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THEORIES OF COLOR VISION

Prof. Vasudev Anand Rao

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,BlueSecondary 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 Theorypostulates 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 YellowIntensity discrimination - ability to detect difference in

brightness.

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