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Colour Blind, a world without colours.
Colour blindness or colour vision deficiency is the inability or decreased ability to see colour, or perceive colour differences, under normal lighting conditions. Colour blindness affects a significant percentage of the population. There is no actual blindness but there is a deficiency of colour vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive colour in light and transmit that information to the optic nerve. This type of colour blindness is usually a sex-linked condition. The genes that produce photo pigments are carried on the X chromosome; if some of these genes are missing or damaged, colour blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a functional gene on only one of the two X chromosomes is sufficient to yield the needed photo pigments).
Colour blindness can also be produced by physical or chemical damage to the eye, the optic nerve, or parts of the brain. For example, people with achromatopsia suffer from a completely different disorder, but are nevertheless unable to see colours.
The English chemist John Dalton published the first scientific paper on this subject in 1798, "Extraordinary facts relating to the vision of colours", after the realization of his own colour blindness. Because of Dalton's work, the general condition has been called daltonism, although in English this term is now used more narrowly for deuteranopia alone.
Colour blindness is usually classed as a mild disability, but there are occasional circumstances where it can give an advantage. Some studies conclude that colour blind people are better at penetrating certain colour camouflages. Such findings may give an evolutionary reason for the high prevalence of red�green colour blindness. And there is also a study suggesting that people with some types of colour blindness can distinguish colours that people with normal colour vision are not able to distinguish.
Colour blindness affects a large number of individuals, with protanopia and deuteranopia being the most common types of colour blindness. The typical human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cones, each containing a different pigment, which are activated when the pigments absorb light. The spectral sensitivities of the cones differ; one is most sensitive to short wavelengths, one to medium wavelengths, and the third to medium-to-long wavelengths within the visible spectrum, with their peak sensitivities in the blue, green, and yellow-green regions of the spectrum, respectively. The absorption spectra of the three systems overlap, and combine to cover the visible spectrum. These receptors are often called S cones, M cones, and L cones, for short, medium, and long wavelength; but they are also often referred to as blue cones, green cones, and red cones, respectively.
Although these receptors are often referred to as "blue, green, and red" receptors, this terminology is inaccurate. The receptors are each responsive to a wide range of wavelengths. For example, the long wavelength, "red", receptor has its peak sensitivity in the yellow-green, some way from the red end (longest wavelength) of the visible spectrum. The sensitivity of normal colour vision actually depends on the overlap between the absorption ranges of the three systems: different colours are recognized when the different types of cone are stimulated to different degrees. Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing the wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue.
Many of the genes involved in colour vision are on the X chromosome, making colour blindness much more common in males than in females because males have only one X chromosome, while females have two. Because this is an X-linked trait an estimated 2-3% of women have a 4th colour cone and can be considered tetrachromats although it is not clear that this provides an advantage in colour discrimination.
Canon 5D Mkii Canon 17-40mm F4L USM
November 16th, 2012
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