One of my earliest memories is of sitting in my parents' 1979 Chevrolet Malibu at the corner of 55th Street and Lake Park Avenue in Chicago, waiting for the traffic light to change. As we sat at the intersection, I asked my mom: "Why do they call green lights 'green lights?'"
My mom couldn't understand my question any better than I could understand her response. "Because they're green," she said.
As far as I was concerned, green traffic lights didn't look green, and still don't. Their color, at least as I perceive it, has absolutely nothing in common with the color of grass, Kermit the Frog, the Incredible Hulk, or anything else that is typically called "green." To me, green traffic lights simply don't have much color at all: almost white, or perhaps a very pale blue. They're easy enough to distinguish from red and yellow lights, especially because (with the exception of one traffic signal in Syracuse, NY), the red light is always on top, or on the left if the signal is oriented horizontally. It just wasn't clear to my 3-year-old mind why anyone would think of calling them "green."
Later, when I started attending school, I managed to frustrate teachers and amuse classmates with my inability to follow seemingly simple instructions such as, "pick up the purple crayon" or "color the triangle orange." Learning to read the labels on the crayons provided me with a trick that could save me from embarrassment -- but unfortunately, sometimes the other students foiled my trick by peeling the labels off.
Eventually, I learned that I am colorblind. That doesn't mean that I don't see colors; I just see them differently from how most other people see them. It's estimated that 7% to 10% of males in the United States have some type of colorblindness. Because the most common types of colorblindness are caused by recessive genes carried on the X chromosome, it's far less common in women.
In order to understand how colorblindness works, it's important to understand the basics of how color vision works. The human eye contains two types of photoreceptor cells: rods and cones. Rods provide monochromatic (grayscale) vision; they are extremely sensitive, and help us to see at night or under low-light conditions. Cones provide color vision.
There are three types of cones, each specialized for light of a given wavelength: long (red), medium (green), and short (blue). The brain combines the signals from these three types of cones to create the perception of color vision. That's why CRT, plasma, and LED displays are able to recreate nearly any color perceptible to humans by using a combination of red, green, and blue phosphors.
There's nothing particularly magical about the number three: most birds, insects, reptiles, and amphibians have four types of cones, and a few have five. These animals can perceive and distinguish between millions of colors that humans can't. If birds had invented color television, CRTs would probably have at least four kinds of phosphors.
On the other hand, many mammals have only two types of cones: one for long wavelengths and one for short lengths. They can see colors such as blue and red with no problem. However, they don't have any cones for medium (green) wavelengths, so they have to rely on the red and blue cones to see these colors. As a result, they aren't able to distinguish very well between mid-range colors. This is why, for example, brown rabbits are able to hide in green grass without being seen by predators such as dogs or cats. On the other hand, it's also why a rabbit may fail to see an orange cat hiding elsewhere in the grass.
That happens to be how I see, too. In humans, a lack of medium-wavelength cones is called deuteranopia. It's also sometimes called Daltonism, in honor of the 19th century British chemist John Dalton, who had this kind of colorblindness, and was the first person to write about it.
Not everyone who is lacking one type of cone has deuteranopia. Some people have blue and green cones, but lack red ones; this is called protanopia. People with this condition have difficulty distinguishing between colors on the long-wavelength end of the visible spectrum. A much less common condition is tritanopia; people with this condition have green and red cones, but no blue ones.
Some people have all three types of cones, but still experience color blindness. This is because the peak sensitivity of one cone type is shifted. In the most common form of this, the sensitivity of the green cones is shifted toward the red end of the spectrum. This is called deuteranomaly, and is the most common form of color blindness. Like deuteranopia, it results in difficulty distinguishing between mid-spectrum colors. Protanomaly and tritanomaly, where the sensitivity of the red or blue cones, respectively, are shifted toward the middle of the spectrum, can also occur, but are less common. Again, all of these are genetic conditions.
By now, you've probably realized that colorblind people don't simply see in black and white. There is an exception to this: achromatopsia, or a complete lack of color vision, occurs when people have only one kind of cone, or no cones at all. This is extremely rare, except on two remote Pacific islands, which neurologist and best-selling author Oliver Sacks wrote about in his book The Island of the Colorblind. It can also, very rarely, be brought about by damage to the cerebral cortex; for example, as a result of a stroke or head injury. In this case, the problem is not with the photoreceptors in the eye, but with the brain hardware that processes the signals from them.
User interface designers ignore the fact that some form of colorblindness affects as much as 10% of the male population at their own risk. Relying on users to distinguish between colors that are fairly close together in the spectrum is a good way to ensure that at least some of them will have difficulties. There are programs (including the open-source Color Oracle) that allow you to see things the way colorblind people see them. This can allow you to spot possible issues. That being said, it's a good practice to use text, shape, and patterns to distinguish between user interface elements, rather than color alone.
That also goes for spreadsheets, presentations, CAD files, and FEA results, among other things. It even goes for electrical wiring: I've been known to stick pieces of tape with the words "red" and "green" on wires so I won't mix them up. Colorblindness is a relatively common condition. If you understand what it is, you can ensure that your user interface, data visualizations, or other graphics are clear to everyone.
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Dave Palmer, P.E., is a licensed professional metallurgical engineer specializing in failure analysis and prevention. He earned his B.S. degree at the Illinois Institute of Technology, and his M.S. degree at the University of Wisconsin-Milwaukee.