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Color Vision

Most people have no idea why color vision is essential. Wouldn’t we get around just as well if we saw the world in black and white? The answer is color vision enables you to distinguish an object from its background more easily.

If your eyes saw only black and white, you could only pick out an object if it was whiter (“brighter”) or blacker (“darker”) than its background. However, if you also use color as a clue, you can recognize an object against a backdrop even if both have the same brightness. It is much easier to recognize and avoid shooting a fellow hunter if he is dressed in red or orange rather than the forest’s green or brown background.

Blue-yellow color vision deficiency exists, but it is extremely rare. Most people with a color vision deficiency – ninety-nine percent – have red-green color blindness. They have difficulty distinguishing between shades of red, orange, yellow, brown, and green. They see these colors as duller than they would appear to someone with normal color vision. Some of them confuse reds with black. Others have trouble distinguishing between shades of purple. Some have problems seeing red apples on a green tree.

Color blindness is inherited and present from birth. It runs through families for many generations. It often skips a generation – a grandson usually has the same color vision defect as his grandfather. Facebook CEO Mark Zuckerberg has red-green color blindness. The Facebook logo is blue because that is the dominant color he sees.

There is no treatment for color blindness, but most men with a color vision defect have few, if any, problems. They can do most normal activities, including driving. The only difficulties they have are buying matching clothes, determining whether meat is fully cooked, and distinguishing ripe from an unripe fruit. They also have somewhat limited career choices.

An object has color because it absorbs specific wavelengths of light and reflects other wavelengths. A dress is blue because it reflects blue and absorbs other colors. Green plants flourish under a red light, which they absorb, but they die under a green light, which they reflect. Black objects absorb all light, while white objects reflect all light. This is why a white shirt is cooler on a hot, sunny day, and a black jacket is warmer in the winter.

The human eye is a combination of two different eyes – a daytime eye, composed of cones, which work in bright light and give you color and detailed vision, and a nighttime eye, made of rods, which work in dim light, and detect motion. Eyes that have both rods and cones are not found in all animals. Rats, for example, have only rods, while ground squirrels have only cones.

Cones and rods are not evenly distributed in the human retina. The cones are packed densely into the center of the macula and are low in density in the rest of the retina. Color vision mostly takes place in the macula. This is why patients with macular degeneration often complain they have difficulty seeing colors.

The normal human retina contains three types of cones – red, green, and blue. When all three cones are present, a person has a trichromatic (“three-color”) vision. If either the red cones or green cones are functioning weakly, or not at all, he has dichromatic (“two-color”) vision. I say “he” because people with abnormal color vision are almost always male. Nearly ten percent of men have a red-green color defect – about twelve million Americans.

Most mammals, including dogs, cats, and bulls, are dichromats, i.e., red-green color blind. A bull cannot tell whether a matador’s cape is red, green, or yellow. Presumably, matadors select a red cape to appeal to the audience rather than attract the bull. The only other mammals that have trichromatic vision are the old-world monkeys in African and Asia. This is strong evidence that monkeys and humans evolved from a common ancestor in Africa.

Most birds, some fish, and certain insects have a fourth cone in their retina, which gives them tetrachromatic (“four-color”) vision. Human trichromats can see about 1 million different colors. Tetrachromats may be able to see an incredible 100 million colors. Tetrachromatic vision gives animals an extraordinary ability to pick out an object from its background. This is how a hawk can spot a tiny mouse from high up in the sky.

Many flowers we think of as white and have no color have brilliantly colored ultraviolet patterns to insects. We do not even have names for these colors since our eyes cannot see them. Flowers and the insects that pollinate them have evolved together. Flowers have developed complex colors we cannot see, and the eyes of the insects have evolved to see these colors. These colors tell the insects which plants they should pollinate.

Notice that I have been describing color vision defective people as “he”. Red-green color blindness is “sex-linked”. A defective recessive gene causes it on the X chromosome. Men have one X and one Y. They always get their X from their mother and their Y from their father. A man who inherits a faulty X from his mother will always be color blind. One out of every twelve men is color-blind.

Women have two X chromosomes. If they inherit a faulty X from their mother or father, they will almost always have a healthy X from the other parent to make up for it, so their color vision will be normal. This is why red-green color blindness is rarely found in women. Only 0.5% of women are color defective.

Women who carry a defective X chromosome are not color-blind themselves, but they will transmit color blindness to half their sons, and half their daughters will be carriers. A man who is color-blind will never have a color-blind son since he gives every male child his Y chromosome, but all his daughters will carry his defective X chromosome.

It is essential that all boys have a color vision test during their first eye examination. This can be a matter of life and death. The world’s first color vision test, the Holmgren Wool Test, was developed on the King of Sweden’s orders after the color-blind driver of a train failed to recognize a red signal, crashed his train, and killed many people.

One day an angry, depressed young man came to my office. His life’s dream was to be an airline pilot. Before he signed up for flight school, he went to an ophthalmologist, who checked his refraction, cornea, and retina and pronounced his eyes perfect. He did not check his color vision.

After two years of flight school, he failed a color vision test when he was one month away from graduation. He was told he could not graduate. I tested him and discovered he was color-weak instead of color blind. I wrote a letter to his officer, certifying he would accurately identify the color of landing lights, and he graduated on schedule.

Several professions that initially refused to accept color-blind men now recognize that color vision screening tests often eliminate men with mild cases of color deficiency who can do the job just fine. They currently do more sophisticated color vision testing – similar to the tests I used on that pilot – to distinguish the color-weak from the color-blind. They accept the color-weak but put restrictions on what they are allowed to do.

Until 2013, color-blind men were not allowed to pilot an airplane. Today, color vision testing advances enable color-weak men to become pilots if they meet a minimum standard.

Initially, all color-blind men were prohibited from becoming firefighters. The color of a fire provides vital information about its temperature. A fire with a temperature around 977 degrees Fahrenheit is a barely visible red, while a fire of 1,830 is a clear cherry red. The color of the smoke is also important because it indicates which substance is burning and whether or not it contains toxic chemicals. Today, fire service physicians reach an individual determination on each applicant, depending on their colorblindness severity.

A color-blind man will be accepted in the Army and Marine Corps today if he can distinguish red from green – but he will have certain restrictions. For example, he cannot become a Navy Seal or a Navy Special Warfare Combat-Craft Crewman because these men need to identify the color of signal lights and flares accurately.

It seems apparent police officers must have normal color vision. When they report a suspect’s appearance, they must accurately describe the hair and skin color and the color of the garments.

It is confusing that different countries, states, cities, and police departments have different recruitment regulations. For example, any color blindness disqualifies a man for the New York State Police, but the Los Angeles Police Department only says recruits “must be able to accurately and quickly name colors”. Bottom line: a man who is severely color blind will never be able to become a policeman, but a man with a mild deficiency might have a chance – especially in the U.K., where the regulations are the most liberal.

Normal color vision is vital for meat inspectors, who must detect discoloration in meat; casino dealers, who have to sort colored chips; and diamond appraisers, who must spot subtle gradations of colors in gemstones. Obviously, they will not do well as interior decorators.

Recently, scientists at the Medical College of Wisconsin have invented and patented a simple test for red-green color blindness, using a swipe on the inside of the cheek. This could allow us to screen all boys quickly and accurately.