Tuesday, May 18, 2010

There is not just one color blindness

When people hear about color blindness they usually relate it to the term red-green color blindness, which is interpreted as 'not being able to distinguish the colors red and green'. Unfortunately this is misleading and merely describes the most common form of color vision deficiency.

In 1798, John Dalton (of atomic theory fame) was the first known scientist to describe color blindness in detail, as he discovered to be colorblind himself [1]. After studying his color vision, comparing his results with his brother, who was also colorblind, and talking to other colorblind people which he found during his research, he put together a list of Characteristic Facts of our Vision (by our he refers to the group of colorblind people including himself and his brother). This list includes the following points:
1. In the solar spectrum three colors appear, yellow, blue, and purple. [...]
2. Pink appears, by day-light, to be sky-blue a little faded, [...]
5. There is not much difference in color between a stick of red sealing wax and grass, by day.
John Dalton was suffering from deuteranopia [2] - one specific form of red-green color blindness - and his findings already show us, that this is not only about having troubles with the colors red and green but affects the whole color spectrum.

During the 19th century many scientists such as Young, Helmholtz, Maxwell, and Wilson became interested in the topic of color vision and color blindness. These scientists formed the fundamental research paving the way to our understanding of the perception and misperception of colors. In 1855 Maxwell wrote in a letter to Wilson: "The mathematical expression of the difference between colour-blind and ordinary vision is that colour to the former is a function of two independent variables, but to an ordinary eye, of three; [...] If we find two combinations of colours which appear identical to a colour-blind person, and mark their positions on the triangle of colours, then the straight line passing through these points will pass through all points corresponding to other colours, which, to such a person, appear identical with the first two. We may in the same way find other lines passing through the series of colours which appear alike to the colour-blind. All these lines either pass through one point or are parallel, according to the standard colours which we have assumed, and the other arbitrary assumptions we may have made." [3]

Color vision

In order to understand the source of the different types of color vision deficiency, it is important to understand how our color vision works. The human eye consists of approximately seven million cone cells and one hundred twenty million rod cells. Cone cells are responsible for color vision and function best in high levels of illumination; this is the reason color vision is absent at night. Rod cells function in low levels of illumination, and play a secondary role in color vision as they can only distinguish between lightness and darkness.

The average person has three types of cone cells, referred to as trichromacy, which differ in their peak sensitivity along the color spectrum. Long-wave sensitive cones have a maximum absorbance at 560nm, medium-wave sensitive cones at 530nm and short-wave sensitive cones at 420nm. These wavelengths are very close to the the primary colors red, green and blue and therefore are often referred to by those three colors [4].

Mixing the input of those three types of cones makes up our entire visible color spectrum. With this knowledge it is easy to understand that someone with more than three types of cones would have an increased perception of color; an example being tetrachromacy with four cone types. Whereas, on the other side, if there is a problem with one of those cone types or if one is missing the perceived variety of colors can be reduced dramatically.

MacAdams Ellipses
Diagram 1: MacAdam ellipses in the CIE 1931 chromaticity diagram.
Even people with normal color vision, or tetrachromacy for that matter, cannot distinguish differences within the entire color spectrum. This is important to understand as otherwise even people with no defect could be classified as colorblind.

In the 1940s MacAdam conducted a series of tests and introduced the discrimination ellipses, also known as MacAdam ellipses (see Diagram 1), into the CIE 1931 chromaticity diagram [5]. Every ellipse encloses colors which cannot be discriminated at the same level of luminance. These ellipses provide the tolerance limits for color specification and color reproduction. MacAdam estimated that a trichromat can distinguish about seventeen thousand unique colors at each level of luminance, or about three million perceivable colors overall.

Color vision deficiency

With the theory of color vision in the backpack, understanding the different types of color blindness is one step away. As our color vision is based on the three types of cones - red, green, and blue - a slight shift of the maximal sensitivity of one of those three types of cones results in less possible color mixtures, and reduction of the perceived color spectrum. This form of color blindness is an abnormality of the normal trichromatic color vision and referred to as anomalous trichromacy.

If one cone type is missing completely there are only two receptors left to mix the perceived range of colors, resulting in a dramatic reduction of the amount of visible colors; this type of color vision deficiency is called dichromacy.

If we keep removing cones we end up in forms of color blindness where two or even all are missing. In this case the term 'color blindness' is really correct, as only shades of gray or slight hints of colors can be seen. This complete form of color blindness is called monochromacy and is accompanied by a severe light sensitivity as only rods are used to retrieve visual information.


Table 1: Classification of congential color vision deficiency [6]

Table 1 shows the various types of color vision deficiencies. As there are three unique cone types, and each type can have a shifted peak of sensitivity or be missing completely, dichromats and anomalous trichromats are further subdivided into protans (defective long-wave red cones), deutans (defective medium-wave green cones), and tritans (defective short-wave blue cones).

Protan and deutan defects are the most common forms and often grouped together into one description known as red-green color blindness. The reason for this is they share similar areas of color confusion in the red/green portion of the spectrum. By far the most common type is deuteranomalous trichromatism of which around 5% of men are tested for positively. Other forms cannot easily be measured as they are rarely observed.


Table 2: Prevalence of different types of red-green color blindness [6]
The difference between prevalence rates among men and women is based on the fact that every form of red-green color blindness is a sex linked recessive trait. This means that women are often carrying a defective X chromosome but do not show any sign of deficiency, as the normal X chromosome acts dominantly.

Confusion lines and dichromatic convergence points

Dalton described his color blindness to be affecting the entire color spectrum. Pitt estimated in 1935 that the number of distinguishable spectral hues can be as low as 17 when suffering from deuteranopia, compared to about 150 in normal color vision [7]. Maxwell also thought about lines of color confusion for certain color spaces as early as 1855.

Today, we know that confusion lines, or dichromatic isochromatic lines, exist and converge at a single point for each type of color vision deficiency [8]. Colors represented along such a line are perceived as identical if no luminance contrast is present. The same lines are also true for the corresponding types of anomalous trichromatism, whereas the confusion zone is shorter and the full range of chromaticities is not included. These confusion lines are often represented in the CIE 1931 color space.

The CIE chromaticity coordiantes (x/y) for each type of color vision deficiency are as follows:
  • Protanopia (0.7465, 0.2535)
  • Deuteranopia (1.4, -0.4)
  • Tritanopia (0.1748, 0.0)

Diagram 2: Protanopia, deuteranopia and tritanopia confusion lines in the CIE 1931 color space.
Confusion lines and dichromatic convergence points put color vision deficiency into a mathematical framework. For this reason this information is useful for many types of applications:
  • Color blindness tests including pseudoisochromatic plate tests (also known as Ishihara plates), arrangement tests and modern computer based tests often use this data to calculate the type of color vision deficiency and severity.
  • Color blindness simulation algorithms make use of confusion lines by utilizing the fact that colors along these lines are perceived as identical to colorblind individuals. Using this knowledge, color blindness can be simulated by shifting the colors which are confused together along the lines of confusion to a single color.
  • Daltonization algorithms utilize confusion lines to compensate for color blindness. This is accomplished by shifting colors away from confusion lines towards colors visible to the colorblind individual. For example, daltonizing for protanopia involves shifting red values towards the blue end of the spectrum.
References
  1. Dalton J. Extraordinary facts relating to the vision of colours. Memoirs of the Literary Philosophical Society of Manchester: 1798. 5:28-45.
  2. Mollon JD, Dulai KS, Hunt DM. Dalton’s colour blindness: an essay in molecular biography. John Dalton’s Colour Vision Legacy. Selected proceedings of the international conference. Dickinson CM, Murray IJ and Carden D, Eds; Taylor and Francis Ltd 1997: 15-33.
  3. Judd DB. Fundamental Studies of Colour Vision from 1860 to 1960. Mechanisms of Color Vision. Proc N A S: 1313-1330.
  4. Dartnell HJA, Bowmaker JK, Mollon JD. Human visual pigments: microspectrophotometric results form the eyes of seven persons. Proc R Soc (Lond) B 220, 1983: 115-130.
  5. MacAdam DL. Visual sensitivities to colour differences in daylight. J Opt Soc Amer 32, 1942: 247-274.
  6. Birch J. Diagnosis of Defective Colour Vision. Oxford University Press, 2nd ed. 2001.
  7. Pitt FHG. Charateristics of Dichromatic Vision. Medical Research Council Special Report Series no 200, HMSO, London, 1935.
  8. Birch J. Dichromatic convergence points obtained by subtractive colour matching. Vision Res 13, 1972: 1755-1765.

14 comments:

  1. QUESTION:

    What are the colors seen along the various confusion lines?

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  2. This comment has been removed by the author.

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