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Defective Color Vision and Perception

(July 2004) posted on Wed Jun 30, 2004

Coudray discusses color vision and describes some of the optical conditions that might influence what you, your staff, and your clients see.


By Mark A. Coudray

Prepress comprises more than just separations, film, and screens. It also entails the creation of art and the decisions about how the art will be reproduced. The viewing and interpretation of color is a critical part of the reproduction process. Many pairs of eyes will look at the same piece of art as it moves through prepress--the artist, client, account rep, prepress technician, and agency contact are just a few who might be involved.

In addition, we might view a hard copy, like an inkjet proof, color laser print, or original media. The image is transformed through digital image capture (photography, scanning, or software image editing), viewed on a variety of monitors, and inspected under different lighting conditions and on a range of output media. There are plenty of opportunities for things to go awry. Even with a perfectly managed color workflow, you're still dealing with the human element--the perception of color vision. This month, I would like to introduce some of the critical elements of human color perception, common deficiencies, and the testing options available to determine if the people making color decisions are capable.

Real-world examples

My own experience is peppered with examples of color-deficient individuals making decisions that cost me money. There was the time a legally blind client rejected a print run because the green was not the right shade. Forget the fact that it matched the Pantone sample perfectly. The judgment was being made by someone who saw this particular shade of green in a completely different way than the normal population. It was only after we joked, "What are you, blind?" that he admitted that he was, indeed, legally blind.

It was the end of my third year of college studying graphic communications before I was tested for color deficiency. One of my close friends not only failed the test, he had the all-time lowest score ever recorded on it. He had no idea he was so color blind. As far as he was concerned, when someone said something was red, he associated his visual sensation with that descriptive name. He never considered what he saw as red was different than what everyone else called red.

These are but two examples of how color can be seen differently. They clearly show that knowing more about color deficiency and how to detect it would be helpful to anyone in prepress. The knowledge that everyone in your company has been tested and certified color competent also can be a serious benefit and selling point to your clients. At the very least, it can provide a point of discussion to make sure your clients are color capable as well.

Understanding color vision

The human eye has two primary photo receptors: rods and cones. Rods are used under low light conditions. This is our night vision or scotopic vision. It is primarily devoid of color. We see tone, but color is significantly degraded under low light. We are most sensitive to blues at night and least sensitive to reds.

Our daylight vision is called photopic vision. During the day, the cones provide the primary color rendering. Cones are special cells that contain photosensitive pigments. There are three varieties that are sensitive either to red, green, and blue wavelengths. The overlapping of these sensitivities gives us the perception of full color. Rods are largely inactive under normal daylight illumination. We're most sensitive to bright yellow-green (chartreuse) during the daylight hours.

As we transition from light to dark, the sensitivity of the cones diminishes, while the rods become progressively more sensitive. This causes our perception of color to change. We need to be aware of this change as we work under dimmed lighting levels in the prepress area.

When one or more of the cone receptor types is damaged or does not function, our perception of color is impaired. This can happen for many reasons. The main cause is a defective gene on the X chromosome. Men have an X and Y chromosome while women have two X chromosomes. You may have heard that only men are color blind. This is not true, but men do suffer from color abnormalities at a much higher occurrence than females. Typically, up to 10% of males have some form of color deficiency, while fewer than 1% of females are affected. Vision also can be impaired by diet, age, and certain types of disease. These causes are more rare than the typical genetic visual defect.

The type of deficiency is classified based on the sensitivity of the cones. A normally sensitive individual is called a trichromat because all three types of cones are functioning. A dichromat has two cones working, and a monochromat has visual color sensitivity in only one cone type. About 90% of all color-deficient people are trichromats, but they have reduced sensitivity in one type of cone. This means they will see all colors, but certain ranges of color will be affected.

While there are many types of color deficiencies, the most common types fall into four categories. All of these problems impair the ability to distinguish between reds and greens. Vision deficiencies that affect the yellow/blue range are much more rare. The four most common types are protanopia, which affects 1% of males and 0.02% of females; dueteranopia, which affects 1.1% of males and 0.01% of females; protanomaly, which affects 1% of males and 0.02% of females; and deuteranomaly, which affects 4.9% of males and 0.38% of females.

People who suffer from protanopia have severe difficulty distinguishing reds and greens. The reds also appear dimmer or darker than an equivalent green. This makes it extremely difficult--or impossible--to distinguish dark red, violet, and maroon from brown or black.

Those with dueteranopia have severely reduced red and green discrimination; the difference being that both reds and greens appear at the same luminance levels. In this case, subtle variations of color are completely lost. Large groups of colors appear to be the same color.

Protanomaly is characterized by reduced ability to distinguish reds and greens. It is not nearly as severe as protanopia and is characterized by reddish colors appearing darker than normal. Subtle variations of reds, oranges, browns, and maroons blend together or are lost. This can have a big effect on the range of colors commonly found in images depicting fleshtones, baked foods, woods, or textiles.

The most common type of color defect is deuteranomaly. Affected individuals cannot adequately distinguish between reds and greens, as in the other examples. However, the reddish colors appear at their correct value or luminance. When color separating subtle, earthtone, neutral, or natural colors, people who have this problem can't tell the difference between objects. Contrast can vanish, and objects can disappear into the background or foreground.

Testing for defective color vision

Many tests are available to determine whether an individual's color vision is defective. One family of tests is referred to as confusion charts. They are typically made up of many different colored objects, each with different color values. The subject sees different patterns in the chart as vision progressively degrades. The degree of defect is determined by which patterns the subject describes.

The Ishihara Charts Test, named after Shinobu Ishihara, who developed it in 1917, uses a very similar approach. The charts consist of fields of dots and figures of the same color, but with varying degrees of color saturation and luminance. On these fields are dot-shaped letters of other colors that are easily readable by people with normal color vision. The letters cannot be seen by color-deficient subjects--or, in some cases, be seen only by them. The charts also give a good indication of what form of color defect the viewer has. The full test consists of 38 charts, but the existence of a deficiency is usually clear after only four charts are viewed.

Another very common test is the Farnsworth-Munsell 100 Hue Test. This method of determining color abnormalities and color discrimination has been used by government and industry for more than 50 years. It provides reliable data that can be applied to many psychological and industrial color-vision problems.

The test consists of four trays that contain a total of 85 removable pegs. These pegs have varying color caps of incremental hue variation on one side. The reverse side is numbered. Color aptitude is determined by a subject's ability to place the color caps in hue order. The color sequence starts with yellow, moves through green, onto blue, purple, and finally red. After the pegs have been arranged to the subject's satisfaction, the tray is inverted and the number sequence is compared. Each transposition is scored and charted. The type, frequency, and location of the out-of-sequence pegs determines what type of abnormality is present.

Using the results

Knowing something about color perception and defective color vision is a very important part of the prepress process. First and foremost, it puts everyone on the same page. It allows you to determine fundamental capability for making color decisions.

Most opthamologists have the ability to test for color deficiency. How many actually do this as part of a regular exam? My guess is not many. You have to request the test. This may seem like an incredibly basic concept, but the reality is most people have never been fully tested for color deficiency.

When a small degree of deficiency is present, the subject will be totally unaware there is a problem at all. For this reason, radical color blindness isn't what concerns me. I am most interested in the marginal subject affected by deuteranomoly. This is the individual who cannot tell the subtle differences in colors. How many times have you been challenged over a fleshtone that was not quite right, yet you felt you had nailed it? How about the product shot where the client insists the color of the sweater is just not there? Without a doubt, the most frustrating is the Pantone color on which the client or the production department cannot agree to a match.

All of these examples occur because we haven't come to a common understanding of color-viewing capability. Knowing the kinds of color anomalies that exist helps you to be aware of certain patterns. When those patterns begin to appear on a regular basis, you have good right to suspect a problem with the viewer's vision. At the very least, the Ishihara Color Charts and Farnsworth-Munsell Test act as a point of communication. It helps to establish your level of professionalism and understanding of a very complex problem.

©2004 Mark Coudray. Republication of this material in whole or in part, electronically or in print, without the permission of the author is forbidden.


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