upon his mood, what he recently ate, or even his body temperature. Finally, physical variables exist with which to contend such as the viewing time, angle, and/or distance.
Once again, it is important to standardize things like the color of the background, viewing distance, angle, and time used by all evaluators. The same applies even if using a color measurement device. The device must be properly calibrated and configured using the same measurement parameters (i.e., light source, angle, spectral inclusion or exclusion, etc.) as the device used for final approval.
Object
The majority of problems arise from misunderstanding the physical characteristics of the object. (For this discussion, object and sample are synonymous.)
When the manufacturing methods and/or materials used to create the visual standard vary from those used to create the object (sample) that is being evaluated v. that standard, numerous problems arise. Therefore it is important that a visual standard is obtained that most accurately represents the “target” (i.e., compare apples v. apples, not apples v. oranges).
For example, problems arise when the following conditions are present:
- The standard has a matte finish and the sample is glossy.
- The standard is made of natural materials and the sample is synthetic.
- The standard is painted and the sample is printed.
In each case, it is important to realize that the range of colors that can be reproduced by a given process is limited by what is commonly referred to as the color gamut of that process.
Injection molding, painting, powder coating, hot stamping, screenprinting, offset printing, and pad printing all have their own, unique color gamut. Sometimes colors that are created using one process cannot be recreated using another process. When that happens, such colors are said to be out of gamut.
When color matching, if the previously mentioned variables are not, or cannot be, controlled, the best that can be achieved is a metameric match.
Metamerism
A metamerism is the situation in which two color samples with different spectral power distributions (reflected wavelengths) appear to be the same color when viewed side by side under one set of conditions, but not necessarily under another. Such colors are referred to a metamers.
Metameric matches are extremely common. In fact, the basis for nearly all commercially available color image reproduction processes such as photography, television, printing, and digital imaging is the ability to make metameric color matches. Four types of metameric failure exist, including illuminant, observer, geometric, and field size.
Sound familiar? That is because the terms correspond directly to the three members of the color triplet. An illuminant metameric failure is a result of varying lighting conditions (i.e., colors that match in cool white fluorescent light do not match in natural sunlight).
An observer metameric failure is the result of having two different observers (i.e., the decorator and the customer, two different color measurement devices, etc.).
A geometric metameric failure results when material attributes vary between the standard and the sample/object. Normally, these attributes (translucency, gloss, or surface texture) are not considered in color matching. However, geometric metameric failure can occur when two samples match when viewed from one angle, but then fail to match when viewed from a different angle. A common example is the color variation that appears in pearlescent auto finishes.
A field size metameric failure results when the size of the two color samples varies significantly. For example, two 1 square color samples may appear to match when viewed side by side, but when the sample colors are viewed as 1″ square and 4″ square samples, they dont appear to match. This is a result of human physiology: the perception of the sample color is affected by colors that are reflected to the eye from the periphery (area around the sample).
Color Measurement Devices
Assuming you have a measurable standard and sample, color measurement devices are useful tools, even when metameric failures exist.
The benefit of color measurement devices is that they can be configured to eliminate variables involving light source, viewing angle, viewing time, field size, and gloss level. It is when they are relied upon as the sole means of qualifying a color that problems arise.
A spectrophotometer works in guiding you to a successful color match the same way a global positioning system works to guide you to a destination. Sometimes the computer will say that you have an acceptable match when, in reality, you dont just like a GPS can say youve arrived when youre actually in the wrong neighborhood.
If a human, and not a computer, is the one using the product, shouldnt the humans have the final say in whether a color is acceptable or not?
As decorators, we must educate our customers as to how color is perceived, and assist them in eliminating as many variables from the process of obtaining an acceptable color match as possible. Work with them to identify and document specifications for light source, standard and sample field size, viewing distance, viewing angle, viewing time, and (where applicable) color measurement device configurations. Failing to do so may be a costly mistake.
Sources:
Wyszecki, Gunter and Stiles, W.S. (2000). Color Science Concepts and Methods, Quantitative Data and Formulae 2nd edition, New York: Wiley-Interscience.
R.W.G. Hunt. The Reproduction of Color 2nd edition, Chichester: John Wiley and Sons, 2004.
Mark D. Fairchild. Color Appearance Models, Addison Wesley Longman, 1998.
John Kaverman is the technology coordinator for Innovative Marking Systems of Lowell, Mass. He holds a degree in printing from Ferris State University and has over 20 years of combined screen and pad printing process experience. He may be reached at [email protected].
