But before we learn more about them we have to have a look at how color vision actually works. We have to do so because the functionality of the eye is closely related to the three main types of color blindness.

How color vision works

To see anything at all we need some tiny little helpers inside our eyeballs, the so called photorecptors. There are two different types of them: rods and cones. Both of them are sitting on the retina and pass information of light on to our brain. There are about 120 million rods which are very sensitive to light but not to color.

The cones are the photoreceptors which are responsible for our color vision. They are only about 6 to 7 million of them, gathering together very closely in the center of the retina, the so called fovea centralis.

Cone Absorption Curves — © 2009 by Bruce MacEvoy

And here comes the clue: Each of those cones is carrying one out of three different photopigments and therefore reacts differently on colored light sources. For each of this three types there exists a specific color absorption curve with peaks at different points in the color spectrum.

• S-cones: sensitive to short wavelength light with a peak at ca. 420nm (blue)
• M-cones: sensitive to medium wavelength light, peak at ca. 530nm (green)
• L-cones: sensitive to long wavelength light, peak at ca. 560nm (red)

Mixing together the information of those three different types of cones makes up our color vision. This is also the reason that only three main colors are needed if we want to mix together all visible colors, because we only have three sources of information for mixing our whole color spectrum.

Types of color vision deficiency

Based on this knowledge about our visual system we easily can put together the list of different forms of color blindness. All of them have a direct relation to the available photoreceptors in your eye and are accordingly categorized.

• Monochromatism: Either no cones available or just one type of them.
• Dichromatism: Only two different cone types, the third one is missing completely.
• Anomalous trichromatism: All three types but with shifted peaks of sensitivity for one of them. This results in a smaller color spectrum.

Dichromats and anomalous trichromats exist again in three different types according to the missing cone or in the latter case of its malfunctioning.

• Tritanopia/Tritanomaly: Missing/malfunctioning S-cone (blue).
• Deuteranopia/Deuteranomaly: Missing/malfunctioning M-cone (green).
• Protanopia/Protanomaly: Missing/malfunctioning L-cone (red).

For a better understanding you can also call them blue-, green-, or red-weakness respectively -blindness. Unfortunately this terms didn’t really made their way and are not used very often.

You could ask know: “What about red-green color blindness or blue-yellow color vision deficiency? These are the ones I know but I can’t find them in your lists above.”

That’s right. The problem with this well known terms is, that they are not telling the truth! Many people think that if you suffer from blue-yellow color blindness this are the only colors you can’t distinguish. But that’s wrong. Color blindness doesn’t relate to just two color hues you can’t distinguish, it is the whole color spectrum which is affected. More on this a little later in this article and in the next chapter of Color Blind Essentials where we will have a closer look at red-green color blindness.

But to solve the puzzle: blue-yellow color blindness relates to tritan defects and red-green color blindness to all types of protan or deutan defects.

The above list shows you the prevalence rates of each type of color vision deficiency. The ratios between the most frequently occurring types for men can simply be remembered as: 1 protanope to 1 protanomalous trichromat to 1 deuteranope to 5 deteranomalous trichromats.

We already learned in the last chapter of Color Blind Essentials: What is color blindness?, that because of our genes more men than women are colorblind. Adding up all the numbers results in a total of 8% of men and 0.5% of women who are suffering from some type of color vision deficiency.

What do you see if you are colorblind?

We learned now a lot about the different types and categories of color vision deficiencies. But what does it really look like if you are colorblind? How do you see the world if you are colorblind? The four pictures below should give you a first impression.

Normal Color Vision	Protanopia
Deuteranopia	Tritanopia

If you have normal color vision you might realize that in the case of red-green color blindness (protanopia/deuteranopia) not only red and green colors are affected but the whole color spectrum is perceived differently. The same is of course true for blue-yellow color blindness (tritanopia). This is based on the fact, that all colors are perceived as a mixture of the three different cone types, and if one of them is missing the whole color spectrum changes.

The simulation below shows how the color spectrum changes. The shown lines are just meant as guides. Any line which ends in the so-called copunctal point connects the colors of confusion for a certain type of color vision deficiency. A more severe color blindness simply results in thicker and longer confusion bands.

Simply put you can say, color blind people see the world like people with normal color vision see it at dusk or dawn. At this time of the day colors start to fade away which is comparable to a color vision deficiency.

To learn more about how color vision works and to get a lot of interesting details about this topic, visit the excellent web site of Bruce MacEvoy on Color Vision.

In the next part of Color Blind Essentials we will have a closer look at the most common type of color vision deficiency: Red-Green Color Blindness.

The book consists of around 30 pages and it gets directly to the point:

“It is an invisible handicap, and those afflicted by it will certainly not shout it from the rooftops. That is probably the main reason why this anonymous group is hardly taken into account, except in a negative sense; ther still are quite a few jobs that are not accessible to the colorblind.”

Color-Blind: seeing the world
through different eyes

The author of the book is the founder of Blind Color, a company based in the Netherlands which tries to make the general public more aware of the existence of color blindness. They also help you to make your products to be color-proof for the colorblind and support companies, families and teaching institutes in all kind of topics related to color vision deficiency.

Starting with color in everday life, especially in a colorblind life, the book also covers the topic of how to do better when using colors for many different tasks. The last part is called color bilndness, its scientific explanation and focuses also strongly on color vision in general.

What I like very much in the booklet:

• It includes many good examples about color blindness in real life.
• A lot of color blindness simulating pictures strongly support the message.
• There is a whole list of hints and tips for different industries what they have to think about when working with colors.
• A very detailed description of color vision and the source of color blindness.
• An explanation of the different types of color vision deficiency and their possible severities.

What I think could be done better:

• Unfortunately the author doesn’t tell us a lot about possible career choices, which is often asked for by many people.
• There is no information on different color blindness tests, color vision enhancements/tools, or about possibilites to cure color vision.
• Some of the simulated pictures don’t seem to be correct. The color red is sometimes rendered much to dark (I’m strongly red-blind and still can spot a huge difference between the original and the simulation).

Overall it is a compact booklet about color blindness with some really good tips. You can easily learn the basic facts of color vision and its deficiencies, but if you want to learn some more details in certain topics which I mentioned above, you need further sources of information.

The booklet either in English or Dutch can be ordered directly from the Netherlands at info@blindcolor.nl.

My friend from www.kleurenblindheid.nl was so nice to send me an issue of this booklet. Thanks. And if you speak Dutch you should also visit his very comprehensive site on color blindness.

So what actually is color vision deficiency also known as color blindness?

Simply put, if you are suffering from a color vision deficiency you are perceiving a narrower color spectrum compared to somebody with normal color vision.

This short definition raises a few more questions which need to be answered to understand the term color-blind more completely:

• Why am I suffering from color blindness at all?
• What means narrower color spectrum compared to normal color vision?
• Are there different types of color vision deficiency?
• How do I know if I’m colorblind?
• Is there some possibility to cure color vision deficiency?
• Can I just live with it or do I have to be afraid of it?

In this article I will among other things answer the first two of those questions. The others will be looked at in the follow up articles of this series about Color Blind Essentials. But first of all I would like to take you back to the 18th century.

History of color vision deficiency

The first scientific paper about color blindness was written by John Dalton in 1793 entitled “Extraordinary facts relating to the vision of colours“. Dalton himself was red-green colorblind and as a scientist he took interest in this topic. He claimed, that a colored liquid inside the eyeball is the source for a different color perception. This was proved wrong only after his death, when his eyes were examined and no such liquid was found.

After that Thomas Young and Hermann von Helmholtz were the first who described the trichromatic color vision. And once a theory for human color vision was ready, the basics of color vision deficiency weren’t far away.

The cause of color blindness

Color perception in the human eye is build up by three different types of cones. Each type is sensitive to a certain wavelength of light (red, green, and blue) and every perceived color is therefore a mixture of stimuli of those three cone types.

Now, if you one of those peaks of sensitivity is shifted towards another one or if one is missing at all, you perceive a narrower color spectrum—in other words you are colorblind. As a peak can be shifted everything between a little bit and the whole way, any type of severity is possible. The closer the peaks are the more severe is your color vision deficiency: slightly, moderately, strongly, or absolutely colorblind.

The type of affected cones also has a big impact on your color vision deficiency. As there are three different types of color receptors, there are also three different main forms: red (protan), green (deutan), and blue (tritan) disorders. As red and green deficiencies result in quite comparable color vision problems, they are put together and known under the term red-green color blindness. You will find more information on the different types of color blindness in the following two articles of this Color Blind Essentials series.

Much less common possibilities for color blindness are also glaucoma, aging, alcohol missuse, or a hard injury on your head. Those factors often cause some milder form of blue-yellow color blindness (tritanomaly). Also other facts like signal transmission can cause problems in color perception, but this is not fully understood yet.

Why am I suffering from color blindness?

You know now the cause of color vision disorders, but we still have not evaluated why we can be colorblind at all.

We learned that in most cases color blindness is a genetic disease which is inherited from the parents to their children. This means, if one or both of your parents is suffering from some type of color vision deficiency, there is a certain chance that you or your children will have the same vision handicap. The chance is strongly related to the type of color blindness.

Before I show you a sample inheritance pattern, we will have a closer look at our chromosomes. Unfortunately it is not as simple as it could be, because there are different chromosomes involved in color vision. And on top of that even on the same chromosome several different genetic code pieces are participating. The essence you should know is, that red-green color blindness is a sex linked recessive trait and blue-yellow color blindness is a autosomal dominant trait.

• sex linked: encoded on the sex chromosome X, whereas men only have one of them (XY) compared to women (XX).
• autosomal: encoded not on the sex chromosome, equal for men and women.
• dominant: if it is encoded on one chromosome, you really suffer from it.
• recessive: if you have another healthy chromosome, it won’t show up.

If you combine this all together, we have more colorblind men than women. — Why is that?

Color blindness inheritance pattern

The above genetic encodings lead us directly to the inheritance pattern. This will also show us on a glance, why there are more men suffering from color blindness than women.

Red-Green Color Blindness Inheritance Pattern

The diagram on the right shows the inheritance pattern of red-green color blindness, which is by far the most common type of color vision deficiency. As you can see, this is a disorder which is passed on from a grandfather to his grandson, whereas the mother is only a carrier of it. A carrier is not affected because the trait is recessive. This causes much more men to be red-green colorblind, and even more women to be carriers of this color vision deficiency. You can also learn from this diagram, that a woman can only be red-green colorblind if both of her parents are at least carrying the disease encoded in their genes.

Am I the only colorblind person?

No, definitely not. Color blindness is a very common disease which is found all over the world. Different scientific studies show, that roughly 8% of all men and 0.5% of all women are colorblind. This numbers are supported by different studies and are about the same all around the world. The high difference between men and women is resulting from the facts we just learned, that the most common form red-green color blindness is a recessive sex-linked trait.

Knowing this numbers you can also compute some very interesting probabilities in color vision deficiency:

• Approximately every 500st handshake is between two colorblind people.
• It is almost sure (probability: 94%) that at least one out of a football team is colorblind.
• If you pick 100 persons randomly, there is a tiny chance (< 1.5%) that none of them is colorblind.

In the next article of the series Color Blind Essentials, we will have a closer look at the different types of color blindness. The common and also the very uncommon ones.

You will not only learn what color blindness really is, which forms of it exist and of course some details about the most well known red-green color blindness. But you will also have the possibility to read more about on how a color vision deficiency can affect your everyday life, if there is a way to cure or at least soften it and the different possibilities to test your color vision.

This series on Color Blind Essentials includes the following six parts which will be posted once a week on Tuesdays:

• What is color blindness?
• Types of color blindness — publish date: 9th March
• Red-green color blindness — publish date: 16th March
• Color blindness tests — publish date: 23rd March
• Living with color blindness — publish date: 30th March
• Curing color blindness — publish date: 6th April

If you want to learn even more about color blindness and closely related topics, you can either follow some of the links I provide in this series, directly dive into the articles archive of Colblindor, search the whole web site or subscribe to my latest articles.

And if you like this series why don’t you write a comment or drop me some lines through my contact form? I’m looking forward to hear from you and hopefully can answer your questions—if there are still any left after reading the Color Blind Essentials series.

Color Assessment & Diagnosis Test

Because of the lack of reliable, standardised tests and the absence of information on the specific colour vision needs of professional flight crew, the UK Civil Aviation Authority supported by the US Federal Aviation Administration initiated this study.

A team around Prof Barbur from the Applied Vision Research Center in London was mandated to find the minimum color vision requirements for modern flight crew, and a new color assessment and diagnosis test. This was the last part of the study after The Use of Colour Signals and the Assessment of Colour Vision Requirements in Aviation and a Task Analysis which included two operating case studies: the Airbus A321 and Boeing 757.

Dr Sally Evans, Chief Medical Officer at the CAA, says:

“The current diversity in colour vision testing methods and standards demonstrates the need to adopt more objective assessment techniques internationally. If the assessment methods and limits derived from this study were applied as minimum requirements for professional flight crew, 35 per cent of colour deficient applicants would be eligible for medical certification as a professional pilot. The CAA intends to promote this research internationally with a view to gaining acceptance of the CAD test and its incorporation in world-wide medical standards for pilots.”

This sounds very promising for all colorblind pilot applicants! So let us have a closer look at what this new color blindness test is all about and how they reached this new results.

Color Assessment & Diagnosis Test

The current procedures within JAA for pilot applicants are unsatisfactory for at least two reasons.

1. There is no guarantee that the deutan subjects that pass secondary tests can cope with safety-critical, color-related tasks, since the severity of their color vision loss remains unquantified.
2. Many color deficient subjects that can carry out such tasks safely fail the lantern tests and will not therefore be allowed to fly.

This findings and many detailed studies on color vision deficiency resulted in a new color blindness test, the color assessment & diagnosis test (CAD test). The subject’s task is to report the direction of motion of a colored square on a gray square background with dynamic luminance contrast noise. This new developed color vision test has shown in a broad study to be very accurate in identifying type and severity of one’s color blindness.

The subject’s color vision severity is measured in Standard Normal units (SN units). If your result would show a red-green threshold of 2 SN units this would mean, that you need a twice as strong color signal compared to a average standard CAD observer. This threshold can be quit different for deuteranomalous and protanomalous observers as a limit to pass the PAPI test. Details on this are shown in the conclusions.

PAPI Test

The Precision Approach Path Indicator (PAPI) was indicated as the most important, safety-critical task that relies largely on color vision. On this basis a PAPI simulator test was developed to quantify the severity of a pilots color vision deficiency which is still safe to fly. This simulator can be used in controlled laboratory environments.

PAPI Test Simulator

The simulator reproduces both the photometric and the angular subtense of the real lights under demanding viewing conditions when the lights are viewed against a dark background. Since other color-related tasks such as seeing the color of the parking lights or the discrimination of runway, center-line, red and white lights are less demanding, it is assumed that the pilot will also be able to perform correctly these tasks.

The aim was to identify type and severity of color vision deficiency which cause problems with the PAPI test and correlate those results to the CAD test results. In principle, this approach should make it possible to recommend pass/fail limits based on the observer’s ability to carry out the most safety-critical and demanding PAPI task.

Principal conclusions

The very promising results suggest that subjects with minimum color blindness that does not exceed 6 SN units for deuteranomalous observers and 12 SN units for protanomalous observers perform the PAPI test as well as normal trichromats. If these findings were adopted as pass/fail limits for pilots ~35% of color deficient applicants would be classed as safe to fly.

• When the ambient level of light adaptation is adequate, normal aging does not affect significantly either red-green or yellow-blue thresholds below 60 yrs of age.
• Analysis of PAPI results shows that the use of a modified white light results in significant, overall improvements in PAPI performance.The modified white is achieved simply by adding a color correction filter.
• 43 of the 77 deuteranomalous subjects failed the PAPI test. 29 out of the remaining 34 subjects that passed the PAPI test had CAD thresholds < 6 SN units.
• 20 of the 40 protanomalous subjects failed the PAPI test. 13 out of the remaining 20 subjects that passed the PAPI test had CAD thresholds < 12 SN units.

The study also concluded that the administration of the CAD test eliminates the need to use any other primary or secondary tests. When one includes normal trichromats, ~94% of all applicants will pass the so called fast-CAD screening test and be classified as safe to fly. This process is very efficient since the fast-CAD test is simple to carry out and takes less than 30 seconds to complete.

Official CAA news:
CAA research paves the way for more people with CVD to become pilots
CAA Paper 2009/04:
Minimum Colour Vision Requirements for Professional Flight Crew