Procedures for Testing Color Vision Report of Working Group 41 (1981) / Chapter Skim
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2. Classification of Color Vision Defects
2. Classification of Color Vision Defects
Pages 4-13
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NORMAL COLOR VISION Colorimetric Definition There are many ways of producing a given hue sensation.* For example, a ~yellow.
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Because light cannot be physically subtracted, the third primary is added to the test field; the task thus requires matching the mixture of the test color and one mixture primary to the mixture of the remaining two primaries. Different normal trichromats will use slightly different amounts of the primaries to match various hues, but it is the general similarities among normal observers rather than the comparatively small differences that allow us to classify an observer, whose color vision we are evaluating, as either normal or abnormal.
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In population studies of normal trichromats, the distribution of match midpoints describes a bell-shaped or normal curve including only a rather narrow group of settings. An observer who has a unique match that falls within this range can be excluded f rom the most f requent categor ies of color defective vision -- the congenital red-green color defects.
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and has also proved important in evaluating color defects acquired in eye diseases (Lakowski, 1972~. Engelking Trendelenburg Equation.
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In color match performances, different observers have different matching ranges. Observers with good color discrimination tolerate little change in the mixture ratio and have narrow matching ranges.
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male population; they need a higher ratio of green to red pr imary than normal tr ichromats in the Rayleigh equation (Figure 2-2 ~ . The mixture half-f ield that the deuteranomalous trichromat accepts as a color match to the yellow test field would appear greenish yellow to a normal trichromat.
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Extreme anomalous tr ichromats always have a wide matching range that usually includes the normal match. Chromatic Discriminative Ability As indicated by the matching width, many anomalous trichromats show a loss of color discrimination compared with normal trichromats.
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The tritanope, however, will have a reliable match on the Rayleigh equation, and the match will fall within the distribution of matches made by normal trichromats unless, of course, there is a concomitant red-green defect (Pokorny, Smith and Went, 1981~. It has proved difficult to demonstrate a defect comparable to anomalous trichromatism in observers with tritan defect, primarily because of the normal interobserver variability in the EngelkingTrendelenburg equation.
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The common clinical methods of testing color vision are based on tests designed to evaluate the hereditary defects, using observers with normal visual acuity. The assessment of acquired color defects may be complicated by low visual acuity, presence of an undiagnosed congenital color defect, or other concomitant problems.
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These observers may name many object colors correctly, since they were constrained in childhood to use the terminology of the color normal observer, and they tend to use whatever cues possible to do so (Jameson and Hurvich, 1978~. In comparison, acquired or developmental color defects may differ in severity in the two eyes and are usually accompanied by decreased vision and other evidence of eye disease.
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