The retina contains photoreceptor cells called rods and cones. Each cone contains visual pigments that are sensitive to one of the three primary-colour light wavelengths. One group of cones recognizes green, one red, and one blue. By blending these three colours, the eye is able to distinguish all the colors of the spectrum. This is known as normal trichromatic colour vision. Color blindness occurs when one group of cones does not recognize its color properly. This may be caused by a lack of visual pigment, called an anopia, or by faulty visual pigment, known as an anomaly. Individuals lacking visual pigment are termed dichromats and those with faulty pigment are termed anomalous trichromats.
Most people with colour blindness see some color and many see all three primary colours but in the wrong proportions. They may require brighter shades to recognize a particular hue. Some individuals may be red or green blind. A person with red blindness sees reds and oranges as shades of gray or black. A person with green blindness seeds reds, oranges, and greens as much the same shade and cannot distinguish between them. Blue blindness is extremely rare, as is complete colour blindness, a condition usually attributable to other visual problems.
Colour blindness may be detected during the routine ophthalmologic examination. A common screening device called the Ishihara Test uses circles of different sizes and hues to form a mosaic picture. The picture shows a two-digit number in a background field of circles. Those with normal colour vision are able to see the number formed with coloured circles. Those with colour blindness cannot detect a number from the background field. There is no cure or treatment for hereditary colour blindness, although some experimentation is being done with such tinted contact lenses as the X-Chrom lens, and filters. The individual may not be able to see colour with the lenses, but may be able to distinguish between colours better as a result of wearing them.
Acquired colour blindness can occur as a result of ageing or such disorders as retinal optical-nerve conditions, toxic amblyopia, macular disease, cataract, glaucoma, and diabetic retinopathy. The diseased eye is generally the only eye affected, and effective treatment of the illness may result in an improvement in color vision. Drugs can also temporarily alter an individual’s colour vision. Barbiturates given as sedatives and large doses of vitamin A (in a retinol form, not beta carotene) affect the yellow or yellow-green vision. Caffeine has also been shown to affect and alter all colors. The popular drug Viagra causes temporary changes in blue/green colours and increased sensitivity to light in some cases.
