1. THEORIES OF COLOR VISION
Prof. Vasudev Anand Rao

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.

13. 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.

14. THANK YOU