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Introduction
The term blindness is used somewhat misleading in the designation of certain visual conditions in which visual acuity is not significantly reduces. Color Blindness, for example, does not involve a reduction of visual sharpness and should more accurately be called a color-perception deficiency. Color blindness occurs almost exclusively in males and the most common form is the inability to distinguish between certain shades of red and green. Night blindness is the condition of reduced dark adaption, resulting in the ability to see in low levels of illumination. It is commonly associated with vitamin A deficiency or with inherited diseases of the choroid or retina, such as retinitis pigmentosa, a condition involving progressive degeneration of the retina and abnormal deposits of pigment . (Health and Medical Information produced by doctors, 1)
Color Blindness
Color Blindness is a defect in the perception of one or more colors. This condition, which is scientifically known as achromatopsia, may be present at birth or acquired as a result of disease or injury.
Color Blindness is frequently inherited, usually as a sex-linked character. It is fairly common affecting about 4 to 8 percent of the male and 0.4 percent of the female, European population. Many color-blind individuals vary from normal only in the quality of reds and greens which they see; to them as to normal persons all colors may be matched by the appropriate mixtures of the three basic colors. They, therefore, have anomalous trichromatic vision. Other color-blind individuals can match all colors with the correct mixture of only two of the basic colors and so are said to have dichromatic vision. Color deficiency may occur in either reds and greens or yellows and blues. The latter is less common. The third group, extremely rare, comprises those with a monochromatic vision: inability to distinguish many colors. Many animals lack color vision, or are deficient in it, while some reptiles, birds, fish, and mammals have more or less good color vision (Brandt, pp. 75).
Theories of Color Blindness
Many theories have been offered to explain thr color vision, none of which has been accepted as wholly explaining the known facts relative to it and to cases in which color vision is deficient.
Helmholtz Theory: The classical concept of color vision of Hermann von Helmholtz, modified from that of Thomas Young, demonstrates that all colors can be compounded by the appropriate mixture of three basic or primary colors: red, green and blue. It is fundamentally a theory derived from physical concepts and postulates that colors are distinguished in the retina by three separate photosensitive substances located in the cones. This theory does not adequately explain certain types of color-sense deficiency, colored after images, or the sensation of black (Ganong, pp. 165).
Ladd-Franklin Theory: This theory suggests a light but not color-sensitive substance, rhodopsin, in the rods and a slightly modified substance in the cones, which is affected differently by yellow and blue light. This substance would be present primarily in the cones at the periphery of the retina, where these colors are perceived (Ganong, pp. 165).
Physiology of color vision
Any discussion of blindness can best be understood in the light of a generally accepted theory of color vision. According to H. Helmholtz, in 1853, three different photochemical substances are postulated to exist in the human retina, each of which response to stimulation by one of the three primary colors, red, green or blue-violet. Appropriate variations of mixture and intensity of these three colors can produce white or any one of the seven visible colors of the rainbow. Modern color photography makes use of the same principle, the appropriate fusion of the three basic colors yellow, magenta and cyan blue reproducing all other visible colors. Human congenital color blindness can be best explained according to the Helmholtz theory as an inborn deficiency of one or more of these color-sensitive retinal substances. There are many other theories of color vision including that of Ladd-Franklin. The Ladd-Franklin theory postulates a light-sensitive substance, existing in one form in the rods and in a modified form in the cones of the retina, rather than the several color-stimulated substances of the Helmholtz theory. To date, however, histological or chemical proof of these theories is still lacking (Bills, pp. 100).
The perception of color recognizes to take place in certain cells of the retina called cones; simple black-white fray discrimination is handled by other retinal cells called rods. Color vision is perfect at the centre of the visual field which is represented by the fovea of the retina, where the cones are most numerous. Proceeding away from the foveal area, as tested by a perimeter using different colors, the visual field loses in succession green, yellow, red and blue color sensitivity. The far edges of the field of vision, normally, are completely color blind; that is, they register form alone in black-gray-white. In this part of the retina, only rods are present. There are no cones. At low light intensity, the rods alone are found to function, so that the eye registers form without color and are, therefore, functionally color blind. Conversely, color vision is most efficient in bright light, after the eye has become light-adapted. In general, those animals whose retinas shows only rods and the absence of cones have been demonstrated by appropriate tests to be color blind. These include domesticated and laboratory test animals such as dogs, cats, rats and guinea pigs.
Seemingly, only animals that have good vision and are active during the day have color vision. Among mammals, such animals include, in addition to man, the apes and most monkeys. Color visions are also found in day-active birds and reptiles and in fishes, crustaceans, insects and some mollusks that are active in relatively well-lighted places (Gray, pp. 150).
Human Congenital Color Blindness
This condition may be explained based on the Helmholtz theory because it may be separated into three types, classified by the absence of the power of discrimination of one, two or all three of the primary colors mentioned above.
Monochromatism: This term refers to a state of total color blindness which is extremely rare. An individual so afflicted has nonfunctioning retinal cones. All of his vision is in black, gray and white, with a little tinge of blue, as the latter color is partially discernible by the retinal rods.
Dichromatism: Three types are found: Protanopia (Red-Blindness), Deuteranopia (Green-Blindness), Tritanopia (Blue-Blindness).
Hypothetically speaking, one of the three primary color-sensitive substances postulated by Helmholtz is missing in each of these. Red-blindness is common, green-blindness is less so, and blue blindness is rare. Protanopes usually see red as green. Also recognized as red-green color blindness. This type is known as Daltonism, named after the chemist John Dalton, who suffered from the condition and was described in 1794.
Anomalous Trichromatism: This error of color vision was discovered and described by Lord Rayleigh, in 1882. All three primary colors can be discerned; nu there is a relative lack of fine discrimination in the appreciation of the intensity of either red (partial protanopia) or green (partial deuteranopia). Partial tritanopia (blue perception) is rare. These three anomalies may be considered as transitions between normal and dichromatic vision. Partial deuteranopic (green shade discrimination) is the commonest type, accounting for about 4.6 percent of the population, or over half of all types of color blindness (Birren, pp. 75).
About eight percent of all males and 0.5 percent of all females show some form of color perception defect. Most forms of color blindness are inherited as sex-linked characters. Though transmitted through females, the trait is recessive in females. This explains the fact that the condition is more prevalent in males. Thus, genetically, color blindness resembles hemophilia. No pathological change, gross or microscopic, can be found in the eye or any part of the nervous system in the congenital types of color blindness. However, acquired color blindness frequently accompanies degenerative disease of the retina, the choroid, or the optic nerve, such as toxic amblyopia, retinal detachment, and optic nerve atrophy. Injury very rarely causes color blindness (Zoltán Kövecses, pp. 32).
Testing of Color Vision
Accurate color vision is assuming greater importance in many industries, with the increasing use of color in the coding of fuels, electronic wiring, and various types of assembly. Discrimination of red from green signals is mandatory in the safe operation of cars, trains, planes, and the piloting of ships. The Holmgren test for color blindness consists of matching skeins of yarn of various colors and shades. Lacking standardization, it has given way to the use of pseudo-isochromatic plates, published by Ishihara and also by Stilling. A series of twelve test plates consist of dots of various sizes and colors. A design, letter, or number is outlined by dots in those colors with which the test colors are likely to be confused. This test is almost too stringent, since people with only partial color blindness, of a non-disqualifying degree, may fail to pass. For the various types of partial color blindness, Rayleighs test is better. In this test, the subject matches a pure yellow beam of light by fusing red and green in a second beam, until the two yellows match. An individual with partial protanopia uses redder than the normal in this test; with partial deuteranopia, greener issued than the normal individual requires. The spectroscope may also be used for testing color sense (Jeffries, pp. 182).
Prevalence and Incidence
In the United States, approximately 10 million people have irreversibly impaired vision. Of these, about 1.5 million cannot read ordinary newspaper type even with the aid of optical correction . The prevalence or number of existing cases, of legal color blindness in the United States is estimated at 500,000. However, most of these individuals have some usable residual vision. The incidence or rate of occurrence, of legal color blindness in the United States is believed to be between 45,000 and 50,000 new cases per year. (Giere, pp. 39)
Although it is very difficult to ascertain the global prevalence of color blindness, the International Agency for the Color Blindness has estimated that there are about 23 million color blind people in the world today (Giere, pp. 39).
Conclusion
Unfortunately, no treatment or cure exists for color blindness, although so-called cures continue to be foisted on the public. It is possible, however, to train a red-green color blind individual to discriminate between the two colors, as seen in traffic lights, by associating depth of shade of the intermediate confusion color with the true color as seen by a normal eye.
Works Cited
Bills, Arthur Gilbert. General Experimental Psychology. Kessinger Publishing, 2006.
Birren, Faber. Color Psychology and Color Therapy: A Factual Study of the Influence of Color on Human Life. Kessinger Publishing, 2006.
Brandt, Herman F. The Psychology of Seeing. Kessinger Publishing, 2007.
Ganong, William F. Review of medical physiology. McGraw-Hill Professional, 2005.
Giere, Ronald N. Scientific perspectivism. University of Chicago Press, 2006.
Gray, Peter O. Psychology. Worth Publishers, 2006.
Health and Medical Information produced by doctors. Web.
Jeffries, Benjamin Joy. Color-Blindness: Its Dangers and Its Detection. BiblioLife, 2008.
Zoltán Kövecses, Bálint Koller. Language, mind, and culture: a practical introduction. Oxford University Press US, 2006.
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