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Color Vision Testing

Humans perceive light with photosensitive cells (called cones and rods) in the retina. The rods, located in the peripheral retina, give us our night vision but can not distinguish color. The cones, located in the center of the retina (called the macula), are responsible for encoding color. Each cone contains structures or visual pigments sensitive to one of three wavelengths of light - red, green, and blue. All other colors are variations of these three, and are distinguished from one another by the amount of pigmentary sensitivity and response within the cones. Defects in color vision occur when one of the three cone cell color coding structures fails to function properly, due to either injury or genetic miscoding.. One of the visual pigments may be present and functioning abnormally, or it may be absent altogether. As a result, certain wavelengths of light may be perceived differently than they would be otherwise, or they may not be perceived at all.

Causes Of Color Vision Deficiency

Most color vision deficiencies arise genetically, carried on by the female X-chromosome to children. Ocular injury or disease, such as macular degeneration or optic nerve disorders, can also cause color deficiency. Certain medications are also linked to color vision loss.

Testing Methods

Pseudo-isochromatic plates are the most commonly used method to test color vision. To a color-deficient person, all the dots in one or more of the plates will appear the same. To a person with normal color vision, some of the dots will appear dissimilar enough from the other dots to form a distinct figure on each of the plates. The original card version of this test (by Dr Shinobu Ishihara) was designed to be carried out in a room adequately lit by daylight. The presence of direct sunlight or artificial light may produce some discrepancy in the results because of some alteration in the appearance of shades of color. This electronic version may also produce some discrepancies as the images have been optimised for web-based delivery and with a monitor resolution of 800x600 and 256 colour display or greater. The results of this test are not to be considered a valid medical test for color blindness and merely serve to illustrate the tests available.

Plate #1 The correct image is 12. Because the image is on a grey background, even a color-deficient individual will answer this correctly.

Plate #2 The correct image is 8. Red-green deficient people see a 3.

Plate #3 The correct image is 7. Blue-yellow deficient (more rare) people are unable to see an image.

The Color-deficient Person's Vision

Color vision deficiencies bother affected children from the earliest years. At school, coloring can become a difficulty when one has to take the blue crayon - and not the pink one - to color the ocean. When cooking, red deficient individuals cannot tell whether their piece of meat is raw or well done. Neither can they make the difference between green and ripe tomatoes or between ketchup and chocolate syrup. They can however distinguish some citrus fruits. Oranges seem to be of a brighter yellow than that of lemons. To them, purple looks like a dark blue that is almost black and red is a very dark gray that is also almost black. Some of them cannot appreciate a rainbow because they can only see two light colors, blue and yellow. However, they distinguish faded and gray colors much better and they even appear attractive to them. When driving, they only distinguish traffic lights from street lights in terms of brightness. Moreover, rear lights make it difficult for them to tell whether a car is coming or going away.

Jobs That Exclude Color-deficient People

In many countries, one cannot aspire to some jobs and careers prior to passing color-vision tests. Jobs that currently require perfect color vision are listed below:

Transportation:

· in the Army, Air Force, Navy: pilots, engineers
· civil aviation: pilots, mechanics and engineers
· air traffic control
· railways: mechanics and train drivers
· public transport: bus drivers...

Public safety:

· police (certain grades)
· customs officers
· firefighters

Other jobs:

· electrician and electronics technicians
· in labs: technicians and druggists
· textile and printing industries
· color photographer, paint maker
· jobs where one selects fruits, vegetables and meat.

In many other occupations such as those dealing with geology, cartography and chemistry, color perception deficiency is also a handicap.

Treatment

Currently, there is no available treatment for color vision disorders. When diminished color vision is linked to medications, discontinuation of the drug's) will aid in preventing further loss of color sensitivity - in many cases, however, color vision will not be completely restored. Treatment for any ocular diseases associated with color-sensitivity disturbances will also help to prevent further loss of color vision.