version 0.10 (c) 2007 celest visi n brightness contrast classroom presentation pre-experiment how...

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VISINBRIGHTNESS CONTRAST

CLASSROOM PRESENTATION PRE-EXPERIMENT

How many black dots can you count?

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WHITE’S ILLUSION

How many shades of gray are in the picture?

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Are squares A and B the same shade of gray?

ADELSON’S CHECKERED SHADOW ILLUSION

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COMMON MISCONCEPTION ABOUT VISION: EXTRA-TRANSMISSION

Some people believe that when they look at something energy or waves are emitted from the eyes – extra-transmission

“Extra” means out

“transmission” means to send or move

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If vision is dependent on light

entering the eye,

CAN YOU SEE THINGS IN THE DARK?

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HOW DO WE SEE?

However, vision is a perceptual process involving many parts of the brain that do not work like a camera.

Because sight is based on light entering the eyes, it might appear that our eyes work like cameras to produce images of objects.

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Consider this:

When we turn on a light bulb it produces light

Not all objects produce light, but all visible objects reflect light into our eyes.

This reflected light enables us to see the object

Luminance : Amount of reflected light energy that enters the eye.

Illuminant: Light source.

Illuminance: Amount of light energy emitted from a light source. Illuminance is measured in lux (lx).

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When light falls on a surface, some of it is absorbed and some of it is reflected.

HOW DO WE SEE OBJECTS THAT DO NOT PRODUCE LIGHT?

reflected

absorbed

Our eyes can detect the reflected light.*The wavelengths of light reflected determine the

color(s) we see.

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DARK AND LIGHT SURFACES

absorb most of the lightthat falls on them, andreflect little.

reflected

absorbed

reflected

absorbed

Dark surfaces Light surfacesabsorb little of the light that falls on them, andreflect most.

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If vision is dependent on light

entering the eye, CAN YOU SEE THINGS IN THE

DARK?

No light… …no sight!

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There are two types of photoreceptors in the human retina, rods and cones

Rods handle vision in low light, and cones handle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The chemical that is formed (activated rhodopsin) creates electrical impulses in the optic nerve.

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EDGE DETECTORS!

Visual perception is sensitive to the difference or contrast between two surfaces.

In terms of vision, contrast is the comparison between the reflected light of one surface and another.

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WHAT IS “PERCEPTION”?

Some perceptions are illusory.

Perception is the process of acquiring, “interpreting,” selecting, and organizing sensory information.

Do you always see what is “really” there?

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How does the human brain and eye

accomplish the task of perceiving?

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THE HUMAN EYE

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The eye is actually an organ in the shape of a ball. From the outside you can see the pupil and the iris. The pupil is an opening that permits light to enter the eye. The iris is the muscle that controls the size of the pupil.

Pupil

Iris

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The light bends when it passed through the cornea, then enters the pupil. The lens bends the light to

focus an image on the retina at the back of the eye. The photoreceptors receive the light image and

send information to the brain via the optic nerve.

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VISION AND THE BRAINLight stimulates photoreceptors sending messages to

the visual cortex.

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THE VISUAL PATHWAY

Adapted from Frisby 1979

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BASIC DEFINITIONS

Luminance: Amount of reflected light energy that enters the human eye after being reflected from the surface of objects. Luminace is measured in candela per square meter (cd/m2).

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REFLECTED LIGHT

Illuminant

IlluminanceLuminance

Photoreceptors on the retina

Illuminant: Light source.

Illuminance: Amount of light energy emitted from a light source. Illuminance is measured in lux (lx).

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BASIC DEFINITIONS

Reflectance: Physical property of an object’s surface. Calculated by a ratio of luminance divided by illuminance. Reflectance is measured in percentage (%).

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REFLECTED LIGHTReflectance

Photoreceptors on the retina

BrightnessPerception

Brightness: A subjective measure of the perception of luminance.

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MODELS

The most abstract models are mathematical representations

where variables represent realities.

We can develop a better understanding of vision

using models.

A model is an abstract representation of an everyday experience.

Some models look very similar to what they represent,

other models are more abstract--like a diagram of the

situation.

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=

ILLUMINATION, REFLECTANCE, AND BRIGHTNESS

R(a) =

E1

I1

Illuminance: I1

Luminance: E1

a R = E

I

Brightness constancy is due to discounting the illuminant.

In general:

Reflectance = LuminanceIlluminance

The reflectance has the same value under different lighting

conditions. =

E2

I2

Illuminance: I2

Luminance: E2

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=R(b) R(b) R(a)

E(a)

a

b

E(b)

RE

I

E(a)/I

E(b)/I=

E(a)

E(b)

BRIGHTNESS PERCEPTION IS BASED ON COMPARING TWO SURFACES

R(a)

R(b) =

= = E(a)

I E(b)

Ix

E(a)

IE(b)I

÷

The contrast between two surfaces is compared as a ratio of their luminances.

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CALCULATING CONTRAST

Inner A appears brighter than inner B because of higher contrast.

2.0 = 200% 0.5 (50%)

A B

Inner Inner

Outer Outer

E(inner A)E(outer A)

E(inner B)

E(outer B)Contrast =

50

25

50

100

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MODELS OF BRIGHTNESS CONTRAST

PHYSICAL SIMILAR ABSTRACT

R(a)

R(b) R(b) =

E(b)

I

R(a) = E(a)

I

BRIGHTNESS CONTRAST

REFLECTANCE

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WHITE’S ILLUSION

How many shades of gray are in the picture?

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WHITE’S ILLUSION

Only one!

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Are squares A and B the same shade of gray?

ADELSON’S CHECKERED SHADOW ILLUSION

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Yes!

ADELSON’S CHECKERED SHADOW ILLUSION

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BRIGHTNESS CONTRAST REVISTED

A B

Inner

Outer Outer

Can you believe that the reflectance of Inner(A) is EQUAL to the reflectance of Inner(B)?

Why did the inner boxes appear to be the same now?

Inner Inner