Color Perception

by ACHARYA NARENDRA DEV COLLEGE

WHAT IS COLOUR? “Colour is the visual perceptual property corresponding to the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors.”

VISIBLE PART OF THE E.M. SPECTRUM

color

wavelength interval

frequency interval

red

~ 625–740 nm

~ 480–405 THz

orange

~ 590–625 nm

~ 510–480 THz

yellow

~ 565–590 nm

~ 530–510 THz

green

~ 500–565 nm

~ 600–530 THz

cyan

~ 485–500 nm

~ 620–600 THz

blue

~ 440–485 nm

~ 680–620 THz

violet

~ 380–440 nm

~ 790–680 THz

SPECTRAL SENSITIVITY OF HUMAN EYE Three types of cones are there on the basis of the wavelength they absorb: • • •

Short Medium Long

back

EXPERIENCE OF COLOUR • HUE- identification of colour • BRIGHTNESSintensity of colour • SATURATION- purity of a colour

COLOUR PRODUCTION • Opaque

• • • •

objects

reflection absorption scattering

Transparent and translucent objects Fluorescence and phosphoresence Chemiluminescence Incandescence

MIXING OF COLOURS Additive colour mixing: •Addition of two or more light sources of different hues •Basis of T.V. and computer screens and partition mixing in clothes

Subtractive colour mixing: • Selective removal of wavelengths of light • Basis of dyes, paints, inks etc

ARRANGEMENT OF NEURONS IN RETINA

ACTIVATION OF RHODOPSIN

GENERATION OF AN ELECTRICAL SIGNAL Transducin - GDP Metarhodopsin II Transducin - GTP

Phosphodiesterase

cGMP

back

GMP

Decreased Na+ conductance

Hyperpolarization

TRANSMISSION OF ELECTRICAL SIGNAL The signals are carried to the brain via three channels: • Red – Green & Blue –Yellow channel which sense colour • Intensity channel

COLOUR PERCEPTION BASED ON “OPPONENT THEORY”

REQUISITES FOR COLOUR VISION •

•

Two different spectral classes of receptors, with each class defined by the sensitivity of the receptor to the wavelength of light. Realised through: – – –

Expression of different opsins by different genes. Use of more than one chromophores. Oil droplets screening individual receptors.

EVOLUTION OF COLOUR VISION

COLOUR VISION

• A 2 cone system provides advantage in the form of visual contrast i.e. recognition of difference in a given space. • Dominant and offset wavelength detection. • More the no. of photoreceptors, greater the sensitivity.

DIVARIANT COLOUR VISION

• About 2% of human males have only 2 types of cones; long and short wavelength sensitive. • Neighbouring retinal areas are compared in the brain to detect border contrast.

TRIVARIANT HUMAN VISION

Pink circles represent substitutions. White circles represent conserved residues.

DIVARIANT SYSTEM LONG WAVE-ON SYSTEM ACTIVE SHORT WAVE SYSTEM ACTIVE SHORT WAVE SYSTEM INACTIVE

LONG WAVE-OFF SYSTEM ACTIVE

BLUE

GRAY

WHITE

BLACK

BROWN

YELLOW

TRIVARIANCE M-CONE-ON SYSTEM ACTIVE L-CONE-ON SYSTEM ACTIVE L-CONE-OFF SYSTEM ACTIVE

DIVARIANT GREEN

M-CONE-OFF SYSTEM ACTIVE

RED DIVARIANT

ELSEWHERE

Gonodactylus smithii (Mantis Shrimp)

A BEE-UTIFUL WORLD

ADVANTAGES OF COLOUR VISION • Allows detection of subtle differences in surroundings. • Better survival of organisms due to being sensitive towards a particular organism/nutrition. • Co-evolution of the colour expression in plants.

•

Colour therapy

COMPLEMENTARY CHROMATIC ADAPTATION

THANK YOU! • Manoj Kumar • Vikrant Suri • Manish Grover • Navneet A Vasistha