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ECE 6397:Visual System:
Physiology, Computation, and Methods
Title
Prof. Valery KalatskyDept. of Electrical & Computer Engineering
University of Houston
Lecture XX
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We can only see the visible spectrum.
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Visible Spectrum
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The range of EMR visible to humans is also called Light.
What is Light?
e v s e spec rum c ose y resem es e range o a ers roug
our atmosphere from the sun.
Other creatures see different ranges of visible light, for example bumble-bees
can see ultraviolet light and dogs have a different response to colors than dohumans.
The sky is blue because our atmosphere scatters light and the shorter
.illuminated by a blue light but in fact that light is scattered from the sun. Thelonger wavelengths like red and orange move straight through the atmosphere
which makes the sun look like a bright white ball containing all the colors of thevisible spectrum.
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Human Visual System
The Eye: iris and pupil
The eye can be compared to a TV camera
attached to an automatically tracking tripoda machine that is self-focusing,
,has a self-cleaning lens, andfeeds into a computer with parallel-processing capabilities
The colored part of the eye, the iris, controls light levels inside the eye.
The round opening in the center of the iris is called the pupil.The iris is embedded with tiny muscles that dilate and constrict the pupil size.
Range of pupil diameters 1 8 mm
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Cornea
The cornea is the transparent, dome-shaped window
covering the front of the eye.
It is a powerful refracting surface, providing 2/3 of the eye's focusing power.. . .
There are no blood vessels in the cornea, it is normally clear and has a
shiny surface. The cornea is extremely sensitive - there are more nerveendings in the cornea than anywhere else in the body
The adult cornea is only about 1/2 millimeter thick and
has a diameter of about 12 mm
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Lens
The purpose of the lens is to focus light onto the back of the eye.
Optical power 20 diopters (relaxed). Whole eye 60.
Change in power due to accommodation 8.Index of refraction 1.39 1.41 (high in center, low at edges)
The lens is about 5 mm wide and has a diameter of about 9 mmfor an adult human
The lens is encased in a capsular-like bag and suspended
within the eye by tiny ligaments called zonules.
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Retina
The retina is a very thin layer of tissue that lines the inner part of the eye.
It is responsible for capturing the light rays that enter the eye.Much like the film's role in photography.
These light impulses are then sent to the brain for processing,via the optic nerve.
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Retina: Gross View
Human retina through ophthalmoscope
Optic nerve, a circular to oval white area
Approximately 17 degrees (4.5-5 mm), to the left of the disc, can be seenthe slightly oval-shaped, blood vessel-free reddish spot, the fovea,
which is at the center of the macula.
measuring about 2 x 1.5 mm across.From the center of the optic nerve radiatethe major blood vessels.
A circular field of approximately 6 mm around the fovea is considered the
central retina.
The total retina is a circular disc of approximately 42 mm diameter.
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Retina
The retina is approximately 0.5 mm thick
Retina consists of about 5 layers
Light passes through all these layers beforereaching the photoreceptors
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Retina: Structure
Pigment epithelium
Photoreceptor, outer segments
Photoreceptor nuclei, outer nuclear layer
Network of connections (plexus), outer plexiform
layer
Bipolar cells, inner nuclear layer
Network of connections, inner plexiform layer
Ganglion cells
Nerve fibers
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Rods & Cones
Rods Cones
Sensitivity to light High, night vision Lower, day vision
Photopigment More Less
Amplification High Lower
Temporal resolution Low, slow response High, fast
Acuity Low, not present incentral fovea
High, concentrated infovea
Achromatic: 1 type
510 nm
Chromatic: 3 types
430, 530, 560 nm
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Spectral Sensitivity
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Dynamic Range
Typical ambient luminance levels (cd/m2):
Absolute threshold: 10-6
Starlight: 0.001Moonlight: 0.1
Indoor lighting: 100
Sunlight: 10.000
Maximum intensity of common CRT monitors: 100
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Extraocular Muscles
The six tiny muscles that surround the eye
and control its movements are knownas the extraocular muscles.
The primary function of the four rectusmuscles is to control the eye's
movements from left to right and up and down.
The two oblique muscles move the eye rotatethe eyes inward and outward.
All six muscles work in unison to move the eye.As one contracts, the opposing muscle relaxes, creating smooth movements.
In addition to the muscles of one eye working together in a coordinated effort,
the muscles of both eyes work in unison so that the eyes are always aligned.
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Accommodation
Far point Near point
Objective distance 1S = 1 12cmS =
2 2
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Visual Pathways
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Visual Cortex
Occipital Lobe
Primary Visual Cortex
V1
Area 17
Striate Cortex
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Sensory Modalities
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Stereopsis
The image cast on our retinas is two-dimensional, but we look out on a
three-dimensional world.
away things are.
Stereopsis is the process in visual perception leading to perception of
the depth or distance of objects.
It comes from two Greek roots, stereomeaning solidity, and opsismeaning vision or sight.
That means it could refer to any sort of visual depth perception,but since about the 1960s it has come to refer to depth perception from
binocular vision, requiring two eyes.
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Monocular Cues
1. Relative size
. n erpos on
3. Linear perspective
4. Aerial perspective
5. Light and shade
6. Monocular movement parallax
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Relative size
Retinal image size allow us to judge distance based on our past and
present experience and familiarity with similar objects.
A retinal image of a small object is interpreted as a distant object
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Interposition
Interposition cues occur when there is overlapping of objects.
The overlapped object is considered further away
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Linear perspective
When objects of known distance subtend a smaller and smaller angle,
it is interpreted as being further away.
Parallel lines converge with increasing distance such as roads, railway lines
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Aerial perspectiveRelative color of objects give us some clues to their distance.
Contrast of objects also provide clues to their distance.
,is perceived as distant.
Mountains are perceived to be closer when the atmosphere is clear.
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Light and shade
Highlights and shadows can provide information about an object's
dimensions and depth.
Because our visual system assumes the light comes from above,a totally different perception is obtained if the image is viewed upside down.
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Monocular movement parallax
When our heads move from side to side, objects at differentdistances move at a different relative velocity.
Closer objects move "against" the direction of head movement
and farther objects move "with" the direction of head movement.
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Other Monocular Cues
1. Accomodation
2. Convergence
3. Texture Gradient
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Binocular Cues
Observer fixates on P images of P fall onto
foveas (F)
Q is another point in space, which appears to the
observer to be the same distance away as P,and suppose QL and QR are the images of Q on
the left and right retinas.
QL and QR are corresponding points on the two
retinas.
The surface made up of all points Q, the sameapparent distance away as P, is the horopter
through P.
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Binocular Cues
If Q' appears closer to the observer than Q,
then the images of Q' (QL' and QR') will be
farther apart on the retina in a horizontaldirection than they would be if they
were corresponding points.
If Q' appears farther away, QL' and QR' will be
horizontally displaced toward each other
Contrast
f b
M
f b
I IC
I I
=
+
For the foreground If and background Ibluminance, the Michelson contrast is
In order for us to see an object, it must have a contrast higher than a certainthreshold
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Color
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give additional information about the environment.