Color and Vision Questions and Answers

  1. What is color?
  2. How do we see in color?
  3. What are color vision deficiencies? How common are they?
  4. What does it mean to be color normal
  5. What parts of the eye are important for color vision?
  6. What color processing takes place in the eye? the brain?
  7. What spatial and temporal processing takes place in the eye? the brain?
  8. What is chromatic adaptation?
  9. How do we characterize a person's color vision?
  10. How do we select names for colors?
  11. How many colors can we see?
  12. What is object color? What other modes are there?
1. What is color?
Color is that characteristic of a visible object or light source by which an observer may distinguish differences between two structure-free fields of the same size and shape, such as may be caused by differences in the spectral composition of the light concerned in the observation.[ref 1, p 723] In other words, color is that perception by which we can tell two objects apart, when they have otherwise similar attributes of shape, size, texture, etc.

OK, that's the textbook answer. This is admittedly unsatisfying, because color is an inherently subjective experience. Color only exists in our minds, and putting a scientific definition together of no easy task. The usual definition, given above, is really a circular argument. It amounts to: "Color is that attribute of an object leftover when you eliminate all attributes except color." So, if an two objects look different, but have the same size, shape, texture, etc., then the way you are telling them apart is their color. Still not satisfied? Here's another answer for you.

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2. How do we see in color?
In the retina of our eye are photoreceptors that are sensitive to light. When light is absorbed by the photoreceptors, the light energy is converted into electrical and chemical signals that the neurons in our eye and brain process. There are two kinds of photoreceptors in the retina: rods and cones. Rods mediate vision at lower levels of illumination. Cones mediate vision at higher levels of illumination. There are three types of cones with each type differentially sensitive to a different region of the visible spectrum. They are known as the Short-wavelength sensitive cones, the Middle-wavelength sensitive cones and the Long-wavelength sensitive cones. Sometimes they are referred to as R-, G-, and B-cones but these are misnomers based on the colors in the spectrum. For example, very short wavelength light can uniquely stimulate the S-cones but the sensation associated with this light stimulation has a reddish and bluish component. Fundamentally our color vision derives from comparisons between the amount of light being absorbed by each cone type. Our visual system compares the outputs of the cone types to process color. In addition, color appearance is influenced by the ratios of cone excitations in surrounding regions and by the overall levels of cone excitation caused by the prevailing illumination. These comparisons occur at different stages of processing that start in the retina and continue to the cerebral cortex of the brain.
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3. What are color vision deficiencies? How common are they?
Color vision deficiencies result from either a lack of one or more cone types, or cones that behave somewhat differently from average. Those lacking long wavelength cone pigment suffer from protanopia. A related condition is anomalous protanopia, or protanomaly. Here, long wavelength cones are present, but their sensitivity is shifted spectrally to shorter wavelengths, so they interpret certain stimuli differently than normal observers. Similarly, deuteranopia (deuteranomaly) is the lack (spectral shift to longer wavelengths) of the middle wavelength cone pigment, and tritanopia is the lack of short wavelength cones (tritanomaly is incomplete tritanopis). It is important to remember than none of these conditions should be referred to as true color blindness. This is not simply a politically correct statement. In fact, those suffering from any of these conditions do experience color, but not in the sense that a color "normal" observer does.

Other less-common deficiencies are rod monochromacy and cone monochromacy. With rod monochromacy, there are no cones present, only rods. Persons suffering from this are truly color blind. With cone monochromacy, a person has only one cone type. For more details on these and other vision deficiencies, see reference 3.

From reference 2, percentages of the more common deficiencies:
Type Male % Female %
Protanopia 1.0 0.02
Deuteranopia 1.1 0.01
Tritanopia 0.002 0.001
Cone monochromatism ~0 ~0
Rod monochromatism 0.003 0.002
Protanomaly 1.0 0.02
Deuteranomaly 4.9 0.38
Protanomaly ~0 ~0
Totals 8.0 0.4
4. What does it mean to be color normal?
A color normal individual is one whose color vision is not greatly different from the average person. This may seem obvious, but remember that the color vision deficiencies are, in most cases, not simply on/off conditions. There is a continuum of, for example, deuteranomalous people. Some will have only a slight shift in middle-wavelength cone sensitivity and others may have so large a shift that middle-wavelength cone behavior is no longer distinguishable from that of long-wavelength cones. A series of color-matching experiments can determine what type of deficiency a person might have. The end result is usually simply an understanding of approximately how far from average an individual might be. To state a cutoff between normal and anomalous vision assumes some criterion which is application specific. In other words, just how normal you are depends on just how normal you need to be for your task. If you are a taxi driver, you certainly need to determine the color a traffic signal. If you are an interior designer, you probably need somewhat better color vision to please your customers.
5. What parts of the eye are important for color vision?
Click on the image for a larger view
Use by permission from reference 4.

Most of the important parts of the eye are labeled on the diagram. The cornea and lens focus the image onto the retina. The retina is the part that actually detects incoming light. The iris adjusts in width to partially account for light levels. The fovea is the central focal point of the eye. (That is, when we look at something, we are casting its image onto the fovea.) The fovea is the are where we get most of the spatial detail and color in what we see. The optic nerve is a bundle of nerves which carries the visual information to the brain. Not shown is the macula, which is a filter over the fovea. It serves to limit the damage that might be cause to the fovea if we accidentally focus on intense light sources, such as the sun.
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6. What color processing takes place in the eye? the brain?
During the development of the embryo, part of the neural tube which develops into the central nervous system forms outcroppings that extend and develop into the retinas. Therefore the retina is considered part of the brain: a part that is easily accessible for study. The retina consists of 5 layers of cells and between each layer there are extensive interconnections in which visual processing takes place. There are about 125 times as many receptors in the retina as there are ganglion cells in the optic nerve, which connects the eye to the brain. This gives an indication of some of the processing that must go on. Because the retina is so accessible, much is know about the processing of color. The signals from different cone types are segregated in opponent pairs during this processing so that ganglion cells have specific receptive fields that are excited by one cone type in the center and inhibited by another in the surround. Although the effects this organization can be evidenced in psychophysical experiments that measure different aspects of visual function, our conscious experience of color does not relate well to this organization. It is at the later stages of processing in the brain in which our subjective experience of color is processed.
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7. What spatial and temporal processing takes place in the eye? the brain?
Subjectively, we experience many different qualitative aspects of vision, for example, form, color, motion, depth, etc. This information is all input into the visual system through the photoreceptors in the retina. Therefore the retina has to process the information to preserve all these different aspects of the visual stimulus. The term multiplexing is often used to describe the way the early visual system simultaneously sends this varied information to the brain. Specialized pathways are determined very early in visual processing. The second layer of retinal cells already has specialized neurons that process form by creating center-surround receptive fields. Different types of temporal response are seen in cells with either sustained or transient responses in the next layer of processing. Two streams of visual processing have been identified at the optic nerve level. One responsible for fine detail and color and the other responsible for detecting rapid temporal change with high sensitivity to changes in contrast. (In other animals, cells that respond to directional motion are already present in the retina.)

In the brain we see evidence of hierarchical and modular processing of visual information. At the early stages of cortical processing, cells exist that respond to stationary or moving edges and bars of light contrast, evidence of form processing. At later stages of cortical processing we find areas of the brain that seem to be specialized for the processing of specific visual attributes. For example area MT in the temporal lobe shows a specialization for motion detection. Not only do cells here respond best to moving stimuli with specific characteristics (direction and speed) but also studies with animals has demonstrated that artificially stimulating these cells can influence the perceptual judgments of motion. In monkeys, an area called V4 has been identified as being critical for the processing of color. It has been shown that humans with brain damage in the area homologous to this one lose color vision.
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8. What is chromatic adaptation?
Chromatic adaptation is the ability of the human visual system to adjust itself in response to varying illuminant conditions. In other words, we adapt to the color of the light source in order to better preserve the color of objects. For example, if viewed under incandescent light, white paper has a decidedly yellow cast. However, we have the ability to automatically account for the yellowish light, and we therefore see the paper as white. If you think about it, this makes a lot of sense. It would be a very confusing world if objects were changing color every time the light source changed. From an evolutionary point of view, we still need to know if the fruit is ripe whether it is morning, noon, or evening. Chromatic adaptation makes this possible.
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9. How do we characterize a person's color vision?
There are several test available, depending on what aspects of color vision you wish to focus on. The Farnsworth-Munsell 100 Hue Test presents the observer with 80 color disks and a few anchor points. The observer must order the disks between each anchor point. The color span the whole circle of hue space, and mistakes made are plotted on a polar graph, the angle corresponding to hue, and the distance from center increasing with error in disk placement. General trends, such as large errors in the red/orange region, can be mapped to specific vision deficiencies. Observers with poor performance throughout the hue circle are not necessarily color deficient, but they do lack good color discrimination.

Another popular test are psuedoisochromatic plates. The most common of these are Ishihara's Tests for Color-Blindness. These plates, common in grade school vision testing in the U.S., consist of dots of various colors. Plates contain a number or letter which is only visible to observers with the ability to distinguish between the various colors of the dots. This test is not designed to accurately predict specific vision deficiencies, but rather as a general screen for color vision defects.
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10. How do we select names for colors?
This question has been of interest not only to scientists who study color and the visual system but also to linguists and philosophers. The conventional wisdom used to be that culture and language determined our use of color names. This view began to change in 1969 when Berlin and Kay published a book that showed that there is a high degree of universality in the use of color terms across cultures and languages. Now many investigators believe that there is a physiological basis for the use of certain basic color terms and the parsing of color space into categorical regions denoted by these basic color terms (black, white, gray, red, green, blue, yellow, purple, orange, brown and pink). There is evidence from animal studies and studies with infants to support this categorical view of color.

However, people do use many more color words; hundreds of different terms have been catalogued. It seems however that the non-basic terms are used without the same generality and consistency as the basic terms. For example, cyan may have a specific meaning to a couple of printers working together in a print shop but the man on the street may have an altogether different notion of what cyan is. Yet everyone, within the limits of the homogeneity of normal color vision, will agree about the meaning of orange or pink.
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11. How many colors can we see?
This is a very popular question and it is usually answered vaguely by, "Millions and millions!" However, it is better to ask the question in a more specific way to get a more comprehensive answer.

Many of us can select a setting for our computer monitors that displays millions of colors and we see an improvement in image quality with this setting. However, if you select the colors correctly you can reduce the number of colors to a couple of hundred or even fewer (depending on the image) without noticing a degradation in quality. This would indicate that we can't see millions of color variations simultaneously.

One way to answer the question is to measure the ability of people to discriminate colors. Many researchers have investigated chromatic discrimination by varying the wavelength of two monochromatic lights until they are just noticeably different. Other studies have used the variability of color matching to gauge discriminability and yet others have directly measured threshold differences throughout color space. Using these measures we find that our visual systems can discriminate millions of colors.

In our laboratory we are interested also in larger than threshold color differences: the type of differences that would make you reject a certain touch-up paint because it's not a close enough match to the color of the paint of your car. Even with such a metric there are close to a million discriminable colors on a computer monitor which only can reproduce a fraction of the colors we can see out in the real world.
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12. What is object color? What other modes are there?
There are several important modes of viewing color. These can basically be divided into two forms. If we perceive a stimulus to be light reflected off or through something, we are viewing in object mode. If we perceive a stimulus to be the light itself, we are in illuminant or illumination mode. To get a feeling for the difference, suppose we put a large yellow image in a computer monitor. If asked the color, any observer would say "yellow." Now suppose we reflect that same yellow light off a sheet of paper. When asked the color of the paper, observers would say "white." We intuitively know that the yellowness of the paper is due to the light, and we compensate for that when we are viewing in object mode. If we were to take the yellow light reflecting off the paper, and view it through a small opening, observers would again switch, and claim the color was yellow. The mode has changed, and the perceived color changed with it.


  1. G. Wyszecki and W.S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae 2nd Ed., Wiley, New York, 1982.
  2. R.W.G. Hunt, Measuring Colour 3rd Ed., Fountain Press, England, 1998.
  3. R.M. Boynton, Human Color Vision, Special Limited Edition, Optical Society of America, Washington D.C., 1992.
  4. M.D. Fairchild, Color Appearance Models, Addison-Wesley, Reading, Massachusetts, 1998.

Frequently Asked Questions

Why color machine vision?

Monochrome (black and white or gray scale) machine vision is now well established as a tool for alignment, gauging, and optical character recognition. Color is often useful to simplify a monochrome problem by improving contrast or separation. However, the greatest potential uses of color vision, human or machine, are for cost-effective object recognition, classification and assembly inspection. For a few of the many applications.

What do we mean by color?

Color is a visual object attribute which results from the combined output of three sets of retinal cones each sensitive to different portions of the visible part of the electromagnetic spectrum. The cones have peak sensitivities in the red, green and blue portions of the spectrum respectively. Any perceived color may usually be created by a variety of sets of "primary" colors when combined in the correct proportions.

What is a color space?

A color space is a means of representing the three components of a color in terms of a position in a (usually) three dimensional space. RGB, HSI, LAB and CIE are some of the many color spaces which may be used, depending on the particular purpose of the analysis. A very complete discussion of color spaces for video and computer graphics may be found at Rick Davis's website as well as many other fine sites on the internet.

What is RGB color space?

Most video monitors use red, green, and blue color sources as the primary colors for color image generation. RGB color space uses a rectangular coordinate system with one coordinate axis assigned to each of three color components, red, green, and blue. RGB coordinates are commonly used for specifying and coding computer generated images.

What is HSI color space?

For most practical machine vision purposes, HSI may be regarded as the same space as RGB but represented in a different coordinate system. This diagram shows how the RGB cube would appear viewed from the black end, looking in the direction of the black-white diagonal.

The HSI coordinate system is cylindrical with the intensity (I) axis coinciding with the black-white diagonal of the RGB system. Saturation (S) is the radius from the intensity axis and hue (H) is the angle with respect to (usually) the red direction. When we assign a single name to a color it is usually the hue to which we refer. A number of slightly different representations of the exact transformation between HSI and RGB are in common use.

I've been told that it is very important to work in HSI space rather than RGB space. Why does it matter what color space is used?

Many traditional applications of color machine vision are aimed at differentiating single color objects from the background for alignment and gauging purposes. As long as the colors in the image are reasonably well saturated hue will tend to remain relatively constant in the presence of shadows and other lighting variations. In such cases an image based on hue alone may work better with standard alignment and gauging tools than traditional gray scale analysis.

When colors have low saturation (lie near the black-gray-white axis) hue may be difficult to determine accurately; when saturation is zero hue is undefined. For systems which must be able to differentiate all colors, saturated and unsaturated, HSI representation can introduce significant problems. We have found that for general recognition and classification of objects which may be multicolored, contrary to conventional wisdom, the disadvantages of HSI space will almost always outweigh any possible advantages.

Which color space does WAY-2C use?

WAY-2C, like the human visual system, uses RGB space. Our patented analysis methods use the probability distribution of the complete 3-dimensional color vector rather than the one-component-at-a-time approach of most other systems. Although our methods are equally applicable to RGB or HSI space, experience and theory both show that, in contrast to conventional wisdom, RGB is almost always preferable.

What is supervised classification?

Supervised classification is classification in which a system, living or machine, learns to recognize objects or situations by the example of a trainer who knows the "correct answers". In contrast, unsupervised training involves the person or machine learning on their own without guidence from a trainer. WAY-2C uses supervised classification.

What is information theory?

Information (or communication) theory is based on the pioneering work (1) of Claude Shannon of Bell Labs and MIT during the 1940's. It is concerned with understanding, measuring, and optimizing the efficiency of information transfer. Information theory is central not only to modern communication technology, but also to the understanding of natural languages, and the communication of information by other natural information processing systems. Organisms which transfer information with less than optimum efficiency from sense organs such as the eyes to the portions of the brain which must take appropriate action may have a significantly lowered chance of long term survival.

What is Minimum Description Classification?

Minimum description is an information theory based approach to supervised pattern recognition which handles complex data distributions well. Its unifying theme is data analysis based on minimizing the amount of information necessary to describe a set of observations (2). Based on comparison of probability distributions, minimum description is consistent with both maximum likelihood estimation and the time honored philosophy of the simplest explanation being the best. It also lends itself to simple geometrical interpretations.

I've heard a lot about the need for complex and specialized algorithms for color based recognition. Don't I need very powerful computers or special expertise for this type of application?

Our experience shows that most practical color based classifications can be performed with the simple, consistent, minimum description approach and are easily handled by modern personal computers. After all, birds, bees and other creatures with small brains and little formal mathematical training are adept at recognizing objects based on color distributions.

Why does WAY-2C succeed where the competition fails?

All competing systems of which we are aware offer color as an extension to their traditional monochrome machine vision products. In contrast, WAY-2C is designed specifically to recognize objects based on their color distributions. It is the product of over a decade of research by a team of Ph.D's with expertise in optical physics, information theory, and data analysis.

Most existing color machine vision systems which offer statistical matching analyze the three color components separately. However both common sense and a rigorous analysis show that this approach throws away critical color information thus dramatically decreasing the number of color patterns which can be distinguished and increasing the probability of errors.

Also, traditional statistical matching methods for color machine vision are based on simplifying assumptions about the nature of the color distribution. These assumptions are valid in only a tiny fraction of the potential applications of color machine vision; when they are invalid the classification methods based on them are more likely to produce the wrong answers.

By analyzing all the color components simultaneously, and using a more appropriate patented matching criterion based on information theory, WAY-2C generally produces classifications very consistent with those of human inspectors.

WAY-2C has been proving itself since 1992 in a wide variety of challenging industrial, government, and military inspection and process control applications.

If you are disappointed with the performance of your present color machine vision system, and are serious about wanting to improve it, contact us. We'll give you an honest, no obligation, appraisal of whether WAY-2C can help. Then, if you choose, we'll work with you to see how a WAY-2C system can be implemented with the minimum possible disruption of your existing inspection process and the maximum use of your present hardware and software.

Does WAY-2C require tedious and expensive threshold setting?

NO. After all, humans and animals don't use thresholds for classification. Why should your color machine vision system? Some old-fashioned systems which rely on separate analysis of the color components try to classify complex color distributions by a few manually set thresholds. Instead, WAY-2C learns almost instantly by example, automatically setting hundreds of parameters to be used in the identification. Experience shows that WAY-2C is more reliable over the long term than human inspectors, and is is at least as good, if not better than humans in recognizing subtle color differences in side by side comparisons.

In the verification/anomaly detection mode, when only a single reference class is used, WAY-2C does allow adjustment of a threshold indicating the how far the object being inspected can differ from the reference before being rejected. However, even here, a theoretically defined default threshold value handles most cases.