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Variation in Colour Perception in Normal Individual

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The Ishihara test is a color perception test for red-green color deficiencies, the first in a class of successful color vision tests called pseudo-isochromatic plates (“PIP”). It was named after its designer, Dr. Shinobu Ishihara, a professor at the University of Tokyo, who first published his tests in 1917. The test consists of a number of colored plates, called Ishihara plates, each of which contains a circle of dots appearing randomized in color and size. Within the pattern are dots which form a number or shape clearly visible to those with normal color vision, and invisible, or difficult to see, to those with a red-green color vision defect. Other plates are intentionally designed to reveal numbers only to those with a red/green color vision deficiency, and be invisible to those with normal red/green color vision. The full test consists of 38 plates, but the existence of a severe deficiency is usually apparent after only a few plates. There is also an Ishihara test consisting 10, 14 or 24 test plates. Being a printed plate, the accuracy of the test depends on using the proper lighting to illuminate the page.

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A “daylight” bulb illuminator is required to give the most accurate results, of around 6000-7000K temperature (ideal: 6500K, Color Rendering Index (CRI) >90), and is required for military color vision screening policy. Fluorescent bulbs are many times used in school testing, but the color of fluorescent bulbs and their CRI can vary widely. Incandescent bulbs should not be used, as their low temperature (yellow-color) give highly inaccurate results, allowing some color vision deficient persons to pass.Proper testing technique is to give only three seconds per plate for an answer, and not allow coaching, touching or tracing of the numbers by the subject. The test is best given in random sequence, if possible, to reduce the effectiveness of prior memorization of the answers by subjects. Some pseudo-isochromatic plate books have the pages in binders, so the plates may be rearranged periodically to give a random order to the test.Since its creation, the Ishihara Color Blindness Test has become commonly used worldwide because of its easy use and high accuracy. In recent years, the Ishihara test has become available online in addition to its original paper version.

Though both media use the same plates, they require different methods for an accurate diagnosis. The United states Navy uses the Ishihara plates (and alternatives) for color vision screening. The current passing score is 12 correct of 14 red/green test plates (not including the demonstration plate). Research has shown that scores below twelve indicate color vision deficiency, and twelve or more correct indicate normal color vision, with 97% sensitivity and 100% specificity. The sensitivity of the Ishihara test varies by the number of plates allowed to pass, which can vary by institutional policy. Sensitivity also may be influenced by test administration (strength of lighting, time allowed to answer) and testing errors (coaching by administrators, smudges or marks made upon the plates). Visual impairment and blindness has remained as one of the most important health issues in the Eastern Mediterranean countries [3], as there are 40.5 million living with visual impairment and 5 million afflicted with blindness which in some cases could have been prevented with prophylactic and screening measures. World Health Organization has estimated the prevalence and causes of visual impairment in the region.

Congenital colour vision defects affect 8% and 0.5% of males and females, respectively on a global basis. The high prevalence of colour blindness necessitates early diagnosis, since these individuals with this disorder cannot accurately make colour discrimination which will impact their performance both personally and professionally. Therefore, colour vision assessment is essential in a complete visual examination.

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Many tests have been developed and distributed worldwide to diagnose colour vision defects. Colour defect tests are performed mostly for three goals, the first for screening for the presence of congenital or acquired defects, the second to diagnose the type and severity of the defects and the third to assess the impact of the defect on a specific profession or employment. Generally speaking, a precise, easy, and cost effective test is needed to diagnose visual colour defects accurately.

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