Color Vision Testing
Color vision testing is the assessment of chromatic discrimination ability and the diagnosis of any perceptual deficiency according to its severity and quality (see Paramei and Bimler, “ Protanopia”; Paramei and Bimler, “ Deuteranopia”; Bimler and Paramei, “ Tritanopia”; Rodríguez-Carmona, “ Environmental Influences on Color Vision”). Tests vary in sensitivity, specificity, ease of use, and time required for administration [1, 2, 3, 4]. Many were designed primarily for vocational screening for congenital deficiency, an issue in any occupation where color-coding conveys information (e.g., railways, aviation, electronics) [5, 6]. Testing is also important for assessing and monitoring acquired color abnormality, appearing as a manifestation of visual-system pathology resulting from ophthalmological diseases (e.g., glaucoma, ocular hypertension), systemic or neurological diseases (e.g., diabetes, Parkinson’s), or the effects of medications or exposure to environmental/occupational toxins (see Paramei, “ Color Perception and Environmentally Based Impairments”).
Color vision tests fall into several broad categories [1, 2, 3, 4]. Pseudoisochromatic plates and arrangement tests both directly address an observer’s confusions between color pairs along a given confusion axis. Both types of tests have the advantage of rapid administration and ease of interpretation, making them suitable for field/epidemiological applications. Generally they distinguish between the protan, deutan, and tritan forms of deficiency. Their results emphasize a dichotomous outcome: whether a subject’s color sensitivity is (vocationally) impaired. Versions of the tests exist for testing color vision in children. More recent computerized developments measure variations in color perception along a continuous range. Matching tests and naming (lantern) tests follow different principles. Below, the most widely used tests are described in more detail.
The Ishihara test, used most widely, is intended for diagnosis of congenital red-green deficiency (“daltonism”), differentiating its two types, protans and deutans, and severity (mild, moderate, or extreme) [6, 7] (see Paramei and Bimler, “ Protanopia”; Paramei and Bimler, “ Deuteranopia”). The Hardy-Rand-Rittler (HRR) test contains additional six plates designed to detect tritan defects and gauge their severity [7, 8]. There also exists a Farnsworth F2 plate designed specifically for revealing tritan abnormality (see Bimler and Paramei, “ Tritanopia”; Paramei, “ Color Perception and Environmentally Based Impairments”).
Color Arrangement Tests
Arrangement/panel tests use a set of color stimuli (“caps”) which sample a color circle (see Green-Armytage, “ Color Circle”) at regular intervals. The subject is requested to arrange them in sequence, so that each color lies between the two colors most similar to it. Transpositions of the caps, departing from a normal trichromat’s sequence, are recorded as errors. These departures can be plotted graphically to measure the angle of the confusion axis (if any) and summed to quantify the severity of any deficit.
Two shorter versions, the Farnsworth Dichotomous D-15 test and the Lanthony Desaturated D-15d, each contain only 15 movable caps plus a fixed “pilot cap” as the start of the sequence  and take about 5 min to complete. The D-15 is designed to diagnose moderate to severe color defects. The D-15d test uses color samples that are lighter and paler . It was designed specifically to capture mild or subclinical color defects in observers who pass the standard D-15 test. Errors can include diametrical circle-crossing transpositions, indicating the protan, deuten, or tritan confusion axis when plotted graphically (see Fig. 1 in Paramei, “ Color Perception and Environmentally Based Impairments”). Outcomes of both tests can be summarized as a color confusion index (CCI), where 1.0 corresponds to perfect color arrangement and CCI values greater than 1.0 indicate progressive impairment of color discrimination. These tests are often used in conjunction, though the more sensitive D-15d is widely employed for early detection of mild acquired dyschromatopsias.
The Lanthony New Color Test [3, 4] comprises four panels of 15 caps each at four levels of saturation, to examine color similarities at four different scales. Like the D-15d, it can be used to track the progression of acquired dyschromatopsias.
Although not strictly an arrangement test, the City University Test [2, 3, 4] is derived from the D-15. It is a forced-choice test consisting of 10 panels, each presenting four colored dots in a quincunx around a central dot; the subject has to indicate which of the four colors most closely resembles the central one. The colors are selected so that protan, deutan, or tritan deficiencies affect which dot is subjectively most similar to the center.
Within color-matching tests, the “gold standard” of color deficiency diagnosis are anomaloscopes, which present colors as monochromatic light rather than on a computer monitor or via reflective pigments. Compared to the pseudoisochromatic and panel tests, anomaloscopy requires a skilled examiner.
The Moreland anomaloscope serves to assess tritan discrimination. One half of the 2° hemipartite circle is a cyan standard (480 nm light tinged with a small admixture of 580 nm), which must be matched by mixing indigo (436 nm) and green lights (490 nm) in the other half-circle, known as the Moreland equation . Decreasing discrimination along the tritan confusion lines increases the range of mixtures which perceptually match the standard. Notably, at 8° only complete tritanopes accept the full range of color mixtures as a match to the cyan standard (see Bimler and Paramei, “ Tritanopia”).
More recently, computerized equivalents of pseudoisochromatic tests have become common, displaying a series of colored mosaics in which the elements vary in luminance spatially as well as dynamically to leave only chromatic cues. Employed on a calibrated monitor under strict psychophysical protocols, the tests allow precise measurement of chromatic sensitivity.
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