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What is
Vision ?
VISION
“A process that produces
form images of the external
world, a description which
is useful to the viewer and
not cluttered with the
irrelevant information.”
David Marr
What is
Light ?
LIGHT
Part of
Electro Magnetic
Spectrum to which the
Eyes are sensitive
What is
Electromagnetic
Spectrum ?
TV
9
200nm
10,000nm
Optical
Radiations
Radar
6
X-rays
1
UV
Micro
3
IR
Gamma
-3
Cosmic
-6
400nm
Visible
Light
Radio
12
780nm
Properties of light
• Travels in straight lines
Properties of light
• Reflects from surfaces
angle of incidence (qi)
=
angle of reflection (qr)
Properties of Light
• Refracts at boundaries of transparent
materials
sin (qi) = n sin (qr)
qi
n is refractive index
of dense material
air
dense
medium
surf ace
qr
n ≈ 1.1 - 1.3
(depends on material)
Frequency corresponds to
colour
OPTICAL MEDIA
REFRACTION
Human eye
Aqueous
humor
Retina
Vitreous
humor
Pupil
Fovea
Optic nerve
fibers
Cornea
Iris
Lens
Refraction quiz
A
B
C
n1
n2
1
2
3
Refraction review
n1
n2
n1<n2
The eyes
receivers of information
Shaping and transforming light
into sensory perceptions
The eye - receiver of
information
• Light sources: sun, light bulbs, candles,
moon
• Light reflects off of objects in environment
sun
obse rver
The eye - receiver of
information
• Light sources: sun, light bulbs, candles,
moon
• Light reflects off of objects in environment
objects effectively become light sources
sun
obse rver
Photoreceptor
“receptor of light [photons]”
• photoreceptor cell transforms light into
nerve impulses
incoming light ray
photoreceptor
output of cell
(signal along axon)
Implications for vision
• Does not matter if an object is a source or
a reflector of light
• Strength of reflection is a function of:
– color of object
– smoothness of object
– relative orientation between light rays, surface
normal, and observer
Human Eye
• lens eye
• array of photoreceptors: retina
– rods and cones
• focusing: cornea plus crystalline lens
• photoreceptors are “backwards”
– axons (nerves) leave through blind spot
Representing objects by edges
• Very efficient compression
• 100 million axons from photo receptors per
eye
• 1 million axons from ganglion cells per eye
• Reduction by a factor of 100!!
LIGHT
Retina
Receptors
Brain
Process
Vision
Sunset
What will happen if
Colors are not
perceived ?
RECEPTORS
100 Million Rods & Cones
Billion Cells in Three Layers
10 Billion Calculations /Sec
1.2 Million Axons
Optic Nerve
> Dozen Vision Centers
Three Dimensional
World
\FantasticTrip.pp
s
Global Facts
BLIND
VISION 3/60 OR LOW
IN BEST SEEING EYE
WHO ESTIMATES
42 – MILLION BLINDS IN
THE WORLD
HALF ARE DUE TO
CATARACT
CATARACT WITH VISION
3/60 OR LOW
20 M WILL INCREASE TO 40
M IN YEAR 2020
• Globally, at least 100 million eyes
have visual acuity < 6 / 60 due to
cataract.
• Annually, at least 25 million eyes
develop visual acuity < 6/60 due
to cataract.
ONE MILLION OPERATED
EVERY YEAR
BUT
1.5 M ADDED EVERY YEAR
SO
WORLD IS BECOMING MORE
BLIND
NUMBER OF EYE
SURGEONS
UK - 14 PER MILLION
AFRICA – 1 PER MILLION
CATARACT SURGURY
RATE
CSR
UK – 4500 PER MILLION
AUSTRALIA – 6500 PER MILLION
AFRICA – 200-400 PER MILLION
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