Definition of Color Blindness
The term color blindness is a bit misleading. It doesn’t necessarily mean the person can only see in black and white, although there are instances in which this is the case.
More often than not, the term is used to refer to someone who has difficulty making out colors of a certain color family, like blues and greens or red and pinks.
Why this happens is actually quite curious. Let’s talk about the retina first, to understand how we see in color.
The retina is like film in a camera. It takes the images we see and sends them to the brain for processing. But, the retina is really part of the brain itself. I liken it to one huge neuron with a really long axon, or tail.
The retina is actually made up of ten layers. Since we’re not in med school, we don’t need to know all ten layers, but there is a layer in particular that affects our color vision, and that is the photoreceptor layer. This is where the rods and cones are found.
The Rods and Cones
The Photoreceptive Layer
There’s a reason this layer is called the photoreceptive layer; it’s where the image is actually captured. This layer is comprised of cells known as photoreceptors, also known as the rods and cones.
The rods are the cells that give us our peripheral vision, and they actually look like little rods. They don’t see in color, just black and white.
They are densely packed in the periphery of the retina and are completely absent from the macula. These cells are also responsible for our ability to see at night, which is termed “scotopic vision.”
The macula allows us to see fine detail and to do things like read up close. The macula is densely packed with cones.
These cells are highly specialized, and they actually look like cones. Each cone is associated with what is called a photopigment, or pigment of a certain color. There are three types of cones: red, blue and green. Cones are responsible for our daytime or “photopic vision.”
Wavelength of the Different Colors
Facts About Color Blindness
Colors are created by wavelengths of light. For example, the wavelength of the color red is about 450 nanometers. Color variations are caused by a variation in wavelength and intensity of light. If a person is lacking cones that react to the color red, that person cannot distinguish the various shades of the red color family. The same applies to blue and green.
668 - 789 THz
380 - 450 nm
606 - 668 THz
450 - 495 nm
526 - 606THz
495 - 570 nm
508 - 526 THz
570 - 590 nm
484 - 508 THz
590 - 620 nm
400 - 484 THz
620 - 750 nm
Different Types of Color BlindnessClick thumbnail to view full-size
There are three different types of color blindness caused:
- Protanopia (someone who sees the spectrum in blues and yellows, and easily confuses red and green; these people also have difficulty distinguishing red light)
- Deuteranopia (someone who cannot clearly see purplish red and green)
- Tritanopia (someone who cannot distinguish blue and green or yellow and violet)
These three types are caused by a loss of one or more of the photopigments. These same deficiencies can be caused by mutated cone cells. The mutations are called protanomaly, deuteranomaly, and tritanomaly respectively.
What Causes Color Blindness?
Very rarely a person may suffer from a mutation of the cone cells, which will also result in a deficiency in color vision.
Those who are color blind are typically lacking one or more of the photopigments. There are several different photopigments.
- Rhodopsin - found in the rods only
- Photopsins - found in the cones (there are three sub pigments in this group including red, green and blue pigments)
- Melanopsin - responsible for the circadian rhythm and pupilary light reflex. Interestingly, melanopsin has been linked to the photophobia (light sensitivity) caused by migraines.
Someone who is only missing one pigment is said to have dichromatic vision. Someone who is missing two (or all) of the pigments is said to have monochromatic vision.
Someone missing all three pigments relies on the rod cells for their vision, and because the macula is comprised of only cone cells, someone who is completely color blind also has severely reduced visual acuity.
They will never have perfectly clear 20/20 vision. Complete color blindness is very rare and is almost always accompanied by nystagmus (a condition that causes the eyes to move rapidly from side to side or up and down and do so constantly).
Color blindness is hereditary, and males are most commonly affected by it. Interestingly, the gene for color blindness is passed on by the female who is usually not affected. About eight to ten percent of men are color blind, whereas about 0.4 percent of females are affected by the disorder.
Ishihara Plate Comparison
Sometimes color vision deficiency can be caused by cataracts or medications. It can also be caused by brain damage or a syndrome known as achromatopsia. This syndrome is hereditary and is usually associated with:
- Photophobia (light sensitivity)
- Amblyopia (lazy eye)
- Nystagmus (rapid, constant movement of the eyes from side to side, or up and down)
- Hemeralopia (day – blindness)
In achromatopsia, the patient is completely color blind, and this affects their ability to see clearly. In bright daylight, their vision is terrible because the cones in the macula do not work. However, in low light conditions, they see very well because the rods allow us to see at night and in low light.
Color Blind Recessive Gene
Ishihara Color PlatesClick thumbnail to view full-size
Diagnosis of Color Deficiency
Someone who is color blind is easily diagnosed. There are several tests that can be performed by your ophthalmologist to determine quickly and painlessly if you have a color deficiency.
The most common test used for this purpose is the Ishihara Color Plates. These color plates are used to both diagnose color blindness and determine the type of color deficiency.
There are several other tests to diagnose a patient with this disorder.
The Farnsworth Munsell 100 Hue test is another diagnostic test to determine both if a person has a color deficiency and if so, which one. This test is quite lengthy, and can be difficult. There is a shorter version of it called the D-15.
Other tests are available, but are rarely used. However, in cases of color blindness caused by medication, a visual field test may be given using a red stimulus to determine if a patient has lost sensitivity to the color red.
Farnsworth Munsell 100 Hue Test
Treatment of Color Blindness
Unfortunately, there currently is no treatment for color blindness that is inherited. Sometimes color blindness can be caused by the aging process.
Cataracts, for example, can cause a change in color vision because of the yellowing of the lens. Once the cataract is removed, however, the patient’s color vision is restored.
In some cases, color vision deficiency can be caused by medications. A patient with rheumatoid arthritis, for example, may be taking Plaquenil to alleviate symptoms and halt progression of the disease.
After long term use of this drug, the patient can develop a red – green deficiency because of a side effect known as Plaquenil toxicity. The only treatment is to stop taking the medication. But, most of the time the benefit of the medication is worth the risk.
Patients have told me that they have “heard” color contacts can fix a color vision deficiency. This is completely incorrect. Color contacts are 100 percent cosmetic.
The color of the lens is limited to the iris only, and does not cover the pupil; therefore it does not affect the ability of the retina to see colors.
I have also been asked about glasses specifically designed to eliminate glare. Again, this is misinformation. Glasses that eliminate glare have a yellow tint to them.
These glasses alter color perception (which is why the military does not allow them to be used as safety glasses), but they do not fix a color deficiency in someone who cannot see color, especially someone with tritanopia (blue – yellow deficiency).
If you think you have a color deficiency, or other eye problem, see your ophthalmologist right away. If you take medications like Plaquenil or methotrexate, make sure your doctor knows, and have your color vision tested regularly.
It’s important to see your ophthalmologist on a yearly basis, even if you don’t have any eye problems, in order to catch eye diseases before they become a problem.
© 2012 Melissa Flagg COA OSC