In this two-part Geospatial Frequently Asked Question (G-FAQ), I turn my attention to a topic that perhaps more cartographers should consider: making maps that are just as usable for the color blind as they are for people with normal vision. While you may not have considered this topic in the past, it is important when you consider that, (1) colors are often a very large part of the visual cues we associate with maps and that (2) nearly 5% of the population is color blind. In the first part of this G-FAQ series, we will explore the medical condition known as color blindness with a focus on this set of core questions:
How does the human eye see color? What causes color blindness? What are the types of color blindness and how prevalent are each?
How The Human Eye Sees Color
The human eye is much like a camera as it collects light (i.e. photons) and focuses them on sensors behind the lens. Specifically, photons enter the eye by passing through the cornea, into the pupil and then striking the back of the eye, or the retina (see Figure 1 for a more detailed diagram of the parts of the eye). The retina contains roads and cones which detect specific wavelengths of photons. When struck by a photon at the correct wavelength, the rods/cones are stimulated, producing an electrical pulse which travels to the brain along the optic nerve. It is the optical lobe that interprets these signals to produce a color image of the world around us.
The human eye has more than 100 million rods in the retina. Rods detect white, black and shades of grey. Rods are less sensitive to small changes in shades of grey as between white and black. In low light, there are not enough photons to excite cones, only the rods are excited in these conditions. So in low light, we only see in black and white.
The human eye sees color when red, green and/or blue-sensitive cones are excited by photons. There are far fewer cones in the eye, with over 6 million color photoreceptors in the average eye. Cones have pigments that absorb photons over a small range of wavelengths (see Figure 2 and its caption for a short lesson on photons, wavelengths and energy). The wavelength range that each type detects overlap slightly so that certain colors cause two or even three cone types to fire at once. In fact, the secondary colors (i.e. yellow, cyan and magenta) which are formed by mixing the primary colors our cones see (i.e. red, green and blue) are brighter as two cones are excited at once by these photons. Sensitivity to photons varies by cone type as well. For instance, blue cones need about twice as much light to react at any given light intensity, so that blue-violet colors are darker than the other colors we see. There are also more red cones than green cones, and also more green than blue cones – this accentuates problems of color reproduction in the human eye.
Now a final note on the eye and color that I could ignore, i.e. the spatial component to our visual perception. Even if light is evenly spread across the retina, we still perceive colors at the center of our vision differently than at the edges. This is an advantage as then we can determine information about the objects we are looking at even if it is evenly lit. So for example, we can see the edges of a brightly lit building so we do not run into it.
The Causes of Color Blindness
Someone that is color blind has an abnormality which prevents them from seeing colors as a typical person would. In some cases, those who are color blind cannot see specific colors; and in other cases they have reduced ability to distinguish between various shades. For most color blind people, the cause is congenital, meaning that the condition developed before they were born. By far, the most common congenital cause is inherited genetics, and specifically the X chromosome. Since males have only one X chromosome, if they have the recessive allele which causes color blindness, then they will exhibit the condition. Women have two X chromosomes so it would be rare for them to have two X chromosomes with the mutated allele. For this reason, women are more often carriers of the genes that cause color blindness but can still see color normally. Color blindness can also be caused by recessive alleles on non-sex chromosomes as well as by prenatal brain damage.
The other broad category for the causes of color blindness are those which afflict a person after they are born. These causes are most often linked to pharmaceutical drug use and specifically to Plaquenil which is used to treat rheumatoid arthritis. Severe trauma to the head and/or body has been linked to color blindness as have degenerative diseases and Vitamin A deficiencies. Disruptions to the optic nerve and/or parts of the brain that interpret optical signals from this nerve can also cause color blindness.
The Types of Color Blindness
When trying to categorize any topic, you invariably end up with outliers from the classification scheme you develop. With this in mind, here is the most encompassing categories I could develop after looking over various online resources. The most widely accepted classification appears to be based on the number of functioning eye sensors, the three categories are: monochromatism, dichromatism and trichromatism.
This is a type of color blindness where those afflicted only have one rod and/or cone that functions properly. The sub-categories are as follows:
- Rod Monochromatism – the condition of only having rods in the eye so that the world is seen in shades of grey.
- Cone Monochromatism Type I – the condition of having only one type of cone and rods that function. With this condition, you can see colors but not distinguish between various shades.
- Cone Monochromatism Type II – the condition of having only one type of functioning cone. Unlike those with Type I, you cannot see at night with this condition.
These types of color blindness are very rare, impacting about 0.001% of people.
Those afflicted with this type of color blindness have two functioning cones as well as functioning rods. The sub-categories are as follows:
- Deuteranopia – the condition of missing green cone pigment. This impacts 1% of males and 0.01% of females.
- Protonopia – the condition of missing red cone pigment. This impacts 1% of males and 0.02% of females.
- Tritanopia – the condition of missing blue cone pigment. This impacts 0.002% of males and 0.001% of females.
To get a better sense of how a person with dichromatism sees the world, please refer to this month’s Free for All.
This is the most common type of color blindness and those afflicted with it have three cones but one of the color pigments is altered and thus they see colors differently than would an unafflicted person. The sub-categories are as follows:
- Deuteranomaly – the condition of being less sensitive to green light than a ‘normal’ person. This impacts 5% of males and 0.04% of females.
- Protanomaly – the condition of being less sensitive to red light than a ‘normal’ person. This impacts 1% of males and 0.01% of females.
- Tritanomaly – the condition of being less sensitive to blue light than a ‘normal’ person. This is not sex-linked so it impacts 0.0001% of all people.
Overall, about 8% of males and then 0.5% of females exhibit some form of color blindness, be it the total lack of color perception or altered color reproduction. Given that color blindness impacts nearly 1 in 20 people, creating maps that can be viewed as effectively as possible by all people is an important consideration for any cartographer. As such, in our final part of this G-FAQ, we will look at tips for creating maps that everyone can use effectively.
Do you have an idea for a future G-FAQ? If so, let me know by email at firstname.lastname@example.org.
Find Out More About This Topic Here
- Abelard – The Theory of Color
- David Hubel – Eye, Brain and Vision
- Macalester College – Two Type of Color Blindness, Part I / Part II
- National Eye Institute – Diagram of the Eye
- University of Utah – Cognitive Psychology
- University of Washington – Contemporary Optics
- Western Michigan University – Color Reproduction
Brock Adam McCarty