What Does "Colorblind" Mean? It's important to make four points at the outset:
(1) So called "colorblind" people are almost never insensitive to all color; they simply perceive colors differently, or with less discrimination, than "normal" people.
(2) Colorblindness is not two or three different kinds of visual differences with specific, easily recognizable characteristics; it is a wide range of color vision anomalies — some genetic, and some the outcome of degenerative diseases, poisoning or physical injury.
(3) Colorblindness affects the perception of hue, lightness and chroma in different combinations and to different degrees. It is not simply a matter of a hue ("color") deficiency.
(4) There is measurable variation among individuals with "normal" vision, and this variation is so large that the boundary between "normal" and "colorblind" vision is arbitrary. What colorblindness means for the individual depends on the context and their life activities. (Color acuity is clearly mandated for certified gem appraisers, but is not much of an issue for journalists.)
These points suggest some other term, such as "color biased" or "color deficient," would be more accurate. I will use the term color deficiency here.
Three Types of Color Deficiency. The standard approach to color deficiencies evolved in the context of the trichromatic theory, where normal color vision is based on inputs from three color receptors: the R, G and B cones.
Within the trichromatic framework colorblindness can be defined and studied as the functional absence of one or more of the three cones. This is a straightforward way to think about "pure" forms of genetic color deficiencies. However it also had an important scientific justification: individuals who lacked either the R or G cones were used experimentally to define the response curve for the remaining color receptor.
This history also produced quite a lot of controversy (principally between the advocates of Helmholtz and Hering), which led Johannes von Kries to suggest the now established labels for single cone color deficiencies:
• Protanopia. A color deficiency equivalent to a complete lack of R or long wavelength cones. Individuals with this deficiency are called protanopes.
• Deuteranopia. A color deficiency equivalent to a complete absense of the G or middle wavelength cones. Individuals with this deficiency are called deuteranopes.
• Tritanopia. Color deficiency equivalent to an absence of B or short wavelength cones. Individuals with this deficiency are called tritanopes.
There are also cases of monochromatic vision in which only one cone type, or only rods, are present.
What Do Color Deficient Persons See? For most people, the most interesting question is simply, what does the world look like through color deficient eyes? There are different ways to answer that question, depending on what "looks like" means.
Hue Discrimination. The most straightforward meaning is simply color discrimination: what are the colors that appear the same to color deficient people?
A color vision test suitable to answer that question is the Farnsworth Munsell 100 Hue test. (You may be able to find an optometrist or ophthalmologist in your area who can administer this test to you.) The test consists of 84 stubby cylinders, about the diameter of a nickel, with a color sample painted on one end. These color samples define equal, barely different hue increments around the hue circle of the Munsell Color Order System at a low chroma (the colors appear to be very muted, mid valued pastels). The hue circle is divided into four trays, and all colors within each tray have a constant lightness. Thus, only hue can be used to discriminate between the color samples in each tray. Subjects are asked to take all the color cylinders within one tray at a time and sort them left to right in a continuous hue sequence (for example, from red to yellow or turquoise to violet).
On the opposite end of each cylinder is a number from 1 to 85, indicating its correct place in the hue sequence around the Munsell color circle. When the subject has finished sorting the cylinders, they are turned over and errors in the ordering are scored as the sum of the differences between the cylinder numbers and their numerical order in the sorted sequence.
When this task is given to "normal" individuals, few score perfectly and many confuse the ordering of several hues, but the errors are distributed randomly around the hue circle and involve hues next to each other in the sequence. When individuals with any of the three main types of color deficiencies take the test, they confuse widely different hues. Moreover, the confusions fall into specific areas of the hue circle, as shown below.
color confusions and the three types of color deficiency
Notice that the confusion patterns for each type of color deficiency are roughly equal in opposite directions around the center: the confusions are roughly around visual hue complements. Yet most color deficient people remark on color discrimination problems among warm hues, less often among cool hues. The reason is not that these cool hue confusions do not occur, but that our color language is much more precise and discriminating for warm colors than for cool, so color naming or recognition errors by "colorblinds" in the cool hues are harder for color normals to identify and correct.
Metameric Colors. We've seen that any visual system based on two cones will be unable to distinguish some spectral hues from white or gray. The same problem appears in dichromats. For protanopes and deuteranopes, the white metamer is around 500nm, or turquoise — the average wavelength where the response sensitivity of the R and B or G and B cones is equal. In fact, the confusion point for deuteranopes is slightly greener than it is for protanopes, at around 510nm, but this difference is not large enough or consistent enough to distinguish one kind of color deficiency from the other. The white metamer for tritanopes — the wavelength where the R and G curves have an equal response sensitivity — is a light yellow.
color confusions for common dichromancies
Yes ... but I still see all colors! In my correspondence with color deficient artists, they often claim that they can see some colors that are supposedly not visible to their type of color deficiency. In fact, since the 1960's there have been several studies showing that dichromats offer trichromatic color descriptions in terms of the unique hues, especially for wide field color stimuli (larger than 4° visual angle).
A recent study in the hue scaling of monochromatic (strongly saturated) lights clarifies both the perceptual and philosophical aspects of this issue.
predicted color labeling in three types of observers
First, the issue of perception. The trichromat hue scaling replicates the basic pattern reported by Hurvich & Jameson; the increased contribution in perceived yellow and blue, and decreased contribution of perceived green and red, at higher luminance is the Bezold-Brücke effect.
The dichromat responses in the range 470 to 520 nm are identical to trichromats. Dichromat color perceptions show three important differences: (1) substitution of "green" for "red" in the short wavelength (<470 nm) perception of protanopes only, (2) a marked reduction in the "green" content, and a corresponding increase in both the "yellow" and "red" content, of monochromatic stimuli above 520 nm, and (3) an enormous luminance effect on the mixture proportions above 520 nm, causing the brighter colors to appear substantially less "red" and more "yellow", and shifting the "yellow" peak deep into the "green" wavelengths.
In long wavelengths, dichromats perceive colors as a yellow/red mixture or variations of gold and scarlet. Protanopes essentially have a normal sensitivity to monochromatic hue differences, while deuteranopes see most long wavelengths as roughly the same orange or scarlet hue. For protanopes, increasing luminance shifts the "orange" hues into longer wavelengths (essentially an exaggerated Bezold-Brücke effect), while for deuteranopes the predominant scarlet hue shifts toward orange and offers a peek of desaturated unique yellow at around 520 nm.
A phenomenological analogy for these effects is that dichromats see the world under a vibrant gold or orange illuminant, and this illumination becomes more saturated as the luminance of the color area increases. They can still "see" the complete range of colors, but under an illuminant bias that collapses long wavelength color discrimination, and perhaps also reduces the saturation of short wavelength colors. (Saturation scaling was not requested in this study.) This illuminant metaphor for dichromatic color perceptions is consistent with the confusions among dull colors (described above), as illuminant tinting becomes more forceful as the saturation of surface colors decreases.
The philosophical question has to do with "what color is", and these results suggest that color categories are cognitive primitives, in the sense that they are not disrupted by genetic deficiencies in the receptor organ, any more than they disappear under narrowband or monochromatic illumination. This is close to Hering's original contention that the unique hues are fundamental neural pathways.
I interpret the color discrimination evidence to show that the color experience of trichromats and dichromats differs at a sensory level in much the same way that the color perceptions of an individual trichromat differ between a "white" and an "orange" illuminant, while the color experience of trichromats and dichromats is the same at a cognitive or labeling level, in the same way that a trichromat will comfortably label color samples as "yellow", "blue", "red", "green" or "white" even when they are illuminated by a strongly tinted orange light. That is, color constancy is a cognitive rather than receptor function in color perception, so it is able to produce meaningful color discriminations under the glowing gold light of dichromatic eyes.
Summary. The reasons why this happens can serve as reminders of how little we actually know about color deficient vision:
(1) Colorblindness is best thought of as a difficulty with color discrimination, not an absence of color. The problem is that it is philosophically and pragmatically impossible to show that two people "see the same color" given that color is inherently a subjective sensation. But is is very easy to show that color deficients say two colors appear the same to them that to color normals appear different, or to show that color deficients cannot consistently match a certain wavelength of light with a single, specific mixture of two or three "primary" colors. And we have no useful way to describe how these confusions may affect their overall color experience.
(2) The trichromatic theory is the standard framework used to describe color deficiencies. It is essentially a retinal theory of color vision. Yet significant aspects of color perception — for example, the three colormaking attributes — only arise much later in the visual system and in the context of a "real world" interpretation of what we see. Color at this stage apparently depends to a considerable degree on the visual context and past experience in a way that allows color deficients to compensate pretty well for their color confusions. The confusions appear so clearly in laboratory hue discrimination tests because the tests are unfamiliar and the color discriminations are presented as isolated displays. (And don't forget, the discriminations are so subtle that many "normals" get some of them wrong, too.)
(3) The assumption that the retinal photoreceptors create the color sensation is physiologically incorrect; sensory qualities arise from the brain's interpretation. This means that a color sensation "blue" could be experienced even if the "blue" receptor cells are entirely absent. In the research record there are instances of colorblind persons reported to see long wavelength light as "yellow" despite the lack of a red or green cone; and the rare case of a man, a dichromat in only one eye, who claimed to see long wavelength light as "orange".
(4) It is difficult to overstate the inadequacy of language to specify colors, or the insensitivity of the average person to color differences. If the color deficient person is an artist, then discussions of color "errors" are often between people who differ in color vision capabilities, precision in the use of color words, and awareness of color nuances based on training and careful looking. It's hard to know what can be reliably defined in a conversation between these two people!
Color Deficiencies and Painting. The final question I receive is: how should an artist cope with color deficiencies?
Most of my color deficient readers recommend the use of a limited palette, in particular a selection of paints that creates minimally confusing color mixtures. It's worth considering the fact that traditional easel painters, because of their historically limited palettes, rendered colors with gamut limitations that are easily as extreme as many types of color deficiency.
However, 20th century painting styles have created and provided justification for extremely individualistic and even arbitrary color schemes in painting. The expressive and abstract uses of color mean that viewers are no longer surprised to see a blue face or a green sky. Among the paintings sent to me by persons identifying themselves as color deficients, I have not seen any uses of color that strike me as bizarre or unacceptable.
For color deficients as for "normal" artists, the same good advice applies: follow your muse, trust your instincts, and paint with feeling. Then let your art speak for itself.
I have received several inquiries from artists who describe themselves as colorblind and wanted more information about their condition. Many explained their eye doctors were unable to answer their questions. Here is a brief overview.
As this diagram shows, protanopes have the greatest difficulty discriminating yellow green and blue violet hues; deuteranopes struggle with deep yellows and middle blues; tritanopes confuse deep reds and turquoises.
Gray is not a color sample in the Farnsworth-Munsell, but if it were, protanopes and deuteranopes would most often insert it into the hue sequence approximately as shown in the diagrams.
The three categories of observers, identified by genetic screening, were asked to judge the proportion of unique hues in a spectral series of monochromatic lights. (They were free to use other color labels if they desired, but none did.) Importantly, the monochromatic series was presented at two luminance levels, to judge the effects of luminance contrast on color perception: either a 230 td light seen against a 10 td background, or a 920 td light seen against a 920 td background.
Last revised 08.01.2005 • © 2005 Bruce MacEvoy
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