Color Visual Evaluation
Author: Jean Hoskin

Visual Evaluation
Color is perception, so it is a unique experience for each individual.  In order to reduce the variation, there are procedures established to standardize the visual process.  In order to accurately describe visual color difference, it is necessary to control the three elements of the “color experience”: light source, object, and observer.

Controlling the Light Source
The best way to control the light source is with a certified light box. The light box should be in an environment where ambient light does not enter the viewing booth.

The Viewing Environment
Since light reflects from the surfaces of the light box, the viewing environment is very important. The surface should be matte, flat (no distortions), uniform in color, and match a gray within Munsell N5 to N7. The light booth should contain no other items, such as submittal pages or extra standards. Reflection from clothing of the observer is also possible, so neutral color apparel is recommended. The intensity of illumination should not deviate more than 25% from the center to the edge of the samples.

The Object:
The colorants in the object modify the light reaching the surface by reflection, absorption, or transmittance. The resulting color is a combination of these interactions. White light (all color) reaches the surface of the object. The light that does not form the perceived color (red) has been subtracted or absorbed (blue and green) by the object. The color perceived is reflected by the object to the eye, then brain, where color judgments are made by comparison. Color perception is relative and influenced by surrounding colors. Because color is so dependent on size, texture, angle and surround, several requirements are defined in visual procedures.

      1. The standard and the submittal should be side-by-side touching. The farther apart two colors are the more similar they appear.

      2. The standard and the submittal must be aligned in the same direction with respect to the weave, knit stitch or other texture (nap).

      3. The standard and the submittal should be folded as many times as necessary to make the sample opaque.

      4. The standard the sample should be illuminated from overhead and observed at a 90 degree angle.

      5. In addition, all samples should be clean, without labels or marks.

      6. Samples differing in size will appear different in color.

The Observer:
Even with a superior score on the Munsell 100 Hue Test, your daily health will impact your color decisions. The amount of rest or the amount of caffeine will have a strong influence.  All color originates in the spectral characteristics of the light that illuminates a surface.  When the light changes, the observer should allow 30 seconds to 2 minutes of adaptation to the new light source in the light cabinet. This allows your visual system to overcome the natural trait called chromatic adaptation. Chromatic adaptation allows us to interpret color within the context of the surroundings and the ability of the eye to adjust to different light sources or light levels.  For example: If you hold a white sheet of paper and walk into a photo dark room that is lighted with only a red lamp, you will still perceive the paper to be white, even though it appears to your visual system as pink.  For some great examples, check out:

• Colorcube.com
• Planetperplex.com

Evaluations should be made as quickly as possible, since prolonged viewing impacts observer ability to evaluate. Focusing on a color for an extended period of time produces an after image of the complementary color. After looking at the red star for one minute, move your glance to the grey box. What color star do you see?

Again focus on the red star for one minute, and then move your glance to the yellow star.  What color appears where the shapes intersect?

Visual Tolerances:
Unlike instrumental tolerances, which are objective, visual tolerances are very subjective.  Training on visual tolerancing frequently consists of side-by-side evaluation with an experienced colorist. Consistent quality for reference tools is difficult to achieve. The verbal descriptor for the Gray Scale for Color Change (AATCC) is a starting point:

5 = Equal; 4 = Slight; 3 = Noticeable; 2 = Considerable; 1 = Much

The gray scales are expensive, age, and may not easily be translated by the user to chromatic color or to color differences for a standard in a different value range. To add to the complexity, the perception of color difference is dependent on the visual surround. This perceptual phenomenon is called crispening.

Not only do the colors appear different on each ground, but also the amount of color difference between the two swatches is more apparent on the ground that most closely matches the value level of the samples. When an experienced colorist is not available to train you, a spectrophotometer can also serve as a learning tool. The spectrophotometer can help you learn to visualize the difference between acceptability and perceptibility. Study the visual difference compared to the instrumental numbers and your ability to make visual judgments will improve.

Unfortunately, visual evaluation cards are expensive and time consuming to produce.  They are generally for products that have a long life, such as uniforms or automotive materials. However, an approval book or virtual snip card will show the acceptable shade range based on previous decisions.

Summary
Color is the perception by an observer of light that has been modified by an object. All color originates in the spectral characteristics of the light than illuminates the object surface. The color we “see” is the result of the interaction between the incident light and the object. The conditions of the viewing environment and the ability and knowledge of the observer impact color judgment. Light source, object, observer, and surround are the elements to control in any visual evaluation process.

Appendix

Solution #1: Colors are different.

Solution #2: Colors are the same.

Solution #3: After Image. Red star produces a blue green (cyan) complementary after image.

Solution #4: After Image II. Red star produces a blue green after image, which when mixed with the yellow star produces a green intersecting shape.

Impact of Optical Effects
Author: Jean Hoskin

What is the impact of optical effects: after image, crispening, or simultaneous contrast?

After Image – Since the after image is the complement to the stimulus color, the after image will superimpose over the color being evaluated during prolonged viewing. This has the same impact as adding the complement or “dulling” the color. Staring at the pink star on the left will create a yellow-green after image. The yellow-green combines with the pink to create a duller pink, which is visible in the intersection area of the stars.

Crispening – The degree of color difference perceived depends upon the background. Even though these grey scale steps are designed to be consistent from white to black, the intermediate grey steps do not look consistent between the top set and the bottom set. The reason is that the two grey squares that are closest to the background color will appear to have a greater color difference than the other greys in the sequence. In the top example the step appears larger between grey chips 2 and 3. In the bottom example, the largest step between grey chips appears between 5 and 6. You can see how this makes it difficult to give accurate and consistent corrections for magnitude of color difference. Perceived color difference between samples is increased, if the samples and surround are similar. Standardizing and specifying the background improves color evaluation.

Simultaneous Contrast occurs when two colors are perceived at the same time. The difference between the two colors is more apparent or emphasized. A frequent illusion is one color appearing as two. In the example below, the Reds appear to be different. Interrupting the continuous Yellow line makes the Red appear Blue. Interrupting the continuous Blue line makes the Red appear Yellow. The solution is in the Appendix.

In the below figure, the Purple squares are the same, however, they appear different because of the influence of the surrounding colors. The Red and Magenta background makes the Purple chip appear Blue cast, whereas the Green and Cyan background makes the Purple chip appear Red cast. The solution is in the Appendix. You can see that evaluating color in pattern or in a surrounding color that is different than the specified light box grey value will create inconsistent or inaccurate color comments.

Another illusion is when two different colors appear more similar. In the example below, the small yellow-green squares look more alike than they do when placed side by side. The solution is in Appendix. The Yellow-Orange and light Yellow background makes the yellow-green chip “A “ darker and bluer. The Blue Green and Royal background makes the yellow-green chip “B” appear lighter and more yellow.

What does research and industry practice reveal about visual color evaluation?

Unique Hues
Normal color vision has a range of perceptibility. Research at North Carolina State University has explored the concept of Unique Hues. There are generally considered to be four unique hues – red, green, yellow and blue. A unique hue is one that contains no other hues. For example, a unique red would be perceived as containing no yellow or no blue cast. Research has shown that people with normal color vision vary widely in what is considered a unique hue. For more information on this research, check the website for presentations by Rolf Kuehni at http://www4.ncsu.edu/~rgkuehni

Another interesting concept presented by Rolf Kuehni at the AATCC symposium was that “our color visual system is fine-tuned to make narrow distinctions between color stimuli, particularly in terms of hue differences.” However, as yet there is “no neurophysiologic evidence that humans are equipped to assess the physical magnitude of stimulus difference in a quantitative manner.”

Inaccurate comments for small color differences

• According to a research study completed in 2006, Revel reported that
• Color vision acuity does not predict one’s ability to describe color differences seen.
• Age and experience do not predict one’s ability to describe color differences seen.
• Job functions showed a difference preference to approve for reject.
• The ability to identify correctly the major color difference elements improves dramatically above 0.55 DEcmc. 58% of all participants missed the major color difference element in pairs with less than a 0.56 DEcmc.
• The impact of giving the mill inaccurate comments results in extra rounds for submittals and more time.

Source: Revel, Carol T. "Describing Color Differences: How Good are Your Color Comments?" AATCC Review.  January 2007.

Appendix: Solutions

Solution to Figure 7

Solution to Figure 8

Solution to Figure 9