Transcript Theory of color vision

THEORIES OF COLOR
VISION
Prof. Vasudev Anand Rao
1
COLOR – Definition
Color : visual description of an observer by which he
distinguishes two fields of same size, shape and
structure by difference in spectral activity.
- Purely sensory phenomenon and not a physical
attribute
- Perception of colour depends upon spectral
composition of light:
- coming from an object &
- emanating from surrounding
- State of light adaptation of subject
Primary Colors
: Red, Green,Blue
Secondary Colors : Cyan, Magenta,Yellow
Color sense
• Ability of the eye to discriminate between
different colors excited by light of different
wavelengths
• Function of cones
• Better appreciated in photopic vision
• In scotopic vision all colors seen as gray-called
Purkinje shift
COLOUR – The Theory
1.Trichromatic or Young Helmholtz Maxwell Theory
postulates three different receptors maximally sensitive
to wavelength in different regions of visual spectrum.
Three peaks are
• 440 to 450 nm - blue spectrum
• 535 to 550 nm - green spectrum
• 570 to 590 nm - red spectrum
Colour sensation
• Determined by relative frequency of impulses from
each cone system
• A given colour consists of admixture of 3 primary
colors in different proportion
• Eg. yellow color perception is due to simultaneous
stimulation of red and green
Drawbacks – Trichromatic theory
Though it accounts well for laws of color mixing,
has difficulty with other basic phenomena
• Dichromats who confuse red with green can
see yellow
• Difficulty in explaining complementary color
after-images
COLOUR – The Theory…….
2. Herring’s Theory of Opponent processing (Ewald
Herring 1964) updated by Hurvich and Jameson
Two chromatic (red-green and blue-yellow) and one
achromatic (black and white) mechanism
These pair sensation in an opposing or antagonistic
manner
Presence of one of the color of the pair excludes the
other color perception and presence of both nullify
each other
Mutually exclusive colors
• Some colors are not perceived
together (bluish yellow or reddish green)
• Red light which stimulates
red cones simultaneously inhibits green cones
• Color vision trichromatic at photoreceptor level
• Color opponency occurs at ganglion cell onwards
• Accounts for 6 diff color sensations-blue, green,
red, yellow, black and white
• Accounts well for color contrast and color
blindness
• 2 color opponent ganglion cells
Red green opponent color cells
Blue yellow opponent color cells
COLOUR – The Theory…..
3. Granit’s Dominator – Modulator Theory
In addition to the retina's three kinds of cone cells,which
respond to different colors, certain optic-nerve fibers
(dominators) respond either to the whole spectrum or
to specific colors (modulators)
He also proved that light inhibits as well as stimulates
optic-nerve impulses
Factors affecting Color Vision
Lens : In young it absorbs light of shorter wavelength
(<400 nm), so does not affect color vision. In old people
longer wavelength of visible spectrum is absorbed
Retinal Description of Color: The centre of fovea is unique
in having highest spatial density of red and green
cones, with blue cones eliminated from central 1/8 deg
of the visual field
Trichromatic vision extends up to 20 – 30 degrees from
the point of fixation, beyond which the color becomes
indistinguishable
Factors affecting Color Vision
Color Encoding of Cerebral Cortex: The striate cortex
contains group of cells referred to as blobs
In layers 2 and 3 blobs exhibit color double opponency.
An observer’s color vision includes color matching and
color discrimination.
Color Matching : Any physical color can be matched using
additive mixtures of the color and three primary colors.
Color Discrimination
Ability to note that two colors differ in hue or saturation or
both.
Hue discrimination - ability to detect difference in
wavelength.
Saturation discrimination - ability to detect difference in
color content in which different content of white color
has been added.
Pure Color + White Color
Tint Produced
Red
Pink
Green
Yellow
Orange
Yellow
Intensity discrimination - ability to detect difference in
brightness.
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