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Color Vision

What is color?

Color is more than just the property of objects and yet this is contrary to the way in which we use color in everyday language. The association of color with objects in our language, seen in statements such as ''this object is red", is misleading for it is undeniable that the color that we perceive exists only in the brain.
It is commonly stated that color vision is the result of the nature of the physical world, the physiological response of the eye (more strictly the retina) to light, and the neural processing of the retinal response by the brain. The identification of three separate processes in this way is probably artificial, and does little justice to the complex nature of color perception, but the idea is useful and appealing; it turns out that the number "three" has an almost magical association with color vision.

How does the eye work?

Almost the whole of the interior of the spherically shaped eyeball is lined with a layer of photosensitive cells known collectively as the retina and it is this structure that is the sense organ of vision. The eyeball, though no mean feat of engineering itself, is simply a structure to house the retina and to supply it with sharp images of the outside world. Light enters the eye through the cornea and the iris and then passes through the lens before striking the retina. The retina receives a small inverted image of the outside world that is focussed jointly by the cornea and the lens. The lens changes shape to achieve focus but hardens with age so that we gradually lose our power of accommodation. The eye is able to partially adapt to different levels of illumination since the iris can change shape to provide a central hole with a diameter between 2 mm (for bright light) and 8 mm (for dim light).
The retina translates light into nerve signals and consists of three layers of nerve-cell bodies. Surprisingly the photosensitive cells, known as rods and cones, form the layer of cells at the back of the retina. Thus, light must pass though the other two layers of cells to stimulate the rods and cones. The reasons for this backward-design of the retina are not fully understood but one possibility is that the position of the light-sensitive cells at the back of the retina allows any stray unabsorbed light to be taken care of by cells immediately behind the retina that contain a black pigment known as melanin. The melanin-containing cells also help to chemically restore the light-sensitive visual pigment in the rods and cones after it has been bleached by light.
The middle layer of the retina contains three types of nerve cells: bipolar cells, horizontal cells, and amacrine cells. The connectivity of the rods and cones to these three sets of cells is complex but signals eventually pass to the front of the retina and to the third layer of cells known as retinal ganglion cells. The axons from retinal ganglion cells collect in a bundle and leave the eye to form the optic nerve. The backward-design of the retina means that the optic nerve must pass through the retina in order to leave the eye and this results in the so-called blindspot.
The rods and cones contain visual pigments. Visual pigments are much like any other pigments in that they absorb light and have absorption sensitivities that are wavelength dependent. The visual pigments have a special property, however, in that when a visual pigment absorbs a photon of light it changes molecular shape and at the same time releases energy. The pigment in this changed molecular form absorbs light less well than before and thus is often said to have been bleached. The release of energy by the pigment and the change in shape of the molecule together cause the cell to fire, that is to release an electrical signal, by a mechanism that is still not completely understood.

What are scotopic and photopic vision?

Rods are sensitive to very low levels of illumination and are responsible for our ability to see in dim light (scotopic vision). They contain a pigment with a maximum sensitivity at about 510 nm, in the green part of the spectrum. The rod pigment is often called visual purple since when it is extracted by chemists in sufficient quantities the pigment has a purple appearance. Scotopic vision is completely lacking in color; a single spectral sensitivity function is color-blind and thus scotopic vision is monochromatic.
Color vision is provided by the cones, of which there are three distinct classes each containing a different photosensitive pigment. The three pigments have maximum absorptions at about 430, 530, and 560 nm and the cones are often called ''blue", ''green", and ''red". The cones are not named after the appearance of the cone pigments but are named after the color of light to which the cones are optimally sensitive. This terminology is unfortunate since monochromatic lights at 430, 530, and 560 nm are not blue, green, and red respectively but violet, blue-green, and yellow-green. The use of short-, medium-, and long-wavelength cones is a more logical nomenclature.
The existence of three spectral-sensitivity functions provides a basis for color vision since light of each wavelength will give rise to a unique ratio of short-, medium-, and long-wavelength cone responses. The cones therefore provide us with color vision (photopic vision) that can distinguish remarkably fine wavelength changes.

What is chromatic aberration?

The eye cannot simultaneously focus on the three regions of the spectrum where the cone-pigment absorptions peak since refraction at the cornea and lens is greater for short wavelengths than it is for long wavelength. Thus, it is said that the eye is not corrected for chromatic aberration. The medium- and long-wavelength peaks are quite close together and therefore the lens optimally focuses light of about 560 nm on the retina. Since the short-wavelength cones receive a slightly blurred image it is not necessary to provide the same spatial resolution that is provided by the other two sets of cones. The retina contains approximately 40 long-wavelength cones and 20 medium-wavelength cones for every single short-wavelength cone.
The rods and cones are not evenly distributed on the retina. The central part of the retina, the fovea, contains only cones whereas at greater eccentricities there is a greater preponderance of rods. In the fovea the cones are densely packed and it is this part of the retina that provides the greatest spatial resolution under normal viewing conditions.

What is trichromacy?

Since the retina contains four different types of receptor it might be thought that the neural pathways would carry four different signals to the brain, and more precisely to the primary visual cortex which is at the back and rear portion of the brain. It is generally believed, however, that color information is coded by the retinal and post-retinal neural structures as just three types of signals that are often called ''channels".
The idea of ''channels" in the brain is central to the way in which the operation of the brain can be viewed as an information- or signal-processing task. A channel is a conceptual processing route and thus for the visual system we can say that the information from the cones is processed in three separate channels. Remembering that color perception is only one function of the visual system, there are other channels that are responsible for providing other information about the outside world that enables the perception of form, motion, and distance for example. The existence of channels for the processing of color information helps explain the two contradictory theories of color vision that were prevalent during the 19th century: the trichromatic theory and the opponent-colors theory.
The trichromatic theory was postulated by Young and later by Helmholtz and was based upon color-matching experiments carried out by Maxwell. Maxwell's experiments demonstrated that most colors can be matched by superimposing three separate light sources known as primaries; a process known as additive mixing. Although any light sources could be used as primaries the use of monochromatic sources of radiation enables the widest gamut of colors to be obtained by additive mixing. The Young-Helmholtz theory of color vision was built around the assumption of there being three classes of receptors although direct proof for this was not obtained until 1964 when microspectrophotopic recordings of single cone cells were obtained. The roots of trichromacy are firmly understood to be in the receptoral stage of color vision. It is important to realize that a yellow stimulus produced by the additive mixture of appropriate red and green lights does not simply match monochromatic yellow light but is indistinguishable from it. Thus, the trichromatic nature of vision is essential for the operation of many color reproducing processes such as television, photography, and three-color printing.

What is the opponent theory of color vision?

The opponent-colors theory of color vision, proposed by Hering, seemingly contradicts the Young-Helmholtz trichromatic theory. It was advanced to explain various phenomena that could not be adequately accounted for by trichromacy. Examples of such phenomena are the after-image effect (if the eye is adapted to a yellow stimulus the removal of the stimulus leaves a blue sensation or after-effect) and the non-intuitive fact that an additive mixture of red and green light gives yellow and not a reddish-green. Hering proposed that yellow-blue and red-green represent opponent signals; this also went some way towards explaining why there were four psychophysical color primaries red, green, yellow, and blue and not just three. Hering also proposed a white-black opponency but this third opponent channel has been abandoned in most modern versions of the theory. It is now accepted that both the trichromatic theory and the opponent colors theory describe essential features of our color vision with the latter theory describing the perceptual qualities of color vision that derive from the neural processing of the receptor signals in two opponent chromatic channels and an achromatic channel.

What are brightness, hue, and colorfulness?

The perceptual attributes brightness, hue, and colorfulness have been defined by Professor R.W.G. Hunt as follows: Brightness: attribute of a visual sensation according to which an area appears to exhibit more or less light. Hue: attribute of a visual sensation according to which an area appears to be similar to one, or proportions of two, of the perceived colors red, yellow, green, and blue. colorfulness: attribute of a visual sensation according to which an area appears to exhibit more or less of its hue.
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