If you grew up in the United States, there is a high likelihood that one of the 12 boys in your high school art class had difficulty seeing some colors. Most forms of color vision impairment are congenital conditions whose incidence varies geographically. The highest incidence is in the United States and Europe, where about 4% of the population has some degree of impairment, while the lowest incidence (about 2% of the population) is in the Arctic and the equatorial rainforests of Africa and South America (Birren 1983). Red-green color blindness, the most common type of color vision impairment, is actually a generic term for those with any level of severity of red-blindness or green-blindness. Since the genetic condition is sex-linked and encoded on the X-chromosome, it is more common in males than females (8% of males vs. 1% of females in the U.S. are affected).
To see the full spectrum of color we have three different kinds of photoreceptors responsible for seeing three different wavelengths of light (in addition to rods that do not sense color). The S-cones sense blue, or short wavelengths, of light. The M-cones sense the green medium wavelengths of light, and the L-cones sense the long red wavelengths of light. Presence of all three of these cones enable vision of the full color spectrum. Those with color vision impairment may be missing one or more types of those cones, or have malfunctioning forms of one or more types of cones. Those with red-blindness and green-blindness are grouped together into red-green color blindness because they share the same problematic colors, namely red, orange, yellow, green and brown. Those with blue-blindness (often called blue-yellow color blindness) confuse blue with green, and yellow with violet. See Figure 2.cg.19 below for examples of red-, green-, and blue-color blindness.
Regardless of the severity of an individual's color vision impairment, they can see color value, or lightness, differences. This is one reason why it is important, when creating sequential color schemes, to make sure that the color classes vary primarily by color value rather than by hue. With a bit of effort, we can also design qualitative or nominal color schemes that will be readable for map users with color vision impairment by using hues that are less difficult for impaired map readers to see.
Some cartographers have worked to develop color schemes that work well for color vision impaired map readers. We can use color space diagrams to depict color hues that color vision impaired map readers may confuse (see Figure 2.cg.20). Color hues that fall along one of these lines in the CIE space may be confusing. You can create a 'safe' color scheme by choosing hues that do not fall on one line and/or by using lightness differences for hues that fall along one line (see Figure 2.cg.21).
One easier way to find color schemes that work well for color vision impaired map readers is to use ColorBrewer, an interactive, web-based tool that provides color specifications for a variety of color schemes, and indicates if a particular scheme will work for color vision impaired readers. If you want to check a color scheme or design your own, there are several web-based tools that can simulate what color vision impaired individuals see. One such tool is Coblis by Colblindor, which is also a great resource for more information on color vision deficiency.
If you are interested in investigating this subject further, I recommend the following:
- Olson, J. and C. Brewer. (1997) "An evaluation of color selections to accommodate map users with color vision impairments." Annals of the Association of American Geographers. 87(1): pp. 103-134.
- Brewer, C. and M. Harrower. (2002) http://ColorBrewer.org.
- Colblindor. (2010) "Color Blind Essentials." http://www.color-blindness.com/2010/02/23/color-blind-essentials/.