Colour Vision Deficiency
Most cases of CVD are inherited and the person is affected at birth. Overview People who cannot distinguish between certain colours and shades have a condition called colour vision deficiency (CVD) commonly known as colour blindness. Abnormal colour vision may vary from only a slight difficulty distinguishing among different shades of the same colour to the rare inability to distinguish any colours.
Most cases of CVD are inherited and the person is affected at birth. In some cases however the condition results from retinal or optic nerve diseases the development of cataracts certain medications or aging. Diseases such as multiple sclerosis and diabetes can also cause colour vision disturbances.
In a normally functioning eye there are nerve cells in the retina called rods and cones. Rods detect brightness and cones detect colour. There are three different types of cones each of which is sensitive to red green and blue light. People with CVD have either fewer cones for each colour an absence of cones for at least one colour or cones that do not function properly.
Who is affected by colour vision deficiency?
In the inherited type of CVD roughly 1 in 10 men have some degree of abnormal colour vision. This means that one of the three types of cones in their eyes is either faulty or missing altogether. The condition is hereditary and sex-linked: fathers will pass the gene to their daughters (but not their sons) and mothers can pass it to all their children. However because women can be unaffected carriers men are at least 20 times more likely than women to develop colour vision problems.
People with color deficiencies may have difficulty distinguishing certain colors (e.g. a red/green color deficiency means that reds and greens are more difficult to distinguish). But as this photo demonstrates many other colors are just as distinguishable to a person with a color deficiency as to someone with normal color vision.
Below are the various types of colour deficiency.
Problem Official Term % of Males Per 1000 Males Weak in red "protanomalous" 0.5 % 5 No red "protoanopia" 0.8 % 8 Weak in green "deuteranomalous" 3.3 % 33 No green "deuteranopia" 0.6 % 6
The intensity of colour and the brightness of lights also affect a person’s degree of CVD. For example most people who have a red-green colour deficiency would be able to determine that a bright green crayon is green when outside in the sunlight but might think that it is a brown or red crayon when indoors.
Many people might not even realize that they cannot properly differentiate between colours as they are able to function effectively. For safety reasons people with CVD experience exclusion from many occupations such as airline and military pilots and fire and police workers. Using various colour vision tests your eyecare practitioner can diagnose CVD as well as the type and severity of the condition. The colour of dots that make up the numbers are visible to people of normal colour vision but are confused with adjacent colours by those with CVD.
The Ishihara Test
by Dr. Shinobu
Ishihara is a common screening test for colour deficiency. It determines colour problems by means of a series of cards each having coloured dots that form one pattern to the normal eye and a different pattern to the eye that is colour-deficient.
What numbers do you see revealed in the patterns of dots below? Answers are at the bottom of the article.
In mild cases of CVD a person has trouble distinguishing between shades of a certain colour. In more severe cases a person is unable to see a particular colour at all. Complete colour blindness is very rare. It is important to remember that people with colour deficiency generally can see most colours; they just have trouble distinguishing between some shades of red and green.
Colour vision deficiency can affect a person’s daily activities. For example a person affected with CVD who is cooking meat might be unable to see if the piece of meat was raw or overcooked. Distinguishing between ketchup and chocolate syrup or red and green peppers can be difficult or impossible. People with CVD might not see a woman wearing red lipstick and might only be able to differentiate between a red and a green traffic light by its brightness and positioning.
Colour vision deficiency is incurable although people can learn to adapt their lifestyle to their visual condition. In addition wearing tinted eyeglasses or contact lenses might help to distinguish certain colours or to make them appear brighter or dimmer. Electronic devices are available that can identify colours. Known as Electronic Eyes the sensors within the device activate an audio synthesizer that states the name of the colour aloud. Unfortunately the uses of the device are limited as it cannot read text.
Children and Colour Blindness
Early diagnosis of CVD can spare a child from frustration and help reduce stress when doing schoolwork. This is especially true as many materials designed for children rely heavily on colour as a teaching tool. Your eyecare practitioner can counsel you and your child regarding this condition.
Labeling or arranging clothing accessories and other household articles will help a child co-ordinate his or her garments and increase independence. A child can learn the brightness and positioning of traffic lights and the meaning of street signs by their shapes. Inform their teacher that colour-coded school assignments are not suitable and choose books that do not use coloured print on coloured backgrounds. Children’s books specially designed for people with CVD are available.
Did you know?
Colour blind people were used in World War II spy planes to spot camouflaged camps as they naturally looked for outlines more than for colours.
The following numbers are visible in the Ishihara Test by people with normal colour vision:
Left Column Right Column 25 8 29 45 6 56