THE EYE AND THE
PERCEPTION OF
COLOUR
OVERVIEW
What is the colour
 The eye
 The sensation of the colour
 Colour vision effects
 Colour deficiencies
 Closure

WHAT IS THE COLOUR?

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The light result of the feeling produced by the electromagnetic waves in a spectral
field going from 380nm to 730nm.
This area is called: visible spectrum
Each wavelength corresponding has a perceptive feeling called “colour”. We translate
the colour as: red, yellow, green, cyan, blue, magenta
10-14
Cosmic
rays
10-12
X-rays
10-8
U-V rays
10-6
Visible spectrum
Infra red
10-4
1
Radar
10+2 (m)
Radio
WHAT IS THE COLOUR?

The daylight is white. Is it?
730 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
380 nm
HOW WE PERCEIVE THE COLOUR
THE EYE
Is 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 EYE
1. 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
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
Pigment
epithelium
Rods
Cones
Outer limiting
membrane
Muller cells
Horizontal
cells
Dipolar
cells
Amacrine
cells
Ganglion
cells
Nerve fiber
layer
Inner limiting
membrane
Diagram of organization of the retina
THE EYE
Rods & Cones
 The rods
 Are
most sensitive to light and dark changes, shape
and movement and contain only one type of lightsensitive 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
The fovea
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 fovea
Blood vessels and nerve fibers go
around the fovea so light has a
direct path to the photoreceptors.
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

Retina

Optical
centre
Optic
nerve

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
SENSATION OF THE COLOUR
Three main theories describe the colour
vision:
Trichromatic theory
 Hering opponent theory
 Modern opponent colour theory

SENSATION OF THE COLOUR
Trichromatic Theory
three receptor types with different spectral sensitivities
specific colour coded by pattern of responding across
receptors (distributed coding)
The ratios of the signals are used to define the colour
sensation
Light
Cones
Colour sensation
SENSATION OF THE COLOUR
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 COLOUR
Hering opponent theory
 adaptation responsible for afterimages
SENSATION OF THE COLOUR
Hering opponent theory
 The colour receptors had a red/green,
blue/yellow and dark/light response
Light
Colour
receptors
Colour sensation
SENSATION OF THE COLOUR
Modern opponent colour theory
Light
S
M
L
Three
receptor types with different
spectral sensitivities detect the light
Cones
Produce
three processed signals
that are then used to determine the
colour sensation
Cells
Colour sensation
It
is supported by psychological
experiments
COLOUR VISION EFFECTS

These effects and need to be taken into account
when trying to model the colour vision system:
CIELAB, RLAB, CIECAM97s

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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.
COLOUR VISION EFFECTS
Dark and light adaptation
 Dark adaptation
Occurs when the illumination decreases
 Example: walk into a darker room
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Light adaptation
Occurs when the illumination increases
 Example: walk into a dark room into daylight

COLOUR VISION EFFECTS
Simultaneous contrast
Is
the impact of the
surround on the colour
seen, which can make
the same colour appear
different
COLOUR DEFICIENCIES
Is the loss of colour discrimination
Causes:
 Genetic photoreceptor disorders

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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
COLOUR DEFICIENCIES
Types of colour defective vision:

Dichromism:
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Anomalous trichromatism:
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Protanopia
Deuteranopia
Tritanopia
Protanomaly
Deuteranomaly
Monochromatism:
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Rod monochromatism
Cone monochromatism
COLOUR DEFICIENCIES
COLOUR VISION DEFECTS
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 DEFICIENCIES
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.
COLOUR DEFICIENCIES
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
COLOUR DEFICIENCIES
Tritanopia
 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
COLOUR DEFICIENCIES
Normal
Protanopia
Deuteranopia
Tritanopia
COLOUR DEFICIENCIES
NORMAL
DEUTERANOPIA
PROTANOPIA
TRITANOPIA
COLOUR DEFICIENCIES
ANOMALOUS 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
COLOUR DEFICIENCIES
Monochromacy
 Complete inability to distinguish any
colours is called monochromacy.
 It occurs in two forms:
 rod
monochromacy or achromatopsia
 cone monochromacy
COLOUR DEFICIENCIES
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:


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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
COLOUR DEFICIENCIES
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
COLOUR DEFICIENCIES
NORMAL
MONOCHROMACY
COLOUR DEFICIENCIES
Colour defective vision can be addressed
with colour vision tests
Ishihara colour vision tests
Ishihara
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
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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
Thank you for listening
Any questions?