Anatomy and mechanism

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The Human Visual System
The Eye

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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
Exposure
Control
Detection Processing
•Cornea
•Lens
•Iris/pupil
•Photoreceptor
sensitivity
•Retina
•Rods
•Cones
•Brain

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Human Eye
 Human eye is a complete imaging system.
Ear side (Temporal)
Nose side (Nasal)
Cornea
Aqueous Humor
Pupil
Iris
Ciliary Muscle
Sclera
Fovea
Retina
Optic Nerve
Vitreous Humor
Eyelens
Choroid
Suspensory ligament

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Image Formation
 The curved surfaces of the eye focus the
image onto the back surface of the eye.
Object
Image

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Cornea
 The 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.
Sclera
Cornea

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Iris and Pupil
 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.
Iris
Iris open
Dilated pupil
Iris closed
Constricted pupil
Pupil

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Lens
 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.
Lens
Suspensory
Ligament
Ciliary muscle
Transparent
Fibers
Cross section of the eye lens

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Accommodation
 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.
Relaxed muscle
Taut ligaments
Distant object
Near object
Contracted muscle
Slack ligaments

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Aqueous Humor and Vitreous Humor
 Transparent
gelatinous liquid
filling the eye.
 Provides nutrients
to the cornea and
eye lens.
 Also helps maintain
the eyeball shape
with its pressure.
Vitreous Humor
Aqueous Humor

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Retina
 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.
Retina
Optic Nerve
Fovea

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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
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.
Fovea
Plexiform Layer
Photo receptors
Choroid
Optic Nerve
Light

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Rods and Cones
 Highly sensitive to low light
level or scotopic conditions.
 Black and white.
 Dispersed in the periphery
of the retina.
Synaptic endings
Cell nucleus
Inner segments
Outer segments
Rod 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
 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.
Photopic (cones)
Scotopic (rods)
0 5 10 15 20 25 30
Time in dark (minutes)
Threshold
of
detection
(log
scale)

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Distribution of Photoreceptors
 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).
20 º
40 º
60 º
80 º
20 º
40 º
60 º
80º
0 º
Angle
0 º
20 º
40 º
60 º 80 º
60 º
40 º
20 º
20
40
60
80
100
120
140
160
Number
of
receptors
per
mm
2
Rods
Cones
Blind spot
Visual Axis
Nasal
Temporal

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Human Vision
Human Cone Response to Color
 three cone types (S,I,L) correspond to B,G,R
400 460 530 650
600 700
500
Wavelength (nm)
Relative
response
Blue Cyan Green Red
490
I L
S

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Retina
 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.
To optic nerve
Light Cones
Rods
Bipolar
cells
Amicrine
cells
Ganglion
cells
Horizontal
cells

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Hermann Grid
 Illustrates lateral inhibition.

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Hermann Grid
 Point A looks darker because there are 4 inhibitory
inputs
 Point B looks lighter because there are only 2 inhibitory
A B

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Mach Bands
Actual
brightness
Perceived
by you

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Eye Defects
 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
Object at infinity
Eyes at relax state.

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Myopia - Near sightedness
 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.
Myopic eye relaxed
Blurry
Myopia corrected
with a negative lens
Far object
The virtual image from the diverging lens appears to be closer.
Near object
Myopic eye relaxed
In focus
Far object

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Hyperopia - Far sightedness
 Near objects look
blurry because the eye
cannot accommodate
enough for near
objects.
 Far object in focus.
 Corrected with a
positive lens.
Hyperopic eye
Partially accommodated
In focus
Hyperopia
corrected with a
positive lens
Far object
Light from the converging lens looks as though it is coming from the distance.
Hyperopic eye
Fully accommodated
Blurry
Near object
Near object

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Contact Lens
 Contact lens is an
alternative to
corrective lenses.
 Changes the
curvature of the
cornea by adhering
to the surface with
some fluid.
Cornea
Contact lens
Fluid

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Presbyopia - “Old eye”
 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.
Near objects magnified
Far objects
Concave for distance
correction (if needed)
Convex for near object
correction

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Astigmatism
 The cornea is not
spherical; Focal length
different from one
plane to a
perpendicular plane.
F’ horizontal
F’ Vertical
Object
Image at F’ Vertical
Image at F’ Horizontal
Cornea
Direction of blur

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Astigmatism
 Correction of astigmatism
is done through the use
of a cylindrical lens.
 Cylindrical lens converge
rays in one plane but not
the perpendicular plane.
Cylindrical lens
Rays in the vertical
plane are undeviated
Rays in the
horizontal plane are
focused

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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.
 Loss of vision is usually
permanent

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Common Eye Diseases
 Detached retina - portion
of the retina detaches
from the back of the
eye.
 Symptom: Perception of
flashes, Loss of vision
 Cure: Laser surgery to
reattach retina
 Pink eye - Infection of
the surface of the eye.
 Symptom: Irritation
 Cure: Antibiotics
Ow!

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Your eye care
Go see a doctor if you think
there is something wrong with
your eye-
Early detection is essential to
keeping damage low and
preventing permanent loss
of your vision.