A lot of people think, if you suffer from color blindness you can not see any colors at all apart from black, white, and shades of gray. But they are wrong.

This form of complete color blindness is a very rare subtype of color vision deficiency among much more common ones like the the well known red-green color blindness. You might say, in this case the wording color blindness isn’t chosen appropriate—and you’re right.

Color Blindness begins when you don’t have normal human vision which might be only a very light change of color perception. And it ends with complete color blindness combined under the umbrella term monochromacy.

If you are suffering under monochromacy, you perceive everything just in shades of gray. This makes it really hard to accomplish many everyday tasks, because colors are such a central part of our life. For example, people suffering form monochromacy might mix up the following colors:

• green and blue
• red and black
• yellow and white

Monochromacy—also referred to as monochromatism—can be observed in different forms and even worse, many different names are in use:

• rod monochromacy: typical or complete achromatopsia, total color blindness, day blindness
• blue-cone monochromacy: S-cone monochromacy, incomplete achromatopsia
• cone monochromacy: complete achromatopsia with normal visual acuity
• cerebral achromatopsia: atypical achromatopsia

Before we dig a little deeper into the different forms of complete color blindness you should know the basics of vision and specially color vision.

The human eye has two different receptors inside the eye to perceive light. They are called photoreceptor cells. The rods are very sensitive on brightness and used for night vision. On the other side we have three different types of cones (trichromacy) which are responsible for colored day vision. The three different forms of cones have different peaks on color perception: red, green and blue. And the mixture of those three base colors makes us perceive such a huge variety of colors—or at least almost all of us. Compared to cones, rods play no role in color vision.

Rod Monochromacy

In this case your vision relies solely on the rods and the cones are usually not working at all. Some cases also report that the cones are absent, have some irregular distribution or that they show abnormal shapes. Occurrences are estimated to be between 1 : 30,000 and 1 : 50,000. But even if this numbers look very small, rod monochromacy is still by far the most common type of complete color blindness.

Rod monochromatism is often referred to as achromatopsia and is based on autosomal recessive inheritance. In other words, you need two defective chromosomes and it is equally distributed among men and women. Achromatopsia is characterized by the following symptoms:

• complete inability to differentiate colors,
• severe light sensitivity (photophobia),
• long-sightedness,
• involuntary eye movements (nystagmus),
• and central depressed vision (scotoma).

Also rod monochromats are completely colorblind, they often learn to associate certain colors with objects and to differentiate some colors by their brightness.

Blue-Cone Monochromacy

This rare form of monochromatism is caused by loss or rearrangement of the genes encoding L- and M-cones. Because of that only blue color receptors (S-cones) and rods are transmitting color and/or brightness information. If you suffer S-cone monochromacy your are usually completely colorblind. But in twilight situations—where rods and S-cones are working—color perception can be compared with dichromatic vision.

There are reports about very different forms of blue-cone monochromacy and it is sometimes not easy to differentiate it from rod monochromacy. No exact number of incidences is available. Scientists estimate the frequency at 1 : 100,000—for men. Because red and green cones are encoded on the sex chromosome, inheritance patterns are similar to red-green color blindness. Occurrences in women are unknown.

Cone Monochromacy

Also known as complete achromatopsia, this form is very rare. Only a few cases have been reported and none of them is fully accepted. It is assumed that they are either have working L- or M-cones and inactive or absent S-cones. Compared to blue-cone or rod monochromacy, cone monochromacy doesn’t have reduced visual acuity or any special light sensitivity.

Cerebral Achromatopsia

In this case optical examinations show normally functioning cones and rods. It is not understood in detail yet, but it is thought that the color receptors are working properly only the information is not reaching the brain or is not processed. Only a few cases have been investigated. Cerebral achromatopsia is unlike the other types of monochromacy not inherited but and acquired color vision defect. It may be caused by trauma or illness.

The academical background and many references on all types of monochromacy can be found in Opsin genes, cone photopigments, color vision, and color blindness by Sharpe et al. An internet information portal on achromatopsia is provided by the The Achromatopsia Network.

If you are colorblind it is not just about being colorblind or not. There are many different types and characteristics of color vision deficiency which you can suffer from. Color blindness is an umbrella term for all those different forms—and not even a good one.

I’m 13 years old and have been told I am red-green colour blind but only slightly. I have done research and taken MANY colour blind tests. But I’m still confused because how can I be slightly colour blind? Either I am or I am not. I have failed most of the tests I have taken and my optician has told me I am slightly colour blind towards red-green (the most common) but want to know how I am only slightly colour blind and what the scientific word for being slightly red-green colour blind.

First of all, you shouldn’t rely on the color blindness tests available online. These tests are just used to give you a feeling what kind of color blindness you are suffering from and how severe it is. But because every computer display can have different color adjustments they are not 100% reliable.

This young reader says, that either you are colorblind or you are not. This is true and false. If you go to your eye specialist and take a color blindness test, he will tell you if you have normal color vision or not. So yes, either you are colorblind or not.

But he can also tell you, what type of color blindness you are suffering from and the approximate severity of it. There is a whole terminology of color vision deficiency which I don’t wont to list here on the whole. Just the most important facts.

• Types of color blindness:
  • Red-green colorblind: This is the most common form and according to its name, causes the biggest problems with red and green. There are two different subtypes: Red-blind (or -weak) and green-blind (or -weak).
  • Blue-yellow colorblind: This type of color vision deficiency is not so often seen and actually the colors blue and green (not yellow) are the problem area.
  • Completely colorblind: A very uncommon type which makes you see only in shades of gray. Unfortunately a lot of people think that every colorblind person can only see in shades of gray.
• Severity of color blindness:
  • Anomalous trichromat: You have still three different types of color receptors as someone with normal color vision. But one of them is slightly shifted in its peak of sensitivity and therefore you can’t see the same broad color spectrum as others.
  • Dichromat: Here only two of three color receptors (cones) are working. You have to mix your perceived color just with two signals compared to three with normal color vision. The color spectrum is strongly reduced.
  • Monochromat: Either no color receptors are working (rod-monochromacy) or just one of them (blue-cone monochromcy). Anyway you will have either just grayscale vision or a very very restricted color vision.

This means, that is possible to suffer from any possible severity of color blindness. This because the three different color receptors have a highest peak of sensitivity. With an anomalous trichromacy, one of those peaks is shifted towards another one; usually the red peak towards the green or vice versa.

Because of that shift and the resulting peaks which are closer to each other, less information is available to mix up the final color in your brain. And this makes you either slightly colorblind (peaks still far apart) or more severely colorblind (peaks coming closer).

The academic term for a slight red-green color blindness is either protanomly (red-weakness) or deuteranomaly (green-weakness). Both types are a subtype of red-green color blindness and a form of anomalous trichromacy.

The International Colour Vision Society (ICVS) is an international group of physiologists, psychologists, physicists, geneticists, optometrists, ophthalmologists and others who have a research interest in the many aspects of colour vision and colour vision deficiencies.

This group will hold its 19th Symposium of the International Colour Vision Society from July 27 to 31 in Belém, Brazil.

Here is an introductory excerpt of the Program Book on this years symposium.

A glance at the program reveals that the traditional interests of our Society are well represented this year, including contributions by physiologists, psychologists, physicists, geneticists, optometrists, ophthalmologists, and other related professionals who have a research interest in the many aspects of colour vision and colour vision deficiencies. One special session is dedicated to the Nagel Centennial, the anniversary of the introduction of the W.A. Nagel Anomaloscope in 1907.

The program includes in impressive list of speeches with many topics directly related to color blindness. I’ve chosen some of them in the list below to give you an impression of actual research topics.

• Welcome to the wonderful world of color: Gene therapy treatment for colorblindness.
• The effects of colored lenses on the number of discernible colors perceived by dichromats in natural scenes.
• Surface color perception of color defective observers under dim illuminations.
• Performance of the Lanthony New Color Test by young children.
• Mass screening for color-vision deficiencies in Norwegian children.
• Color deficiency correction – methodology and experiment report.
• The contribution to Rayleigh matches of the third red-green photopigment of color-defect carriers.

As described above it is also the centennial of the Nagel anomaloscope, which was introduced in 1907. The Nagel anomaloscope was introduced to measure severity and type of red-green color blindness. It is since then the reference on color blindness tests and of course evolved along the way to cover different types of color blindness.

Maybe we will have the chance to get a grasp of some of the speeches and catch a fresh breeze of academical research on color vision deficiency. Stay tuned.