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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
SENSATION OF THE CCOOLLOOUURR
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
SENSATION OF THE CCOOLLOOUURR
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
SENSATION OF THE CCOOLLOOUURR
Hering opponent theory adaptation responsible for afterimages
SENSATION OF THE CCOOLLOOUURR
Hering opponent theory The colour receptors had a red/green,
blue/yellow and dark/light response
Colour sensation
Colour receptors
Light
SENSATION OF THE CCOOLLOOUURR
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.
CCOOLLOOUURR VISION EFFECTS
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
CCOOLLOOUURR VISION EFFECTS
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
CCOOLLOOUURR DDEFICIENCIESEFICIENCIES
Types of colour defective vision:
Dichromism: Protanopia Deuteranopia Tritanopia
Anomalous trichromatism: Protanomaly Deuteranomaly
Monochromatism: Rod monochromatism Cone monochromatism
CCOOLLOOUURR DDEFICIENCIESEFICIENCIES
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
CCOOLLOOUURR DDEFICIENCIESEFICIENCIES
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
CCOOLLOOUURR DDEFICIENCIES EFICIENCIES
Normal
Protanopia
Deuteranopia
Tritanopia
CCOOLLOOUURR DDEFICIENCIES EFICIENCIES
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
CCOOLLOOUURR DDEFICIENCIESEFICIENCIES
Monochromacy Complete inability to distinguish any
colours is called monochromacy. It occurs in two forms:
rod monochromacy or achromatopsiacone monochromacy
CCOOLLOOUURR DDEFICIENCIESEFICIENCIES
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
CCOOLLOOUURR DDEFICIENCIESEFICIENCIES
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
CCOOLLOOUURR DDEFICIENCIES EFICIENCIES
NORMAL MONOCHROMACY
CCOOLLOOUURR DDEFICIENCIES EFICIENCIES
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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