Deuteranopic unilateral dichromats report that with only their deuteranopic eye open, they see wavelengths shorter than neutral point as blue and longer than it as yellow. Blue–yellow color blindness
Blue-yellow color blindness includes tritan CVD. Tritan CVD is related to the S-cone and includes tritanomaly (anomalous trichromacy) and tritanopia (dichromacy). Blue-yellow color blindness is much less common than red-green color blindness, and more often has acquired causes than genetic. Tritans have difficulty discerning between bluish and greenish hues. Tritans have a neutral point at 571 nm (yellowish). Tritan (<0.01% of individuals): Lacking, or possessing anomalous S-opsins or short-wavelength sensitive cone cells. Tritans see short-wavelength colors (blue, indigo and spectral violet) as greenish and drastically dimmed, some of these colors even as black. Yellow and orange are indistinguishable from white and pink respectively, and purple colors are perceived as various shades of red. Unlike protans and deutans, the mutation for this color blindness is carried on chromosome 7. Therefore, it is not sex-linked (equally prevalent in both males and females). The OMIM gene code for this mutation is 304000 "Colorblindness, Partial Tritanomaly". Tetartan is the "fourth type" of colorblindness, and a type of blue-yellow color blindness. However, its existence is hypothetical and given the molecular basis of human color vision, it is unlikely this type could exist. Summary of cone complements
The below table shows the cone complements for different types of human color vision, including those considered color blindness, normal color vision and superior color vision.
Lantern
Instead of the Ishihara test, the US Navy and US Army also allow testing with the a lantern, such as the Farnsworth Lantern Test. Lanterns project small colored lights to a subject, who is required to identify the color of the lights. The colors are those of typical signal lights, i.e. red, green and yellow, which also happen to be colors of confusion of red-green CVD. Lanterns do not diagnose colorblind, but they are occupational screening tests to ensure an applicant has sufficient color discrimination to be able to perform a job. This test allows 30% of color deficient individuals, generally with mild CVD, to pass. Arrangement tests
Arrangement tests can be used as screening or diagnostic tools. The Farnsworth–Munsell 100 hue test is sensitive enough that it not only can detect color blindness, but also evaluate the color vision of color normals, ranking them as low, average or superior. The Farnsworth D-15 is simpler and is used for screening for CVD. In either case, the subject is asked to arrange a set of colored caps or chips to form a gradual transition of color between two anchor caps. Anomaloscope
An instrument called an anomaloscope can also be used for diagnosis. These instruments are very expensive and require expertise to administer, so are generally only used in academic settings. However, they are very precise, being able to diagnose the type and severity of color blindness with high confidence.