Transcript The Human Visual System - RIT Center for Imaging Science

The Human Visual System
The Eye
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
In this section . . .
 Anatomy
of human eye
 Image formation by human eye
 Method of light detection
 Retinal processing
 Eye optical defects and diseases
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Human Visual System
Image formation
•Cornea
•Lens
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Exposure
Control
Detection
Processing
•Iris/pupil
•Photoreceptor
sensitivity
•Retina
•Rods
•Cones
•Brain
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Human Eye
Ciliary Muscle
Sclera
Iris
Ear side (Temporal)
Vitreous Humor
Pupil
Eyelens
Fovea
Retina
Optic Nerve
Cornea
Nose side (Nasal)
Aqueous Humor
Suspensory ligament
 Human
Choroid
eye is a complete imaging system.
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Image Formation
Object

Image
The curved surfaces of the eye focus the
image onto the back surface of the eye.
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Cornea
 The
Sclera
Cornea
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outer wall of
the eye is formed by
the hard, white
sclera.
 Cornea is the clear
portion of the sclera.
 2/3 of the refraction
takes place at the
cornea.
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Iris and Pupil
Iris
 Colored
iris controls
the size of the
opening (pupil)
where the light
enters.
 Pupil determines the
amount of light, like
the aperture of a
camera.
Pupil
Iris open
Dilated pupil
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Iris closed
Constricted pupil
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Lens
Ciliary muscle

Lens

Suspensory
Ligament

Transparent
Fibers

Eye lens is made of
transparent fibers in a
clear membrane.
Suspended by
suspensory ligament.
Used as a fine focusing
mechanism by the eye;
provides 1/3 of eye’s
total refracting power.
Non-uniform index of
refraction.
Cross section of the eye lens
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Accommodation
Distant object


Near object
Relaxed muscle
Taut ligaments

The suspensory ligaments
attach the lens to the ciliary
muscle.
When the muscle contracts,
the lens bulges out in the
back, decreasing its focal
length.
The process by which the
lens changes shape to focus
is called accommodation.
Contracted muscle
Slack ligaments
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Aqueous Humor and Vitreous Humor
 Transparent
Vitreous Humor
Aqueous Humor
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gelatinous liquid
filling the eye.
 Provides nutrients
to the cornea and
eye lens.
 Also helps maintain
the eyeball shape
with its pressure.
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Retina

Retina
Fovea


Optic Nerve

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Retina is the photosensitive
“detector” for the eye.
Two types of receptors in the
retina: rods for low light
level, and cones for color.
Located at the center of the
retina, fovea contains a
greater concentration of
cones.
Signals from the receptors
leave through the optic nerve
to the brain.
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Plexiform Layer

The retina is made of three
layers:
Plexiform layer is a network of
nerves which carry the
signals from the photo
Photo receptors
receptors.
 Photo receptors.
 Choroid provides
nourishment to the
receptors, as well as absorb
any light that didn’t get
absorbed by the photo
receptors, like a antihalation
backing in film.
Choroid

Fovea
Light
Plexiform Layer
Optic Nerve
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Rods and Cones
Synaptic endings
Cell nucleus
Inner segments
Outer segments
Rod



Highly sensitive to low light
level or scotopic conditions.
Black and white.
Dispersed in the periphery
of the retina.
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Cone



Sensitive to high light level
or photopic conditions.
Three types of cones
responsible for color vision.
Concentrated in the fovea.
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Adaptation

Threshold of detection
(log scale)
Photopic (cones)
0
Scotopic (rods)


5
10
15
20
25
Time in dark (minutes)
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30
Why can’t you see
immediately after you
enter a movie theater from
daylight?
The threshold of detection
changes with overall light
level.
The switch is quite gradual,
until the sensitivities of
cones and rods cross over
at about 7 minutes in the
dark.
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Distribution of Photoreceptors
Number of receptors per mm2
Temporal
Visual Axis
80 º
60 º
40 º
20 º
0º
160
140
120
100
80
60
40
20
Nasal
80º
60 º
40 º
20 º


Blind spot
Rods
Cones

Cones are concentrated
in the fovea.
Rods predominate the
periphery.
There is a blind spot
where there are no
photoreceptors, at the
point where the nerves
exit the eye (optic
nerve).
60 º 40 º 20 º 0 º 20 º 40 º 60 º 80 º
Angle
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Human Vision
 Human
 three
Cone Response to Color
cone types (S,I,L) correspond to B,G,R
I
L
Relative response
S
400
460 490 500
530
600
650
700
Wavelength (nm)
Blue
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Cyan
Green
Red
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Retina
Light
Cones
Rods
To optic nerve


Bipolar
cells
Amicrine
cells
Ganglion
cells
Horizontal
cells
The retina is made of network of nerve cells.
The network works together to reduce the amount of
information in a process called lateral inhibition.
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Hermann Grid

Illustrates lateral inhibition.
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Hermann Grid
A
B
Point A looks darker because there are 4 inhibitory
inputs
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Point
B looks lighter because there
are
onlyCenter
2 inhibitory

Mach Bands
Actual
brightness
Perceived
by you
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Eye Defects
Object at infinity

Image focuses on the
retina for a normal eye.

Distant objects look
blurry for a myopic (near
sighted) eye.

Near objects look blurry
for a hyperopic (far
sighted) eye.
Normal
Myopic
Hyperopic
Eyes at relax state.
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Myopia - Near sightedness

Far object
Myopic eye relaxed
Blurry
Near object
Myopic eye relaxed
In focus

Far object
Myopia corrected
with a negative lens

Distant objects look
blurry because the eye
cannot relax any farther
so that the image is
focused before the
retina.
Near object in focus
without accommodation.
Corrected with a
negative lens.
The virtual image from the diverging lens appears to be closer.
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Hyperopia - Far sightedness
Far object
Near object

Hyperopic eye
Partially accommodated
In focus
Hyperopic eye
Fully accommodated
Blurry
Near object


Near objects look
blurry because the eye
cannot accommodate
enough for near
objects.
Far object in focus.
Corrected with a
positive lens.
Hyperopia
corrected with a
positive lens
Light from the converging lens looks as though it is coming from the distance.
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Contact Lens
Contact lens
Cornea
Fluid
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 Contact
lens is an
alternative to
corrective lenses.
 Changes the
curvature of the
cornea by adhering
to the surface with
some fluid.
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Presbyopia - “Old eye”
Concave for distance
correction (if needed)
Convex for near object
correction
Far objects
Near objects magnified
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 Lens
hardens with age.
 Eye cannot adequately
accommodate near
objects.
 Bifocals (lens with two
focal lengths) contains
a concave lens for
distance (if needed)
and a convex lens for
near objects.
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Astigmatism

Cornea
Object
The cornea is not
spherical; Focal length
different from one
plane to a
perpendicular plane.
F’ horizontal
F’ Vertical
Direction of blur
Image at F’ Horizontal
Image at F’ Vertical
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Astigmatism
Cylindrical lens


Rays in the
horizontal plane are
focused
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Correction of astigmatism
is done through the use
of a cylindrical lens.
Cylindrical lens converge
rays in one plane but not
the perpendicular plane.
Rays in the vertical
plane are undeviated
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Common Eye Diseases

Cataract - Clouding of the
lens.



Symptom: Loss of vision
Cure: Lens replacement
Glaucoma - Pressure
buildup in the eye,
damaging the retina.


Symptom: Loss of vision first
in the periphery.
Cure: Surgery to drain fluid
from the eye.

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Loss of vision is usually
permanent
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Common Eye Diseases

Ow!
Detached retina - portion
of the retina detaches
from the back of the
eye.



Pink eye - Infection of
the surface of the eye.


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Symptom: Perception of
flashes, Loss of vision
Cure: Laser surgery to
reattach retina
Symptom: Irritation
Cure: Antibiotics
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Your eye care
Go see a doctor if you think
there is something wrong with
your eyeEarly detection is essential to
keeping damage low and
preventing permanent loss
of your vision.
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