The human eye relies on its 6 - 7 million cone cells and 100- 130 million rod cells to produce normal vision. The cones—blue, green, and red—are located in the center of the retina —the part of the eye that receives images from the lens and converts them to electrical signals to send to the brain— and are responsible for color vision, light adaptation, and fine detail. The rods are located in the periphery of the retina and are responsible for night vision, brightness perception, and distinguishing shapes.

A person with normal trichromatic (three colors) vision can identify over 7 million different colors! However, 8% (1 in 12) of the men and 0.5% (1 in 200) of the women in North America have fewer cones than normal and are said to have a color vision deficiency. Because the overwhelming majority of these people can distinguish colors to some extent, it is inaccurate to label them as color "blind."

Red-green deficiency is the most common color vision abnormality. The mildest forms of this deficiency are protanomaly—where blue-green and red-purple are perceived as an indistinct grayish shade—and deuteranomaly—where green and red-purple are perceived as an indistinct grayish shade. Both of these defects are forms of anomalous trichromatism, which means that blue, green, and red cones are present, but the pigments contained within them are altered. Half of the people with anomalous trichromatism can make precise color matches (but not as precisely as those with normal color vision) and are said to have a simple anomaly, while the other half cannot and are said to have an extreme anomaly. A more pronounced form of red-green deficiency is called dichromatism (two colors), in which one of the three cone types is absent. Dichromatism is divided into two subtypes, protanopia—where blue-green and red-purple are perceived as gray—and deuteranopia—where green and red-purple are perceived as gray.

The rarest form of color deficiency is monochromatism (one color), which is divided into rod and cone subtypes. A person with cone monochromatism has good central vision but confuses all colors, because two of the three cone types are missing. A person with rod monochromatism, or achromatopsia (no color), has no cones at all and sees everything in shades of gray. People with achromatopsia have poor vision and difficulty adjusting to bright light, so it’s fortunate that this abnormality affects only 1 person in 33,000 in the United States.

Although color vision deficiencies can be caused by eye diseases or normal aging, the majority are inherited as a gene attached to the X chromosome. This explains why men, who have only one X chromosome, are far more susceptible to color vision abnormalities than women, who have two. An affected man passes the color-deficiency gene to all of his daughters, who then become unaffected carriers; his sons get his Y chromosome and have normal color vision. Each son of a carrier female then has a 50-50 chance of getting the gene from his mother. Therefore, the typical scenario is a man inheriting the color-deficiency gene from his maternal grandfather. For a woman to be color deficient, she would have to have an affected father and mother who was at least a carrier; even then, her odds of getting the gene would be only 50-50.

Color vision is essential for recognizing aircraft position lights, light-gun signals, airport beacons, approach-slope indicators, and chart symbols, especially at night. The 14 CFR Part 67 regulations, revised on September 16, 1996, state that a pilot must have "the ability to perceive those colors necessary for the safe performance of airman duties" for all medical certification classes.

Many student pilots with a color vision deficiency don't know they have it, because it doesn't affect their daily lives. Therefore, most learn about it during their first visit to an aviation medical examiner (AME). This is unfortunate, because the AME is required by the FAA to prohibit anyone failing a color vision test from flying at night or by reference to light-gun signals.

There are numerous Web sites (e.g., www.geocities.com/Heartland/8833/ coloreye.html, www.toledo-bend.com/colorblind/Ishihara.html) that have crude color vision tests you can administer yourself. (The emphasis is on the word crude—these screening tools are not a substitute for an eye doctor's exam). If your performance on one of these tests suggests a color vision abnormality, don't go to an AME yet! Instead, have an optometrist or ophthalmologist evaluate your color vision with pseudoisochromatic ("almost same color") test plates or with an FAA-approved alternative test (see tables below).

If possible, find an optometrist or ophthalmologist who uses the Dvorine or AOC test plates. People with color vision deficiencies usually find these easier to pass than the Ishihara plates. While seeing the eye doctor (pun intended!), have him or her complete the rest of your FAA-required eye exam. That way your vision won't even be an issue when you see your AME.

The FAA-approved alternative tests listed below may be used
in lieu of pseudo-isochromatic test plates.

Absolutely not. All of the tests listed above are approved by the FAA for color vision testing. Most people with color vision deficiencies are only mildly affected and can easily perceive those colors necessary for the safe performance of airman duties. A person who has a significant and potentially hazardous color vision problem will fail his or her color vision examination no matter which test is used.

Although no eyeglasses will correct a color vision deficiency, a contact lens called X-Chrom can partially correct it. The problem is that while X-Chrom lenses can help your vision in one way, they can impair it in other ways. Because of this, the FAA does not allow pilots to wear them.

The best way to remove a color-vision restriction is to successfully complete one of the FAA-approved alternative tests listed above. This is preferable to the other option, the color signal light test, because FAA authorization is not required, no waiver is issued, unsuccessful attempts don't have to be reported, and the color signal light test option remains available.