Trillium PairBinocular Colour Vision

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Visible Spectrum and Colour Perception

Rod Vision
"Colour Blindness"
Normal (Trichromatic) Vision

Black and White (Rod) Vision

Humans have a type of visual receptor called rods (in addition to the cones discussed below).  These see in black-and-white as in the following figure.  Also, some visual processes, such as depth perception, "see" in black-and-white (i.e. no colour information is used - only brightness levels).

Rod Vision (12849 bytes)

Eagles are colour-blind. They are long-distance predators so they need maximum visual acuity to detect small animals, and their small movements, from far away.   Because their prey are drab in colour and/or camouflaged, the ability to see in colour would not add a great deal of useful information - and also tie up information channels from the eye to the brain.
    Deer are also colour-blind.  This is why hunters can wear bright red jackets to avoid being shot by other hunters.  Unlike eagles, they do not rely on visual acuity and have poor shape dicrimination.  However they are very sensitive to movement.  A hunter will not be seen by a deer if they remain very still.  Thus a bow-and-arrow hunter can stay up a tree until a deer gets close enough for an arrow to be effective if they remain still.

"Colour Blindness"
    Cats can use scent to locate their prey.  Their habitat of jungles, forests or grasslands restrict their line of sight.  Vision is best used in the final stages of the hunt.  A little loss of visual acuity in exchange for colour discrimination is a useful trade-off to help separate the prey from the background.  Cats (and humans with the most common types of colour-blindness) will see the world in a restricted range of colours.
    Curiously, it was discovered that colour-blind people could detect camouflage better than those with normal vision.  Hence colour-blind observers were added to flight crews in World War II when attacking ground targets.
    This is an indirect consequence of 'metamerism' found with normal vision.  Two objects can look the same colour in one type of light (indoors with reddish incandescent light) but different colours in another light (outdoors in bluer sunlight).   With colour blindness, it is the detectors (eye cones) that have changed rather than the light source.

Colourblind (15110 bytes)

    The disadvantage of having only two colour receptors in the eye (as indicated by the curves in the figure) is that one particular wavelength of light, or a group of wavelengths clustered around that one, will be mistaken for white light.   Therefore the sensation of colour does not cover the entire visible spectrum.

Trichromatic (Normal) Vision
    Humans eat fruits, berries and seeds.  Ripe fruit can usually be determined by either colour or odour.   Colour vision is a long-range sense while odour is sort-range and dependent on wind conditions.  Humans have a lessened odour sensitivity but a more complex visual sense.  This includes shape discrimination.  (Interestingly, some of the things that are most noticed by the human nose are not edible; e.g. flowers, and rotten food.)
    Having three colour receptors, called cones, allows the entire visible spectrum to be interpreted as differences in colour.  The cones are often described as red, green and blue sensitive, but are best defined as long [L], medium [M], and short [S] wavelength sensitive.  There is some evidence that the cone information is organized into red-green and yellow-blue opponent pairs (with a brightness signal to complete the information).

Spectrum (16641 bytes)

    Visual acuity is dependent on the desity of the cones in the fovea.   Addition of a third colour receptor does not reduce the visual acuity by a large amount because blue sensitive cones are only 5% of the total number in the eye.  In the real world, blue objects are either far away but large (e.g. sky, water) or small but close (e.g. fruit being evaluated as food).  Therefore bluish objects will cover a large visual angle and fewer of the blue sensitive cones are required. 
    It is still possible for two wavelengths of light, called 'Complementary Colours', to combine to create the sensation of white but this is rare in nature.  The visual interaction of complementary colours is one of the underpinnings of Impressionist painting.

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