More than 95% of the world population have statistically normal colour vision. These are the people whose colour matching abilities are represented mathematically by the CIE standard and supplemental observers.
Although people with an atypical colour response are often called ‘colour blind’, there are in fact very few people who have absolutely no colour perception. The proportion of the population with reduced colour discrimination is however surprisingly large. Atypical colour response is sex-linked; while being found quite rarely in Caucasian females and non Caucasians, it occurs in 8% of Caucasian males.
This section is exceptionally brief and serves merely to introduce concepts such as ‘cone’ and ‘chromaticity diagram’, not to fully explain them which would require a much larger work.
The mamalian eye contains two types of light receptor: rods, and cones. Rods are mainly of use at low light levels and give a monochromatic response. There are three types of cones, with slightly different spectral sensitivities; through a complex process of adding and subtracting their outputs, three color signals are sent to the brain. The three types of cones are termed L, M and S for long, medium, and short wavelengths.
The Uniform Chromaticity Scale diagram was recommended by the CIE in 1976 as a reasonably uniform, two-dimensional representation of color (withought lightness, which would require a third dimension). The colors of the spectrum run around the curved outer edge from deepest red (wavelength 700nm or so)at the top right, through yellow (about 575nm), to green (500nm) at the top left corner, blue (475nm), and lastly violet (400nm) at the bottom right corner. The straight line segment represents shades of purple, a mix of red and violet light. All visible colors lie inside this shape. White is somewhere in the middle, depending on the exact type of light that your eyes are currently adapted to.
People with normal colour vision are termed trichromats because they have three functional cone types. Those with reduced functioning of one cone type are called anomalous trichromats.
In some people, one cone type is absent or completely non-functional. Because they have two functioning cone types, they are termed dichromats. Finally, those very rare people with only one functional cone type, who see only in shades of grey, are called monochromats.
Dichromats will see certain colours as the same which are clearly different to trichromats. These confused colours lie along straight lines on a chromaticity diagram, converging on a single point.
As one would expect, there are three types of dichromatism, depending on which one of the three cone types is affected. Each type has a different confusion point.
Protanopia is due to missing or dysfunctional L cones. Protanopes have greatly reduced discrimination of reds from greens, and reds look dimmer than normal. The confusion point P, shown in Figure 1, is at or near u' = 0.61, v'= 0.51, very close to the far red corner of the chromaticity diagram.
Figure 1: Protanopic confusion lines
Deuteranopia is due to missing or dysfunctional M cones. It can also be caused by a lack of the L-M colour difference signal. Deuteranopes also have reduced discrimination of reds from greens, but without any colours seeming dimmer than normal. The dueteranopic confusion point lies well off to the left of the chromaticity diagram, at or near u' = -4.75, v' = 1.31. As can be seen from Figure 2, this means that the lines of confused colours are nearly parallel.
Figure 2: Deuteranopic confusion lines
Tritanopia is caused by a lack or deficiency in the S cones. Because these make (almost) no contribution to the lightness channel, all colours are the same brightness as normal. However there is greatly reduced discrimination between yellows and blue/violets. The confusion point, shown in Figure 3 is at or near u' = 0.26, v' = 0.003, very close to the violet corner on the chromaticity diagram.
Figure 3: Tritanopic confusion lines
In each case, confused colours lie along converging lines; these may be used to select colour schemes that can be used by particular types of colour deficient subjects. No one scheme will meet the needs of all subjects with atypical color vision.
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