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CHAPTER 2 CLASSIFICATION OF COLOR VISION DEFECTS _ This section describes how an individual's color vision is characterized on the basis of color-matching performance and chromatic discrimination capacity. The characterization of normal color vision, which occurs in about 90 percent of men and 99 percent of women, is described first. NORMAL COLOR VISION Colorimetric Definition There are many ways of producing a given hue sensation.* For example, a ~yellow. can be produced by a monochromatic radiation {S89 nm) or by the additive mixture of a yellow-green (545 nm) and a yellow-red (670 nary). White is produced by a continuous source containing radiations of all visible wavelengths, such as the sun, or it may be produced by a mixture of as few as two wavelengths, for example, 475 nm (blue) and 575 nm (yellow}. When we look at a yellow or white object, or at any color field at all, we have no way of knowing the spectral composition of the physical stimulus. The Color-Matching Experiment In a typical example of a color-matching experiment, the observer sees a circular field formed by two semicircular half-fields (Figure 2-1~. The half-fields contain different isolated spectral bands. In the particular example we have chosen, the upper half-field contains a narrow spectral band centered at 545 nm (yellow-green light) and a *A discussion of the facts of calorimetry is beyond the scope of this report. Information that is necessary for an understanding of the design of color vision tests is described in the appendix. Readers interested in a more complete discussion of calorimetry may find it in Bouma (1972), 80yn ton (1979), Graham {1965J, LeGrand (1965), Pokorny et al. (1979), Wright (1946, 1972), and Wyazecki and Stiles (1967~. 4

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s 545 nm`,.~ / / M IXTURE / COLOR TEST COLOR J 670 nm ~589 nm FIGURE 2-1 View of the field as seen by the observer, who sees a circle with a black dividing line. The top half of the field appears red, orange, yellow, yellow-green, or green depending on the relative amount of 670 nm and 545 nm light. The bottom half appears yellow. narrow spectral band centered at 670 nm (yellow-red light). The lower half-field contains a narrow spectral band centered at S89 nm (reddish yellow light). By appropr late adjustment of the quantities of 54 5 nm and 6 70 nm 1 ights, an observer can make the whole field appear to be the same color. The halves of the split field contain dissimilar spectral radiations and yet are seen as the same by the observer. Pairs of such stimuli are known as metamers. Normal observers can match all hues by the appropriate mixture of three colored lights. Hence, normal observers are known as trichromats (tristhree, chroma=color). The match usually requires that one of the three mixture primaries, as they are called, be subtracted from rather than added to the mixture f ield. 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. Some Special Matches In order to define normal trichromacy and diagnose color defect, we have available some special color matches that make use of only two primaries. These matches are relatively quick and easy to perform,

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6 compared with full spectrum color matching. Instruments that allow us to evaluate these special matches in the population are called anomaloscopes. Historically there have been three such special matches used to test color defect: the Rayleigh match or equation, the Pickford-Lakowski match or equation, and the Engelking-Trendelenburg match or equation. Of these matches, the Rayleigh match and Pickford-Lakowski match are the most frequently used today. Rayleigh Equation. The Rayleigh equation differentiates normal trichromats from observers with congenital red-green color defect and allows classif ication of these defects. The Rayleigh equation is a special type of color match that involves matching a spectral light near 589 nm to a mixture of spectral or nearly spectral lights near 670 nm and 545 nm, as shown in Figure 2-1. {The exact wavelengths have differed in various instruments. Wavelenoths seven here are for the Nagel Model 1 anomaloscope in current production). When the 670 nm and 545 nm mixture primaries are added together, normal trichromats see a full range of hues from yellow-green, yellow, orange, to yellow-red, depending on the proportion of 670 nm to 545 nm primary in the mixture. The observer's task is to adjust this Proportion so that the mixture field exactly matches the 589 nm f ield ~ ~ ~ The observer may also adjust the luminance of the 589 nm field to achieve an exact match. A normal trichromat can make the match quickly and reliably; there is a unique proportion of 670 nm and 545 nm, which is matched to 589 nm. Two statistics are taken on the primary mixture: the range and the midpoint of the matches. The matching range includes all of the ratios Of 670 nm and 545 nm that a given observer can match to the 589 nm. Usually little change can be made in the mixture ratio without upsetting the color match, and the matching range is termed narrow. The midpoint of the matching range is the 670 to 545 nm ratio, which lies in the center of the range. 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. We can, however, statistically define two subtypes of normal trichromats whose color vision may make them unsuitable for jobs in color-sensitive industries (Figure 2-2~. The deviant color normal observer is the normal trichromat whose Rayleigh equation lies within normal range but with the midpoint displaced more than + 2 standard deviations from the mean of average observers. Deviant color normal observers comprise 4 percent of the normal population. The weak color normal observer is one whose Rayleigh equation midpoint is within normal range but whose matching range is more than twice the most frequent range (modal value) for the population. Color-weak observers comprise 20 percent of the normal population. The Pickford-Lakowski Equation. A special match that is available in some anomaloscopes is source) to a mixture of 1 match of a white light (from a tungsten nm and 585 nm lights. The match was

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RED-GREEN COLOR EQUATI ON | CLASSI FICATI ON . I' I I ' NORMAL TRICHROMATS ~ 1 1 1 I - ~[ ~ NORMAL 1 1 1 1 I = I I _ I RED-GREEN DEVIANT 1 1 ~ - 1 I ~ I COLOR-WEAK . , 1 1 1 1 I I ~ ~ ~ ~ SIMPLE ANOMALOUS TRICHROMATS 1 1 1 1 1 1 1 1 I I I I PA (PROTANOMALOUS) 1 1 1 1 I I I I _ DA (DEUTERANOMALOUS) 1 1 1 - I lil ~ I ~ I I I I EXTREME ANOMALOUS TRICHROMATS 1 1 1 1 ~ ,, I EPA (EXTREME PROTANOMALOUS) I 1 1 1 1 EDA (EXTREME DEUTERANOMALOUS) I III I ~ I I I I DICHROMATS . . ! P (PROTANOPE) . ~ ~ ~ ~ D (DEUTERANOPE) . . ~ ~ ~ _ -3 SD , +3 SD , STATISTICAL PARAMETERS . . 0.35 1.0 3.0 ANOMALOUS QUOTIENTS . FIGURE 2-2 Characteristic red-green matches made by normal trichromats and observers with congenital red-green defects. Adapted from Lakowski (19691. designed to evaluate the effect of aging (Pickford, 1968) and has also proved important in evaluating color defects acquired in eye diseases (Lakowski, 1972~. Engelking Trendelenburg Equation. This equation involves the match of 490 nm to a mixture of 470 nm and S17 nm. This match was designed by Engelking and modified by Trendelenburg in order to evaluate con- genital blue-yellow color defects (Engelking, 1925; Trendelenburg, 1941~. Other investigators have used different wavelengths. As for the Rayleigh match, the normal trichromat makes a unique match. The match may be strongly affected, however, by the inert pigments of the eye. These inert pigments are the lens and a pigment that occurs in the back of the eye called the macular pigment. The lens and macular pigment absorb short-wavelength light (400-500 nary). Different indi- 1 - r

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8 viduals show great variability in the amount of light absorbed by these pigments. One result of this variability is that the Engelking- Trendelenburg equation shows a wide distribution of match midpoints in the normal population, thereby decreasing the utility of the match as a test for the abnormal color vision. Chromatic Discriminative Ability There is considerable variability among color-normal observers in their ability to discriminate small differences in hue or saturation. This fact may be demonstrated in a number of ways. In color match perform- ances, different observers have different matching ranges. Observers with good color discrimination tolerate little change in the mixture ratio and have narrow matching ranges. Observers with poor color discrimination have wide matching ranges. A clinical method of estimating chromatic discrimination is the Farnsworth-Munsell 100-hue test. CONGENITAL SEX-LINKED COLOR VISION DEFECTS Observers with congenital red-green color defects comprise about 10 percent of the U.S. male population, thus 4 percent to S percent of the U.S. general population (Paulson, 1973~. These observers are of concern to the armed forces and transportation industries, because their defects may inhibit their ability to function appropriately. Red-green defects have X-chromosome-linked recessive inheritance. This term refers to traits carried on the sex chromosomes. The female has two X chromosomes, one inherited from the mother and one from the father; the male has one X chromosome from the mother and one Y chromosome from the father. X-chromosome-linked recessive inheritance is specific to genes occurring at a locus on the X chromosome for which no corresponding gene occurs on the Y chromosome. Thus, in X-chromosome-linked recessive inheritance, the male who inherits a defective gene from his mother will always show the defect because he has no normal gene to oppose expression of the defect. The female, however, must inherit a defective gene from both parents to show the defect. Females who have one defective gene are called carriers. The affected male who gives an X chromosome to daughters and a Y chromosome to sons will pass the defective gene to all of his daughters and to none of his sons. Assuming that the mother has two normal X chromosomes, the daughters will be carriers. The offspring of marriages between color-defective males and color-defective females will have higher incidences of color-defective males, color defective females, and carrier females. Comprehensive reviews of the genetic aspects of red-green defects are given by Bell (1926), Francois and Verriest (1961), Waardenburg (1963), Kalmus (1965), Jaeger (1972), and Franceschetti et al. (1974~. Color matching using the Rayleigh match, allows us to differentiate the various red-green color defects. There are two major subdivisions

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9 of color defect observers according to the severity of the defect; anomalous trichromats and dichromats. Within each of these classes we find two qualitatively different types of red-green defect, the protan defects and the deutan defects. The classif ication given below is that of Franceschetti (1928~. Anomalous Trichromats These observers comprise about 7 percent of the U.S. male population. Color-Matching Classification Anomalous trichromats, like normal trichromats, need three primaries for color mixture to match the spectrum, but their matches differ from those of normal trichromats. According to Franceschetti (1928) there are four subcategories of anomalous trichromats. Each subcategory is defined by use of the anomaloscope. Simple Protanomalous Tr ichromats . The simple protanomalous tr ichromat is given the designation PA. This term refers to a presumed genetic entity. The observers who comprise 1 to 2 percent of the U.S. male population need a higher ratio of red to green primary than normal tr ichromats in the Rayleigh equation (see Figure 2-2~. The mixture half-field that the protanomalous trichromat accepts as a color match to the yellow test field would appear orange to the normal trichromat. In addition, long-wavelength (red) spectral lights appear dim; we say that protanomalous trichromats have a Long-wavelength luminosity loss. Simple Deuteranomalous Trichromats (Genetic Entity DA). These observers comprise about 4 to 5 percent or the U.s. male population; they need a higher ratio of green to red pr imary than normal tr ichro- mats 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. In addition to the deviation of the match midpoint, the ranges of the matches made by simple protanomalous and simple deuteranomalous trichromats are often wider than for normal trichromats (see Figure 2-27. Although a given anomalous trichromat may have as narrow a matching range as that of a normal trichromat (Hurvich, 1972; Alpern and Moeller, 1977), it is more usual to find that the matching ranges are two to three times wider than those of normal trichromats (Willis and Farnsworth, 1952; Helve, 19723. F:~rtr~m Prc~tan~malour: lair trichromats (Genetic Entitv EPA) . This group accepts a wide range or rea-green ratios, usually Inc~ua~ng cuo ratio accepted by normal trichromats and perhaps one of the primaries (usually redJ. The extreme protanomalous trichromat also shows reduced sensitivity to long-wavelength spectral light. t

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10 Extreme Deuteranomalous Trichromats (Genetic Entity EDA). These observers also accept a wide range of red-green ratios, including the normal match and perhaps one of the primaries (usually green). Extreme protanomalous trichromats and extreme deuteranomalous trichromats differ from the corresponding simple anomalous trichromats in the width of the matching range (see Figure 2-2 ~ . 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. There is, however, considerable variability among both protanomalous and deuteranomalous tr ichromats in the manifestation of such discrimination loss, with some of these observers showing no loss of color discrimina- tion. Some anomalous trichromats may make more errors on the Farnsworth-Munsell (FM) 100-hue test than normal trichromats. Their errors do not occur randomly but occur for hues where their color discrimination is poorest. The Farnsworth-Munsell 100-hue test is discussed in detail in Chapter 3, Existing Tests. Dichromats Dichromats comprise 2 to 3 percent of the U.S. male population. Color-Matching Classification Dichromats require only two primaries to match spectral colors. They can match all spectral colors by a suitable mixture of two primaries located on either side of 500 nm; generally a red and a blue are used. Furthermore, when spectral colors and small fields are used, Dichromats can match all wavelengths above 540 nm to a single wavelength. In the Rayleigh match they can match either the 545 nm primary, the 670 nm primary, or any mixture of these primaries to spectral yellow (see Figure 2-2~. On the basis of the radiance at which they set the brightness of the yellow half-field in the Rayleigh match, Dichromats can be differentiated into Protanopes and deuteranopes. Protanopes {Genetic Entity P). Protanopes comprise 1 percent of the U.S. male population and show loss of sensitivity to long wavelengths. On the Nagel Model 1 anomaloscope, Protanopes match spectral red to very dim levels of spectral yellow; they match spectral green to brighter levels of spectral yellow. Protanopes confuse blues with purples, blue-greens with red-purples, and light greens with brown. e Deuteranopes (Genetic Entity D). Deuteranopes, who comprise 1 percent of the U.S. male population, possess a spectral sensitivity

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11 similar to that of normal observers. On the Nagel Model 1 anomaloscope. they match spectral red and spectral green to approximately the same radiance of yellow. A deuteranope will confuse blues with blue-purples, blue-greens with purples and greens with reddish purples. Chromatic Discriminative Ability Dichromats have virtually no wavelength discrimination above 540 nm. They are thought to see the spectrum as shades of blues and yellows separated by a neutral region of grays in the spectral region around 495 to 500 nm. Dichromats usually make more errors on a color discrimination test such as the FM 100-hue test than a normal or anomalous trichromat. Their errors occur for those hues where their discrimination is poorest and show profiles similar to those of the corresponding anomalous trichromats (see Chapter 3~. AUTOSOMAL DOMINANT TRITAN DEFECT In addition to the X-chromosomal-linked color defects, there are some very rare hereditary color defects. The tritan defect is one of these rare defects (minimum frequency estimated to be between 1/13,000 and 1/6S, 000 [Kalmus, 1965;~. The defect shows autosomal dominant inheritance, a term which refers to traits carried on any but the sex chromosomes. In dominant inheritance the defect occurs if only one defective gene is inherited. Autosomal dominant inheritance is characterized by a high frequency of the defect (50%) in the male and female children of an affected parent. The severity of the defect can be quite variable from one family member to another. The tritan defect is characterized by a lack of function of the mechanism that allows normal observers to discriminate colors that differ by the amount of violet or yellow they contain. The dichromatic state of the tritan defect is termed tritanopia. A tritanope can match all spectral colors to a mixture of two primaries, usually located on either side of 565 nm, and will have a wide matching range on the Engelking-Trendelenburg and Pickford-Lakowsk~ equations. 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 Engelking- Trendelenburg equation. There are, however, many cases of ~incomplete tritanopia, n consistent with the typical variability observed in autosomal dominant inheritance. Specialized tests of color matching are required to differentiate the tritanope, incomplete tritan, and tritanomalous observer.

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12 ACQUI RED COLOR VI S ION DEFECT S Normal Color Vision Changes with Age Test performance that depends on detection of small differences in color is at its best in young adults in their early twenties. Fewer observers above the age of 25 show excellent color discrimination. Color discrimination loss shows a characteristic pattern: discrimina- tion on the blue-yellow axis is more affected than discrimination on the red-green axis. Thus, normal trichromats will show considerable widening of their matching ranges for the Engelking-Trendelenburg and Pickford-Lakowski matches as their age increases (Lakowski, 1958; Ohta and Kato, 1976) whereas the matching range for the Rayleigh equation shows little or no change with age. On the FM 100-hue test, more errors are made by older observers, who may show blue-yellow discrimination loss (Ohta, 1961; Lakowski, 1962; Verriest, 1963; Frill and Schneiderman, 19 64 ~ . The loss of discriminative ability with age is primarily but not solely attributable to the aging process in the lens of the eye (Lakowski, 1962~. The lens gradually becomes denser and may accumulate screening pigments, which usually absorb short-wavelength light strongly (the lens may appear yellowish). As a consequence less light, especially short-wavelength light, reaches the retina. These age effects are not trivial. The discriminative loss with age may be important in certain job situations. Effect of Disease, Injury, and Drugs Most individuals with defects in color perception have hereditary color defects that do not progress. However, color defects may occur secondary to disease or injuries. Such defects are termed acquired color defects. Acquired defects may be caused by disease or injury affecting the eye, the optic nerve, or the visual cortex. Some acquired defects result from primary hereditary retinal disorders and probably should be termed developmental color vision defects. Cataracts, too, may cause color defects. Drugs may cause toxic effects on the eye, with some loss of vision and color vision. The acquired defects usually involve discrimination loss and may occur prior or secondary to loss in visual acuity. 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. However, the evaluation of color vision in eye disease can be diagnostically important and is common clinical practice in Europe. (Discussion of the measurement and etiology of acquired color vision defects is given in Grdtzner, 1972, and Pokorny et al., 1979~. It is helpful to summarize differences between acquired and congenital color defects (see Table 4-2~. Congenital color defects usually involve both eyes. There is usually no visual complaint or

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13 evidence of other abnormal visual function. The congenital defects are likely to involve red-green discrimination and to show X-chromosomal- linked inheritance. These observers may name many object colors correctly, since they were constrained in childhood to use the termin- ology 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. These defects are more likely to involve discrimination loss on the blue-yellow axis. PHYSICAL FACTORS AFFECTING COLOR VISION Color vision is not static. Color appearance, color matches, and color discrimination are affected by changes in illumination and field of view. Illumination All color tests are designed to be administered at specific illuminations. Color discrimination is best at medium or moderate levels of illumination. At very high levels (glare), the apparent saturation of colors is decreased. Most observers find glare sources uncomfortable. At very low levels of illumination, discrimination deteriorates in a characteristic manner. If the FM 100-hue test is given at 1/lOOth of the recommended test illumination, observers may show increased errors, primarily on a blue-yellow axis. Field Size The size of the field of view is also important in color vision. The larger the field of view, the better the color discrimination. Normal trichromats show small but systematic changes in the Rayleigh match as field size is changed (Pokorny and Smith, 1976~. Reduction in field size to one-quarter degree of visual angle (the size of a tip of a matchstick at arm's length) from the 1 to 2 (the size of a nickel at arm's length) of a typical anomaloscope will lead to decreased discrimination. Anomalous trichromats and dichromats show considerable improvement in discrimination as field size is enlarged. The wide matching ranges that occur with the usual anomaloscope field become narrower when a sufficiently large test field (8) is used. Many dichromats will not accept a full matching range on an anomaloscope when the field is 8 (Smith and Pokorny, 19777. Population statistics are all based on the use of anomaloscope fields of 1 to 2. Furthermore, most color screening tests depend for their success on discrimination loss in anomalous trichromats and dichromats. Such tests may be less effective if the observer is allowed to increase the field of view by bringing the samples closer. The ability to present a large field in an anomaloscope, however, may be useful in studying acquired color defects.