Color Vision Tests
Confusion lines form the basis of many color vision tests such as the Farnsworth Panel D-15 and Ishihara Pseudoisochromatic plates. Pseudoisochromatic plate tests are also commonly used in the clinic to screen for color vision deficiency. Colors are carefully chosen based on the confusion lines. The most commonly used pseudoisochromatic plate in the clinic would be the Ishihara Isochromatic plates (for screening red-green color vision deficiency) and the Tritan (F-2) plate.
Pseudoisochromatic plates are designed in four ways:
1. Transformation plates: where a person with normal color vision sees one figure and a CVD person sees another (figure 13 a).
Figure 13. (a) The transformation plate of the Ishihara. Normal should see 3 while a CVD person should see 5. (b)The vanishing plate of the Ishihara. Normal should see 73 while a CVD will not read the figures correctly
Figure 14. (a) The hidden-digit plate of the Ishihara. Normal should not see anything while a CVD person should see 5. (b) The diagnostic plate of the Ishihara. Normal should see both the 2 and the 6. Deutan type color vision deficiency should see 2 more easily while a protan type color vision deficiency should see the 6 more easily
The Ishihara color vision test is a screening test and the fail criterion for the Ishihara is typically four or more plates. Further color vision testing will be required to confidently diagnose the type of color vision defect. Another useful screening plate test is the Farnsworth F-2 plate (Pease, 1998). There are many color vision tests available for screening and for diagnosis of which only a few will be discussed here. The colours of the Panel D-15 are also carefully chosen from the CIE diagram so they are all isoluminant (that is, have the same value) as seen in figure 15.
Figure 15. Chromaticity co-ordinates of the colours of the Farnsworth Panel D-15 (from Benjamin, W. J. (Ed), Borish's Clinical Refraction. Philadelphia: W. B. Saunders Company, 1998)
Patients are asked to arrange 15 colored caps in sequential order based on similarity from the pilot color cap (figure 16).
Figure 16. Farnsworth Panel D-15
The type of color vision defect can be detected from their arrangement of the caps. These color caps are arranged in a particular fashion due to the confusion of colors that lie on the confusion lines (figure 17). The criterion for failure in the Panel D-15 test is two or more major crossings (ie, greater than a two cap error). Deutan, protan and tritan will produce characteristic errors (crossings) according to their confusion lines. Rod monochromats are color blind and their Vl peaks at about 507 nm. They arrange the D-15 caps according to the scotopic reflectance of the D-15 caps.
Figure 17. The Farnsworth Panel D-15 results from patients with various color vision defects.
The rod monochromatic results are idealised to illustrate the scotopic axis along 5-14. As a rule, rod monochromats give variable results with a tendency of crossing errors to fall along the 5-14 axis
Other arrangement color tests include the L'Anthony's Desaturated Panel D-15, saturated H-16 and the Farnsworth 100-Hue Test. The desaturated panel D-15 is particularly useful in the early diagnosis of acquired diseases and mild congenital deficiencies. The sequence of test administration include screening for color vision deficiencies with a pseudo-isochromatic plate test such as the Ishihara and F-2 plate. Failure of either of these tests implies proceeding to the panel D-15. A patient who fails the Panel D-15 is said to be a moderate to severe anomalous dichromat, or a dichromat. A patient who then fails the H-16 is a dichromat or very severe anomalous trichromat, whereas a moderate anomalous trichromat will pass the H-16. A subject who passes the D-15, can proceed to the desaturated D-15, that can pick up the mild anomalous trichomats. The very mild anomalous trichromats who may or may not fail pseudoisochromatic plates can be diagnosed with the Nagel anomaloscope.
Figure 18. Farnsworth 100-Hue
The Farnsworth 100-Hue is another arrangement test (figure 18). Unlike the tests mentioned above where the colors are specifically chosen to lie close to the confusion lines, the Farnsworth100-Hue is a discrimination test.
Figure 19 shows where the colors of the 100-Hue lie on the chromaticity diagram. The colors are chosen to have the same Munsell value and chroma. Originally there were 100 hues, but Farnsworth removed 15 to make the series more uniform. Performance on the Farnsworth100-Hue is rated by calculating the total error score.
Figure 19. Colors of the 100-Hue on the chromaticity diagram. Point C represents testing conditions using standard illuminant C and point W is the equal energy white (from Hart W. M. Jr, Acquired dyschromatopsias. Surv Ophthalmol 1987; 32: 10)
Lantern tests have been used since the19th century as a means of assessing color vision especially for occupational reasons. Lantern tests simulate colored signal lights. They usually present pairs of red, white and green lights because these are the signal colors used at sea and in air navigation, and the subject is required to name the colors. There are a great number of different lantern tests which vary quite widely in the level of difficulty they present. The level of difficulty depends on the size of the stimulus and its intensity (see Cole and Vingrys 1982 for a review). The lanterns in current use are the Farnsworth lantern, now superseded by the Optec 900, the Holmes Wright Type A and B lantern and the Beyne lantern (figure 20).
Lantern tests are usually failed by dichromats and anomalous trichromats whose defect is severe enough to cause them to fail the Farnsworth D15 test but the ability to recognise small colored signal lights can vary quite widely among those with mild anomalous trichromasy. A pass at the D15 test or a small range at the anomaloscope does not necessarily mean that signal light colors can be recognised (Cole and Maddocks, 1998).
Figure 20. The different lanters used in occupational testing of color vision