Colour blindness

If one or more types of a person's colour-sensing cones are missing or less responsive than normal to incoming light, that person can distinguish fewer colours and is said to be colour deficient or colour blind (though this latter term can be misleading; almost all colour deficient individuals can distinguish at least some colours). Some kinds of colour deficiency are caused by anomalies in the number or nature of cones in the retina. Others are caused by neural anomalies in those parts of the brain where visual processing takes place.

Species that have colour receptors different from humans – such as bird species, which may have four receptors – can make colour discriminations that humans cannot. A color reproduction system "tuned" to a human with normal colour vision may give very inaccurate results for the other observers, human or non-human.

Text II: History of colour theory

History of colours dates back to the ancient Greek philosophical texts written in dialogues by Plato and Timaeus (390BC), passages in writing of Aristotle (350BC) and De Coloribus (330BC). In that study of colour the interests of artistic painters and their understanding of the behavior of light and colour were taken as a basis.

According to the old colour mixing theory the “simple” or primary colours were white and black or light and dark. From this primaries the “noble” hues of red, yellow and blue were mysteriously derived. By mixing the “noble” hues, the ancient artists got the “composite” hues of orange (gold), green and purple.

In 1390, Cennino Cennini published a description of how artists worked with colour. He described seven colours. Four (black, red, yellow and green) were mineral in character. Three more - lime white, the blues (ultramarine, lapis lazuli and azurite) and orange were colours which needed to be developed artificially.

In the visual arts, colour theory is a body of practical guidance to colour mixing and the visual impacts of specific colour combination. There are also categories of colours based on the colour wheel: primary colour, secondary colour and tertiary colour. Although colour theory principles first appeared in the writings of Leone Battista Alberti (1435) and the notebooks of Leonardo da Vinci (1490). The tradition of "colory theory" began in the 18th century, initially within a partisan controversy around Isaac Newton's theory of colour and the nature of so-called primary colours.

In 1704, Sir Isaac Newton published his Opticks, which made several points about colour. He said that the source of colour was not substances, but light. He demonstrated that the different colours of the spectrum result from light being refracted. He called this attribute of light refrangibility from the Latin word refringere which means the ability to be refracted. He distinguished that each colour had a specific angle of refraction, when light passes through a prism of lens. He argued that orange or violet light were just as “primitive” as red and yellow because they cannot be broken down farther into more basic colour. He identified seven primary colours in this order: red, orange, yellow, green, blue, indigo, violet.

Newton saw the colour spectrum as a close system. He attached the red starting point to the violet end point and made a “circle of colours”where the red and violet overlapped and added the colours of magenta and puple which did not appear in the color spectrum. There was no black and white in the circle, but the center was white, which corresponded to the result of mixing all light colours together. But Newton did not understand the difference between additive and subtractive colour mixing. The knowlege of this is directly connected with the special structure of a human eye, which was discovered later after Newton’s reseaches.

In 1810, Goethe published his comprehensive Theory of Colours in which he described the physiological effects of colour.

In 1801 Thomas Young proposed his trichromatic theory, based on the observation that any colour could be matched with a combination of three lights. This theory was later refined by Jame Clerk Maxwell and Hermann von Helmholtz. Newton's law of mixture were experimentally confirmed by Maxwell in 1856.

At the same time as Helmholtz, Ewald Hering developed the opponent process theory of colour, noting that colour blindness and afterimages typically come in opponent pairs (red-green, blue-orange, yellow-purple, and black-white). Ultimately these two theories were synthesized in 1957 by Hurvich and Jameson, who showed that retinal processing corresponds to the trichromatic theory.