Colour vision is only present in daylight or
bright light (photopic vision), and is absent at night or dark
vision (scotopic vision) where rods function. There are approximately
7 million cones and 120 million rods in the human retina. The two
types of cells are not uniformly distributed. The central retina
surrounding the fixation point, the fovea, is essentially rod-free,
and purely occupied by cones. Rods have their maximum density
slightly peripheral to the fovea, and both types of cell diminish
in number towards the retinal periphery3.
The visual spectrum is made up of many wavelengths.
The perceived hue or colour is related to the wavelength of light
. Short wavelengths are seen as blue
or violet, medium wavelengths
are perceived as green and
yellow, while long wavelengths
are seen as orange or
red. The primary colours of
light are red,
green and blue. Mixtures of these colours in various combinations create different wavelengths, and stimulate the cones in varying degrees.
Normal colour vision is trichromatic. Most
of the spectral hues can be matched by an additive mixture of
three primary colours taken from the long-wave (
red), medium-wave (green),
and short-wave (blue) parts of
the visual spectrum. Although the three primary colours are referred
to as red,
green and blue,
the wavelengths of peak sensitivity do not correspond exactly
with these colour names. The three cone types have overlapping
spectral sensitivity which have maximum sensitivity in these
spectral regions3.
In a normal trichromat, three wavelengths
are required to match a given reference wavelength. This is because
all the hues that the eye can see can be matched by different
amounts of the three primary colours in various combinations. So
to match a certain reference wavelength, a combination of all
three primary wavelengths must be present3.
Dichromacy occurs when there are only two
cones functioning. This means that only two types of cones can
provide colour information to the brain, so only two test stimuli
are needed to match a reference point.
Monochromats and achromats only need one
wavelength to match the reference colour. This is because they can
only match with the brightness of an object, rather than the colour
of the object.
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