o verview what is the colour the eye the sensation of the colour colour vision effects colour...

THE EYE AND THE THE EYE AND THE PERCEPTION OF PERCEPTION OF COLOURCOLOUR

OVERVIEW

What is the colour The eye The sensation of the colour Colour vision effects Colour deficiencies Closure

WHAT IS THE CCOOLLOOUURR?

X-rays U-V rays Visible spectrum Infra red Radar RadioCosmic rays

10-810-14 10-12 10-6 110-4 (m)10+2

The light result of the feeling produced by the electromagnetic waves in a spectral The light result of the feeling produced by the electromagnetic waves in a spectral field going from 380nm to 730nm. field going from 380nm to 730nm.

This area is called: visible spectrum This area is called: visible spectrum Each wavelength corresponding has a perceptive feeling called “colour”. We translate Each wavelength corresponding has a perceptive feeling called “colour”. We translate

the colour as: red, yellow, green, cyan, blue, magentathe colour as: red, yellow, green, cyan, blue, magenta

WHAT IS THE CCOOLLOOUURR? The daylight is white. Is it?

730 nm

380 nm

Newton: is the additive of all the colours of spectrum

Colour is the sensation that the eye receive from the quality and quantity of electromagnetic waves

Basic characteristics of colour:

Hue, saturation, brightness

HOW WE PERCEIVE THE CCOOLLOOUURRTHE EYEIs a remarkable biological invention, a shining triumph of the

process of evolution. Although it was the detector that started us on mankind's exploration of the Cosmos, it has some shortcomings that ultimately limit that exploration:

It has limited size and therefore limited light-gathering power.

It has limited frequency response, since it can only see electromagnetic radiation in the visible wavelengths.

It distinguishes a new image multiple times a second, so it cannot be used to accumulate light over a long period in order to intensify a faint image.

It cannot store an image for future reference like a photographic plate can though the brain can.

THE EYE1. Cornea

2. Anterior chamber

3. Iris

4. Sclera

5. Choroid

6. Retina

7. Aqueous humour

8. Pupil

9. Crystalline lens

10.Posterior chamber

11.Fovea

12.Optical nerve

THE EYE

Diagram of organization of the retina

Rods Cones

Pigment epithelium

Outer limiting membrane

Muller cellsHorizontal

cellsDipolar cells

Amacrine cells

Ganglion cells

Nerve fiber layer

Inner limiting membrane

The retina Photoreceptors that are

sensitive to light

When light is absorbed by the photoreceptors, the light energy is converted into electrical and chemical signals which sent from the to brain through ganglion cells and optic nerves.

There are two kinds of photoreceptors:

rods cones

THE EYE

Rods & Cones The rods

Are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment (scotopic conditions)

Are not good for colour vision. The images generated by rod stimulation alone are

relatively unsharped and confined to shades of grey Are more numerous than cones in the periphery of the

retina The light sensitivity of rods is about 1000 times more

than cone cells There are about 120 million rods in the human retina

THE EYE

Rods & Cones The cones

Are not as sensitive to light as the rods Are most sensitive to one of three different colours (green,

red or blue) and usually referred to as photopic vision Stimulation of these visual receptors results in what is

know as true colour vision There are about 6 million cones in the human retina Signals from them are sent to the brain which then

translates these messages into the perception of colour They work only in bright light Three types: S, L & M

THE EYE

Types of cones Each type is differentially sensitive to a different

region of the visible spectrum S: short-wavelength sensitive, most receptive at 419nm (blue), cyanolabe M: middle-wavelength sensitive, most receptive at 531nm (green), chlorolabe L: long-wavelength sensitive, most receptive at 558nm (red), erythrolabe

THE EYE

Is the region of the retina that provides for the most clear vision. There are NO rods...only cones. The cones are also packed closer together here than in the rest of the retina.Very few s-cones in foveaBlood vessels and nerve fibers go around the fovea so light has a direct path to the photoreceptors.

The fovea

THE EYE

Blind spot Approximately 14o from the fovea No rods or cones Insensitive to light Hence NO vision No problem: - binocular vision

- continuous movement in high speed

FROM THE EYE TO THE BRAIN

The light which is absorbed by the photoreceptors is converted into electrical and chemical signals and through the ganglions is transmitted to the neurons in our eye and brain process

All the nerve impulses generated in the retina travel back to the blind spot

Axons in the optic nerve connect the blind spot with the optical centre

Optical

centre

Retina

Optic nerve

SENSATION OF THE CCOOLLOOUURR

Three main theories describe the colour vision:

Trichromatic theory Hering opponent theory Modern opponent colour theory


Trichromatic Theorythree receptor types with different spectral sensitivitiesspecific colour coded by pattern of responding across receptors (distributed coding)The ratios of the signals are used to define the colour sensation

Colour sensation

Light

Cones


Hering opponent theory Based on observation of colour vision

Red, green, blue, yellow

No colours could be described as a combination

-red+green -blue+yellow

Opponent colours


Hering opponent theory adaptation responsible for afterimages


Hering opponent theory The colour receptors had a red/green,

blue/yellow and dark/light response

Colour sensation

Colour receptors

Light


Modern opponent colour theory

Colour sensation

S M L

Light

Cones

Cells

Three receptor types with different spectral sensitivities detect the light

Produce three processed signals that are then used to determine the colour sensation

It is supported by psychological experiments

CCOOLLOOUURR VISION EFFECTS

These effects and need to be taken into account when trying to model the colour vision system: CIELAB, RLAB, CIECAM97s

There are more than 10 different vision effects that can be compensated for

They take into account functions from the field of view, colour constancy through to the viewing conditions

The most common are dark and light adaptation, and simultaneous contrast.


Dark and light adaptation Dark adaptation

Occurs when the illumination decreases Example: walk into a darker room

Light adaptation Occurs when the illumination increases Example: walk into a dark room into daylight


Simultaneous contrast

Is the impact of the surround on the colour seen, which can make the same colour appear different

CCOOLLOOUURR DDEFICIENCIESEFICIENCIES

Is the loss of colour discrimination

Causes: Genetic photoreceptor disorders

Usually in males because the genes for the red and green colour receptors are located on the X chromosome, of which men have only one and women have two (red + green)

Damage to the retina Damage to the optic nerve Higher brain areas implicated in colour processing

include the parvocellular pathway of the lateral geniculate nucleus


Types of colour defective vision:

Dichromism: Protanopia Deuteranopia Tritanopia

Anomalous trichromatism: Protanomaly Deuteranomaly

Monochromatism: Rod monochromatism Cone monochromatism


TYPE FORM CAUSE

Red-green defects

Protanomaly trichromatic dysfunctional L cones

Protanopia dichromatic missing L cones

Deuteranomaly trichromatic dysfunctional M cones

deuteranopia dichromatic missing M cones

Blue-yellow defects

tritonopia dichromatic missing S cones

COLOUR VISION DEFECTS


Protanopia the brightness of red, orange, and yellow is much

reduced when compared to normal reds may be confused with black or dark grey protanopes may learn to distinguish reds from yellows

and from greens "primarily” on the basis of their apparent brightness or lightness

not on any perceptible hue difference Likewise, violet, lavender, and purple are

indistinguishable from various shades of blue because their reddish components are so dimmed that they become to be invisible.

CCOOLLOOUURR DDEFICIENCIES EFICIENCIES

Deuteranopia the brightness of red, orange, and yellow and green is

much reduced when compared to normal not on any perceptible hue difference aside from being different names that every one else

around him seems to be in concurrence upon violet, lavender, purple, and blue all appear to be the

same to a viewer with deuteranopia but without the dimming

CCOOLLOOUURR DDEFICIENCIESEFICIENCIESTritanopia see the world in shades of reds and a type of

green/turquoise colour but this varies individuals with blue-yellow defects confuse colours from

yellow through green to blue tritanopes usually do not have as much difficulty in

performing everyday tasks as do individuals with either of the red-green variants of dichromacy

Because blue wavelengths occur at one end of the spectrum, and there is little overlap in sensitivity with the other two cone types, total loss of sensitivity across the spectrum can be quite severe with this condition


Normal

Protanopia

Deuteranopia

Tritanopia


NORMAL

PROTANOPIA

DEUTERANOPIA

TRITANOPIA

CCOOLLOOUURR DDEFICIENCIESEFICIENCIESANOMALOUS TRICHROMATISM need 3 wavelengths to match all colours in spectrum,

just like normal trichromats but they mix colour in different proportions than normal

trichromats do protoanomaly – deficiency in L cone pigments (reduced

sensitivity to reddish light) and the colours looks dim deuteranomaly – deficiency in S cone pigments

(reduced sensitivity to greenish light) but the brightness of the colour is not effected

can determine this condition using anomaloscope


Monochromacy Complete inability to distinguish any

colours is called monochromacy. It occurs in two forms:

rod monochromacy or achromatopsiacone monochromacy


Rod monochromatism or achromatopsia Where the retina contains no cone cells, so that in

addition to the absence of colour discrimination, vision in lights of normal intensity is difficult

Know as scotopic vision Do not provide a sharp image cause:

Adjacent rods are connected by gap junctions and so share their changes in membrane potential

Several nearby rods often share a single circuit to one ganglion cell

A single rod can send signals to several different ganglion cells It is cause of a disease called retinitis pigmentosa


Cone monochromatism Where only a single system appears to be

functioning, so that no colours can be distinguished, but vision is otherwise more or less normal

When all three types of cone cells are stimulated equally then light is perceived as being achromatic or white


NORMAL MONOCHROMACY


Colour defective vision can be addressed with colour vision tests

Ishihara colour vision testsIshihara plates consist of a series of dots the colours of which are arranged so as to represent different numbers

CLOSURE

The human eye is very complicated and sensitive. It is the only way to see the colours and understand the world.

Although the eye has millions of rods and cones, theories are explaining how the colour vision works and scientists are trying to solve the colour defective vision, some people will never see all the colours, will never understand what the rest of us can see.

SUMMARY

Understanding of vision Know how the eye works Appreciation of colour defective vision

REFERENCES In process colour monitoring (module notes): Dr Mark Bohan Colour Theory (module notes): Dr Kuriakos Stathakis –A.T.E.I. of Athens MIT Encyclopaedia of the Cognitive Sciences Robert Wilson / Frank Keil, 1999 http://webvision.med.utah.edu/sretina.html#overview http//www.e-paranoids.com/c/co/color_blindness.html http//:home.wanadoo.nl/paulschils/05.03.html http://www.accessexcellence.org/AE/AEC/CC/vision_background.html http://faculty.washington.edu/chudler/bigeye.html http://csep10.phys.utk.edu/astr162/lect/light/limitations.html http://www.yu.edu/faculty/rettinge/_private/perception/color.pdf http://www.tedmontgomery.com/the_eye/ http://en.wikipedia.org/wiki/Color_blindness http://www.webaim.org/techniques/visual/colorblind#deuteranopia http://micro.magnet.fsu.edu/optics/lightandcolor/vision.html http://www.psych.umn.edu/courses/HoldenJ/psy3031/psy3031day12.ppt#277,16,3

Kinds of Cones, 3 Pigments http://www.achromat.org/what_is_achromatopsia.html

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