Review of the Color Vision Tests Currently in Use

TABLECREATED

Table 1: Scoring criteria for the color vision tests.

Various studies on the efficiency of the Ishihara test have shown that the tester mains the “gold standard” for rapid identification of congenital red-green deficiencies. According to a study by Birch, the mean number of errors for all transformation and vanishing plates is around a maximum of 16 [6]. She found that the sensitivity and specificity for failing the test with a score of 4 or more errors are 98.7% and 94.1%, respectively [6]. She also found that it was challenging to use hidden digit plates. The reason for that is some CVN participants can read them, and the ability to read them depends on the subject’s age. For example, 40% of the CVN participants aged between 20 to 30 yr can see the hidden Figure (6). The sensitivity of these plates was less than 50%. With respect to the classification plates, she found that 81.9% of protanopes and 93.4% of deuteranopes were correctly classified, whereas 18.1% of protanopes and 3.2% of the deuteranopes could not see either figure. In terms of anomalous trichromats, 46.6% of the protanomalous and 57.2% of the deuteranomalous trichromats were correctly classified while 6.7% of the protanomalous and 2.1% of the deuteranomalous trichromats could not see the classification numerals [6]. However, 40% of the protanomalous and 37.5% deuteranomalous trichromats were correctly classified in terms of luminance contrast (they could see both figures, but one was more distinct than the other). Overall, in terms of classification for missing one of the two figures or identifying one figure as more distinct than the other, 83.2% of protans and 94.1% of deutans were classified correctly [6]. Thus, the classification plates in the Ishihara test are precise in their ability to classify red-green defects. Hovis and Almustanyir [7] calculated the agreement between the Ishihara test and the anomaloscope using the AC1 coefficient of agreement value [8,9]. The reported an AC1 value of 0.86.They also found that the sensitivity and specificity were 0.95 and 0.91, respectively [7].

Hardy-Rand-Ritter plates

The HRR pseudo iso chromatic test was developed by Hardy, Rand, and Ritter in 1955 [10]. It is designed to detect tritan as well as red-green CVDs and grade their severity. The fourth edition was well designed as its colors aligned on the confusion lines and was isomeric with the original version of the test [4]. The fourth edition contains 24 plates that present with either one- or two-colored symbols (X, Δ, and O) within a gray background. These 24 plates include 14 plates designed to identify the type of deficiency (10 plates for deutans and protans and four plates for tritans) and grade the severity as mild, medium, or strong. For red-green defects, a defect is classified as severe when the observer makes one or more errors in the two plates with the most saturated colors, medium when the observer makes errors in the next three most saturated colors, and mild when the patient makes error with the least five saturated colors. Table 1 shows the criteria for this test.

Hovis and Almustanyir [7] compared the HRR and anomaloscope results for 65 CVD and 60 CVN participants. The found that the AC1 coefficient of agreement, sensitivity, and specificity for the redgreen plates were 0.88, 0.95 and 0.91 respectively [7]. Another study conducted by Cole et al. reported that the average number of errors madefor red-green defects on the HRR screening plates was 4.97(out of six symbols), but the subjects did not make any errors on the tritan plates [11]. They found that the sensitivity and specificity were 1 and 0.96, respectively, when using two or more errors on the screening test figures as the failure criterion relative to the anomaloscope findings [11]. Eighty-six percent of their subjects were classified correctly as protan or deutan. This was similar to the percentages reported by Birch for the Ishihara. They also found that 31% of the CVDs were graded as showing a mild defect, 43% as showing a medium defect, and 26% as showing a severe defect [11]. The HRR results need to be compared with a standard measure of the severity to determine the validity of the severity grading. The Farnsworth D15 (F-D15) and anomaloscope tests are acceptable measures of severity. The F-D15 test can grade color defective subjects into two groups “pass” and “fail.”Cole et al. found that, with one exception, subjects who were classified as mild by HRR passed the F-D15 test. Among those who were classified as medium by HRR, 40% failed the F-D15 test, and 85% of those classified as strong by the HRR failed the F-D15 [11].

In comparisons with the anomaloscope, Cole et al.found that the average anomaloscope range increased with HRR severity grading [11]. Subjects who were graded as a mild on the HRR test had an average range of 9.2 units on the anomaloscope (all of them had a range less than 30). This shows that they have good color discrimination, which agreed with the mild grade in HRR classification. Although the average range on the anomaloscope increased in the medium and strong grades, some of the subjects showing a small range in the anomaloscope were classified as medium or strong on the HRR. Twenty-four percent of the subjects classified as medium and strong had an anomaloscope range below 20 units, which is 90^th percentile of the mild group. In addition, 37% of the dichromatic participants were classified as medium [11]. This shows that HRR is a suitable test to separate mild from strong, but it cannot separate medium from strong [11].

Waggoner PIP test

The Waggoner PIP test is a new computerized color vision test that screens for red-green and blue-yellow color vision deficiencies. The old version of the test was introduced by Konan Medical. This version of the test is no longer available, and a similar version is available from Waggoner Diagnostics (Rogers, AR) as part of the Waggoner Computerized Color Vision Test (WCCVT). The Waggoner PIP test can classify the type of color vision defect and determine the severity of the defect. The plates used in this test are similar in design to the Ishihara plates. The test has 25 plates that are used to screen for redgreen defects. These are followed by 12 screening and classification plates for the tritan defect. When a total of 5 errors are made, the red-green screening test ends, and the program switches to the blue-yellow plates before starting the red-green diagnostic plates. The tritan plates are to screen for a blue-yellow defect and can also classify the severity of the defect on the basis of the presentation of the plates, which vary systematically in their saturation. Individuals who cannot tell the more saturated colors are classified as showing a severe defect. The red-green diagnostic plates are administered next. This option will be taken automatically if the individual failed the red-green screening plates. Half of the diagnostic plates are for the protan defect, and the other half are for the deutan defect. Similarly, the saturation of the diagnostic tritan plates changes systematically in order to classify the severity of the defect. Each plate is presented for 2 s within a white background. After the number disappears, the patient enters the number that they saw or the “N” option if they did not detect a figure using the keyboard or the touch screen. Table 1 shows the criterion for this test. The failure on the red-green screening plates is 5 errors and more than 2 errors on the blue-yellow screening plates. The program determines the type and severity of the red-green defect based on the maximum number of errors made on the deutan and protan diagnostic plates. The severity of the blue-yellow defect is also based on the total number of errors.

A study conducted by Almustanyir and Hovis [12] validated a prototype of this new test and found that the Kappa coefficient of agreement (k), sensitivity, and specificity of the red-green plates relative to the anomaloscope were 0.95, 0.96, and 0.99, respectively [12]. They also determined the repeatability of the test, and they found that the k value between the first and second visit was high, with a value of 0.98 [12]. Another study by Hovis and Almustanyir (2017) assessed this test using the program on a Microsoft Surface Pro device [7]. They found that the AC1 coefficient of agreement values of the test with the anomaloscope was 0.9 for the red-green plates and 0.8 for the blue-yellow plates. The sensitivity and specificity relative to the anomaloscope were greater than 0.9. They also reported that the repeatability of the test with an AC1 coefficient of agreement values of the first and second visit of 0.98 for the red-green plates and 0.94 for the blue-yellow plates [7].

Rabin Cone Contrast Sensitivity Test

The RCCT is a new computerized color vision test to determine the threshold for letter recognition (cone contrast sensitivity). The principle of the RCCT involves measurement of the discrimination thresholds as a Tri vector from a grey reference using colors that should be missed by each of the three types of color vision deficiencies. Thus, each color is selected so that only one cone type is modulated as the saturation of the letter changes. The RCCT measures the discrimination threshold for the S-cone (detect a tritan defect), M-cone (detect a deutan defect), and L-cone (detect a protan defect) [13].

The test presents a single colored letter in the center of the computer’s screen for approximately 5 s. The patient needs to respond by identifying the letter displayed on the screen. The order of testing is L-cone, M-cone, and S-cone contrast. Table 1 shows the criteria for this test. A sensitivity that is less than 75 for any cone type is considered to indicate failure in the test. The type of defect is based on the minimum cone sensitivity.

Two studies by Rabin in 2004 and 2011 validated this test [13,14]. The two studies reported that the RCCT had a perfect agreement with the anomaloscope with respect to whether the person had a CVN or a red-green CVD. That is, the specificity and sensitivity were both 1.0. Walsh et al. (2016) reported lower values with sensitivities of 0.97 for both right and left eye and specificities of 0.97 for the right eye and 0.96 for the left eye [15]. In a more recent study, Hovis and Almustanyir also determined the sensitivity and specificity of the test relative to the anomaloscope. They found that the sensitivity and specificity values were 0.95. They also calculated the AC1 coefficient of agreement, and reported an AC1 value of 0.9 [7].

Color assessment and diagnosis test

The CAD test is a color vision test that can screen for color vision deficiencies, measure chromatic discrimination around a gray reference, or both [16,17]. The CAD test assesses chromatic discrimination around a gray reference [17]. The CAD test consists of a gray-colored background and color stimulus that are buried in a background of dynamic luminance contrast noise. The background was made up of small individual squares, and the stimulus changes their luminance every 50 ms so that the screen looks as if it is scintillating. The colored stimulus moves in one of the four diagonal directions as the individual squares oscillate in luminances. An individuals’ task is to respond by pressing a button to indicate in which direction the colored stimulus is moving. This test has two options. The first option is to screen for both red-green and blue-yellow defects. The second option is to classify and quantify the severity of the defect. The observer’s task is to detect the direction of the square. There are 4 possible diagonal directions. A four-alternative force-choice procedure is used to determine the observer’s chromatic detection threshold in a specific direction within the CIE chromaticity chart [18]. The observer is instructed to maintain fixation on the center of the square and not to track the moving square in order to obtain the best results. If the observer is unsure of the direction, then s/he is encouraged to make the best guess since a response is necessary to continue the test. Different pass/fail criteria are allowed by this test, and this depends on the specific application of the results. Table 1 shows the recommended criteria for this test.

Barbur et al. (2006) evaluated the color vision of 250 individuals with a congenital red-green defect using CAD system [17]. The blueyellow thresholds overlapped substantially with the CVN results, and these thresholds were not statistically significant. Nevertheless, there was a clear difference in the red-green threshold results. If ascore of >1.8 SNU is used to identify a patient with a red-green defect, the analysis in their Figure 3 suggests that both the specificity and sensitivity of the test were equal to a value of 1.0. Moreover, Seshadri et al. (2005) also reported specificity and sensitivity values over 90% for the WEB-based version [19]. Walsh et al. (2016) determined the specificity and sensitivity for the CAD threshold test, and they found that both values were near 0.85 [15]. Hovis and Almustanyir also reported that the AC1 coefficient of agreement, sensitivity, and specificity relative to the anomaloscope were 0.65, 0.92, and 0.60, respectively [7].

Farnsworth munsell D-15

The F-D15 color vision test was introduced to distinguish CVN and those with a mild CVD from individuals with a moderate-tosevere CVD [20]. The F-D15 is the current test that determines whether a soldier with a CVD has adequate color discrimination to perform his/her duties safely [7]. Subjects who failthe F-D15 are more likely to encounter problems in making color judgments in their everyday life or at work. Canadian military and civilian aviation authorities use the F-D15 test as a secondary test [7]. They test candidates with a screener test, and if the result is a failure, then the candidate must pass either the F-D15 or the Holmes–Wright Lantern Type A to qualify for an unrestricted pilot’s license [7]. If the candidate passes the F-D15, they are considered as having met the color vision requirement to enter pilot training.

The patient’s task is to arrange colored caps according to similarity by placing the color cap that is most similar to the previous cap set in the box. Major mistakes on the test are referred to as major crossings. The colors are approximately equally spaced around the hue circle, and the difference between adjacent colors is considered as an easily noticeable difference [20]. Errors on the F-D15 tests can be a transposition or a major crossing. The transposition occurs when a cap is placed only one to two positions from the correct arrangement, whereas major crossings occur when the caps from opposite positions of the hue circle are placed adjacent to one another. The test is commonly scored by visual inspection of the score sheet. Table 1 shows the recommended criterion for this test. Traditionally, two or more major crossings represents a failure.

The F-D15 can also be scored using Color Differences Vectors analyses [21]. The three parameters are specificity index (S-index), confusion index (C-index), and angular size. The S-index provides a measurement of how regularly the crossing is oriented. A high S-index value indicates a random arrangement. The C-index indicates the severity of the CVD. This index is correlated with the number of crossings and the total error score. The angular size provides a measurement of the type of CVD. The protan angles are larger than zero, and deutan angles are smaller than zero [21].

Fifty-five percent to 53% of participants with CVDs can pass the F-D15 test if one major crossing is allowed [22-24]. However, there is a possibility of a small number of dichromats (i.e., 3%) passing the F-D15 using this criterion. In the dichromat group, the percentage of those passing the test reduced to 1.5% when considering any crossing as a failure [24]. Hovis et al. reported the repeatability of the F-D15 using 116 red-green color defective subjects [23]. They found that if the failure criterion was two or more major crossings, the repeatability of the F-D15 was good with a kappa (κ) coefficient of 0.84 [23]. This value was less than the κ value of 0.96 calculated from Farnsworth’s data. The reason for the difference in the κ values was that Farnsworth’s population included a large number of CVNs, which would improve the repeatability of the test because CVNs rarely fail the test [23].

Waggoner CCVT D-15

The W-D15 program is a computerized version of the F-D15. The test is a part of the Waggoner CCVT test suite. It uses the same principle as the F-D15. The old version of the program requires the patient to drag the colored circle to the top of the display in order to use that color to “fill” one of the empty rectangles. The color selected should be the one that is most similar to the previous rectangle filled. The colors in the rectangles may be rearranged. The new version of the program presented colored circles on the left side of the screen. The patient is required to select a colored circle that is most similar to the previously filled circle and drag it to the first empty circle to complete a circle figure. The patient also has the option to rearrange the order. Table 1 shows the recommended criteria for this test using either the number of crossings or King-Smith indices.

Almustanyir and Hovis (2019) measured the chromaticity coordinates in a normal a* b* space for the F-D15 and W-D15. Figure 1 shows the results. The color difference between each cap of the two tests is listed in parentheses. The small color difference (ΔEs) between the two tests suggests that the tests should have the same level of difficulty [25].

    

    

    Figure 1: Chromaticity plot of a* and b* in normal space for the W-D15 (open
circles) and F-D15 (closed circles) color vision test colors [25].
    

TABLECREATED

Table 2: Summary of all color vision test screening results reviewed in the study. The AC1 value, sensitivity, and specificity were calculated relative to the anomaloscope.

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Citation: Almustanyir A. Review of the Color Vision Tests Currently in Use. J Ophthalmol & Vis Sci. 2020; 5(1): 1036.

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