In order to adequately understand color blindness, you must first understand the basic structure of the eye. Knowing how color is seen will help you better understand what goes wrong in the eye to cause color blindness. The best way to describe the major aspects of the human eye and how they help process light into color is to track the path of light through the eye.

The first structure light hits when entering the eye is the cornea. It is located at the front of the eye, along with protecting the eye from germs and dust, controls the entry of light into the eye. It refracts light into the lens, which works to refract (or bend) the light still further into the retina.

The purpose of the retina is to send signals to the brain, which will interpret these signals as images. The production of signals by the retina is the step at which color blindness occurs. Within the retina are more than 120 million structures called rods, which are more sensitive to light than their counterparts in the retina, called cones. The difference between rods and cones is that rods, while more sensitive and capable of producing vision in low light situations, are not sensitive to color.

In order to process color, the eye uses its six to seven million cones located in the center of the retina. These cones are each sensitive to particular ranges of light wavelengths within which colors can be seen (400 to 700 nanometers). For instance, blue light has a wavelength of 475 nanometers (nm) while red light can be seen by cones sensitive to light with a wavelength of about 650 nm. Within most people’s genetic code are genes which can determine which cones will be sensitive to certain wavelengths. When those genes malfunction, cones will be sensitive to the wrong wavelengths, which will in turn produce color deficiencies. This is because not enough cones are sensitive to particular wavelengths and therefore are not able to process certain colors.

“Color blindness” is a bit of a misnomer, as most people classified as color blind can see in color. In fact, it is very rare that monochromasy, or seeing entirely in black and white, is diagnosed. Like deafness and blindness, which are terms often misinterpreted to mean absolutely the complete absence of hearing or sight, there are degrees of color blindness.  A normally functioning eye sees color through the interconnectedness of cones which in actuality only see the primary colors, red, green, and blue. It is only by mixing these three primary colors along with white and black, that any other color can be made, much like a painter’s pallet. To be mildly color blind means the eye has cones which are not as sensitive to one of the primary colors as they should be. This is referred to as anomalous trichromasy. More severe color blindness is referred to as dichromasy and is the result of the wavelengths to which cones are attuned being far from optimal.

Color blindness is a genetic condition affecting only about 5 to 8 percent of men and .5 percent of women are born color blind, about 99 percent of this group having either protanomaly (red weakness) or deutanomaly (green weakness) color blindness. This rather large portion of color blind people usually find that color blindness has little effect on their lives and many are unaware of their condition until they undergo the Ishihara Color Vision Test, often given in schools.

Protanomaly (1 in 100 men)

A protanomalous person has difficulty seeing red. The saturation and brightness of this color are extremely reduced. This does not mean all red things seen by a person with protanomaly are simply gray, but that the cones within the protanomalous person processes them as having a shifted color, something closer to a pale green. Things which have a violet hue, such as certain flowers, may be seen as blue, because violet is a mixture of red and blue, and since the cones in this instance do not process red, only the blue remains.

Deuteranomaly (5 in 100 men)

A person with deuteranomaly has difficulty seeing green. For this person, colors in the red, orange, yellow and green part of the color spectrum trend toward red in his sight. Unlike the protanomalous person, who has difficulty processing saturation and brightness, the deuteranomalous person has no change in his ability to see a color’s brightness.

Dichromasy – Separated into protanopia and deuteranopia (2 in 100 men)

The condition of dichromasy is a degree of color blindness which is more obvious to the affected person. To these people there is no difference between red, orange, yellow, and green. The dichromalous person’s condition affects them to a much larger degree than those with protanomaly or deuteranomaly because all the aforementioned colors appear as one hue.

Protanopia (1 in 100 men)

Red, orange, and yellow are reduced in terms of brightness to the protanope. This reduction in brightness can cause the protanope to confuse reds with dark gray or even black. Any color composed of red, orange, or yellow light is often seen as blue as a result of red light not being processed.

Deuteranopia (1 in 100 men)

For the deuteranope, all conditions of the protanope apply with the exception of the loss of perceived brightness of colors. The deuteranope still cannot tell the difference between red, orange, yellow, and green.

Summary

Though many people mistakenly believe the color blind person sees life as an old black and white movie, this is not the case. People with protanomaly or deuteranomaly are usually so unaffected by their condition the possibility exists that, if they were not given a color vision test, they would go their entire lives without being aware of it. Of course, the protanope and deuteranope can face life challenges as a result of their condition, but they often learn ways to manage and thrive despite these challenges.

More Information on Color Blindness

Congenital and Cerebral Color Blindness
Tests for Red-Green Color Blindness
A Glossary of Color Science
Accommodating Color Blindness
Vision Simulator
American Optometric Association
The Genetics of Color Blindness Explained
Is Your Child at Risk?
Sight and Hearing Association
Color Blindness Symptoms