00 - Human Visual System Overview

8/6/2019 00 - Human Visual System Overview

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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.

00 - Human Visual System Overview

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