The Physical Stimulus for Vision

Light

A form of Electromagnetic Energy

Two Complementary Descriptions

Particle - Photon is unit of light.

Wave - like a wave in ocean

Electromagnetic spectrum

gamma - 10-14 - 10-12

x-rays - -- ~10-9

Ultraviolet -- ~760 nm (>10-7)

° Visual 760-380 nm

Infrared <10-6 - ~10-3

radar -- 10-1

radio bands > over a mile

Measures of Light

Wavelength relates to color, e.g., the spectrum

Usually measured in billionths of a meter - nanometers or nm

Measures of Light - 2

Amplitude = Intensity relates to brightness

Measures use only that light that is effective in stimulating the human eye.

Important types of measures of Amplitude

1. Illuminance - light falling on a surface

2. Luminance - light coming off a surface

3. Reflectance = luminance/illuminance

4. Contrast Ratio = luminance of brightest area/luminance of darkest area

Structures of the Eye

1. Sclera

2. Cornea

3. Aqueous Humor

4. Iris

5. Pupil

6. Lens

7. Ciliary muscle

8. Retina

9. fovea (pit)

10. blind spot/optic disc

11. Pigment Epithelium

The Retina

Two Types of Receptors

Rods

~120 million/eye

night vision

no color

not in fovea

most about 20deg in periphery

Cones

~7 million/eye

day vision

three types so color vision

most in fovea

The Retina - 2

Slide 7

Slide 8

Craik-Cornsweet: Filling In

Craik-Cornsweet: Described

Minimal Contours

Minimal Contours Described

Accommodation

DEFINITION: The adjusting of the lens thickness to focus at different distances.

Necessary because can only see clearly one distance at a time

Goes very rapidly

Closes can focus in Near Point

Farthest can focus is Far Point

Loose ability to focus as age - moves towards far point

In dark accommodation moves to ~1 meter from face

As fatigue, accommodation moves to this dark focus.

Acuity

DEFINITION: ability to resolve or see fine details.

Visual Angle: DEFINITION: Angle formed by object on retina.

Types of Acuity: what is meant by acuity depends upon the stimulus used to measure it.

Detection: black bar on white field

Resolution: a grating

Recognition: e.g. Snellen, where

you read letters.

Acuity - 2

Measures of Acuity

20/20: can see at 20’ what a normal person can see at 20’.

This is normal, not perfect, vision.

20/200: can see at 20’ what a normal person can see at 200’.

Visual angle of the critical feature in a test, e.g. the width of the bars in a grating.

A typical population average is 1 arcmin (1/60 degree).

Acuity and Retinal Location:

Best at fovea. Falls off rapidly in periphery. Is tied to density of cones.

Contrast Sensitivity

DEFINITION: the minimum contrast ratio that can be detected.

Indicates the smallest difference between shades of gray that can be detected.

Depends on Spatial Frequency

DEFINITION: how many pairs of white and black bars fit into 1 deg. of visual angle.

Low spatial frequency

few bars/deg.

High spatial frequency

many bars/deg.

The Contrast Sensitivity Function

Our sensitivity to contrast depends on this spatial frequency.

Peak sensitivity is 4-6 cycles/degree.

The highest spatial frequency we can see at any contrast is limited by our acuity.

Contrast Sensitivity and Performance

Increasing contrast above threshold will allow for faster identification, up to a point

Beyond a certain contrast ratio - about 3 to 1 or 4 to 1 - increasing contrast ratio has no effect (Krantz, Silverstein, & Yeh, 1992)

Fundamental Limits of Vision

Operating Range of Vision - 1:10¹⁴

if:

1 cent (threshold) =10⁰

Most incomes are between $10,000 and $100,000 = 10⁶ to 10⁷

GNP of U.S.A. for 1 year

= ~10¹⁵

Fundamental Limits of Vision - 2

Duplex Theory of Vision

Two eyes in one. One for day. One for night.

Dark/Light Adaptation

Dark Adaptation

The term applied to the increased sensitivity that occurs when we enter a region of lowered sensitivity.

Two phases:

early rapid phase - lasts ~7 minutes and due to cones.

later slower phase - complete in about 30 to 40 minutes due to rods.

Light Adaptation

Takes 2-3 minutes

Eye Movements

We move our eyes because of the limited field with good acuity.

There are 6 eye muscles

4 rectus

2 oblique

Types of Eye Movements

Version: Both eyes move together

Vergence: Eye move in opposite directions

Eye Movements - 2

Version

Saccades, most common

places object on fovea.

can be > 400 deg/sec.

Takes ~ 200 msec to begin

Smooth Pursuit

track moving objects

relatively slow ~30 deg/sec.

Vergence:

Convergence (together) and divergence (apart)

Allows us to look at closer and farther objects.

relatively slow and also takes about 200 msec to begin

Color Vision I: Color Matching

Elements of Color

Hue: refers to the color name we apply.

Saturation: purity of color, to desaturate add white

Brightness:

Trichromatic Theory of Vision

We have three classes of cones

L or red: peak at 564 nm

M or green: peak at 534 nm

S or blue: peak at 420 nm

Color Vision I: Color Matching - 2

Color Matching in the Trichromatic Theory

Two patches of light will appear the same if the activity across the three cones is the same, regardless of wavelengths making up the two patches.

In general, can match any one color with three other colors

This is where we get three primaries.

Neutral colors - whites and grays

a balance of activity across the three receptors.

Color Vision I: Color Matching - 3

Formalization of the Trichromatic Theory

By CIE originally in 1931

Based on 300 observers

to develop standard

observer.

A set of Equations that

allow predictions of

matching.

Used in photo printing,

TV and film.

Updates in 1960, 1976

Color Vision II: Color Deficiencies

Most can be understood using Trichromatic Theory

Dichromatism: Missing one of the three cones

Dichromats tend to see through camouflage better than thrichromats

Other Types

Monochromatism: One cone or only rods

Anomalous Trichromats: Three cones but one is different.

Color Vision III: Color Appearance

Color Opponent Theory

Four Primaries: red-green, blue-yellow

Arranged in opposition pairs

Red vs. Green

Blue vs. Yellow

Add on to other get neutral color

Never see a mixture of opposition pairs.

Evidence:

complimentary colors, color aftereffect, simultaneous contrast, color naming - try it with just red, green blue and yellow

Cells in visual system respond this way.

Color Aftereffect

Test Color Aftereffect

Color Vision IV: Resolution

Depth Perception

If retinas are flat (2 dimensional) how do we see depth (the 3^rd dimension)?

We use cues: sources of information about depth.

Monocular or one eye cues

Depth Perception - 2

Binocular or two eye cues

Vergence (Only cue to give absolute depth information):

muscular feedback from effort to converge or diverge gives information about depth.

works only for relatively near objects: <20’

Stereopsis

Binocular Disparity: measure of difference of position of an object on the two retinas

DEFINITION: ability to use binocular disparity to see depth.

Basis of 3-D movies

Accuracy of Depth Judgments:

In general, more cues more accurate.

Depth Perception - 3

Size Constancy

DEFINITION: seeing objects as a relatively constant size despite change in retinal image size.

S_{retinal image}a 1/distance to object (a mean proportional to)

S_perceived = Constant

Can be quite useful in object recognition

A Variation is Emmert’s Law for after effects

S_{retinal image}= Constant

S_perceived a distance

Also applies to depth generated by stereopsis

Depth Perception - 4

To experience Emmert’s Law fixate the center of the dot below for about 45 seconds. Then quickly view the next slide and note the size. Then look at surfaces of different distances, also noting the size.

Depth Perception - 5


Light
	A form of Electromagnetic Energy
	Two Complementary Descriptions
		Particle - Photon is unit of light.
		Wave - like a wave in ocean
				Electromagnetic spectrum
				gamma - 10-14 - 10-12
				x-rays - -- ~10-9
				Ultraviolet -- ~760 nm (>10-7)
			° Visual 760-380 nm
				Infrared <10-6 - ~10-3
				radar -- 10-1
				radio bands > over a mile


	Wavelength relates to color, e.g., the spectrum






		Usually measured in billionths of a meter - nanometers or nm


Amplitude = Intensity relates to brightness
	Measures use only that light that is effective in stimulating the human eye.
	Important types of measures of Amplitude
		1. Illuminance - light falling on a surface
		2. Luminance - light coming off a surface
		3. Reflectance = luminance/illuminance
		4. Contrast Ratio = luminance of brightest area/luminance of darkest area


	1. Sclera
	2. Cornea
	3. Aqueous Humor
	4. Iris
	5. Pupil
	6. Lens
	7. Ciliary muscle
	8. Retina
		9. fovea (pit)
		10. blind spot/optic disc
	11. Pigment Epithelium


Two Types of Receptors
	Rods
		~120 million/eye
		night vision
		no color
		not in fovea
		most about 20deg in periphery
	Cones
		~7 million/eye
		day vision
		three types so color vision
		most in fovea


DEFINITION: The adjusting of the lens thickness to focus at different distances.
	Necessary because can only see clearly one distance at a time
	Goes very rapidly
	Closes can focus in Near Point
	Farthest can focus is Far Point
	Loose ability to focus as age - moves towards far point
	In dark accommodation moves to ~1 meter from face
		As fatigue, accommodation moves to this dark focus.


	DEFINITION: ability to resolve or see fine details.
	Visual Angle: DEFINITION: Angle formed by object on retina.
	Types of Acuity: what is meant by acuity depends upon the stimulus used to measure it.
		Detection: black bar on white field
		Resolution: a grating
		Recognition: e.g. Snellen, where
		you read letters.


Measures of Acuity
	20/20: can see at 20’ what a normal person can see at 20’.
		This is normal, not perfect, vision.
	20/200: can see at 20’ what a normal person can see at 200’.
	Visual angle of the critical feature in a test, e.g. the width of the bars in a grating.
		A typical population average is 1 arcmin (1/60 degree).
Acuity and Retinal Location:
	Best at fovea. Falls off rapidly in periphery. Is tied to density of cones.


DEFINITION: the minimum contrast ratio that can be detected.
	Indicates the smallest difference between shades of gray that can be detected.
Depends on Spatial Frequency
	DEFINITION: how many pairs of white and black bars fit into 1 deg. of visual angle.
	Low spatial frequency
		few bars/deg.
	High spatial frequency
		many bars/deg.


	Our sensitivity to contrast depends on this spatial frequency.
		Peak sensitivity is 4-6 cycles/degree.
		The highest spatial frequency we can see at any contrast is limited by our acuity.


	Increasing contrast above threshold will allow for faster identification, up to a point
	Beyond a certain contrast ratio - about 3 to 1 or 4 to 1 - increasing contrast ratio has no effect (Krantz, Silverstein, & Yeh, 1992)


	Operating Range of Vision - 1:10¹⁴
			if:
			1 cent (threshold) =10⁰
			Most incomes are between $10,000 and $100,000 = 10⁶ to 10⁷
			GNP of U.S.A. for 1 year
			= ~10¹⁵


	Duplex Theory of Vision
		Two eyes in one. One for day. One for night.


Dark Adaptation
	The term applied to the increased sensitivity that occurs when we enter a region of lowered sensitivity.
	Two phases:
		early rapid phase - lasts ~7 minutes and due to cones.
		later slower phase - complete in about 30 to 40 minutes due to rods.
Light Adaptation
	Takes 2-3 minutes


	We move our eyes because of the limited field with good acuity.
	There are 6 eye muscles
		4 rectus
		2 oblique
	Types of Eye Movements
		Version: Both eyes move together
		Vergence: Eye move in opposite directions


Version
	Saccades, most common
		places object on fovea.
		can be > 400 deg/sec.
		Takes ~ 200 msec to begin
	Smooth Pursuit
		track moving objects
		relatively slow ~30 deg/sec.
Vergence:
	Convergence (together) and divergence (apart)
		Allows us to look at closer and farther objects.
		relatively slow and also takes about 200 msec to begin


Elements of Color
	Hue: refers to the color name we apply.
	Saturation: purity of color, to desaturate add white
	Brightness:
Trichromatic Theory of Vision
	We have three classes of cones
		L or red: peak at 564 nm
		M or green: peak at 534 nm
		S or blue: peak at 420 nm


Color Matching in the Trichromatic Theory
	Two patches of light will appear the same if the activity across the three cones is the same, regardless of wavelengths making up the two patches.
	In general, can match any one color with three other colors
		This is where we get three primaries.
	Neutral colors - whites and grays
		a balance of activity across the three receptors.


	Formalization of the Trichromatic Theory
		By CIE originally in 1931
		Based on 300 observers
		to develop standard
		observer.
		A set of Equations that
		allow predictions of
		matching.
		Used in photo printing,
		TV and film.
		Updates in 1960, 1976


	Most can be understood using Trichromatic Theory
	Dichromatism: Missing one of the three cones





		Dichromats tend to see through camouflage better than thrichromats
	Other Types
		Monochromatism: One cone or only rods
		Anomalous Trichromats: Three cones but one is different.


Color Opponent Theory
	Four Primaries: red-green, blue-yellow
	Arranged in opposition pairs
		Red vs. Green
		Blue vs. Yellow
		Add on to other get neutral color
		Never see a mixture of opposition pairs.
	Evidence:
		complimentary colors, color aftereffect, simultaneous contrast, color naming - try it with just red, green blue and yellow
		Cells in visual system respond this way.


	If retinas are flat (2 dimensional) how do we see depth (the 3^rd dimension)?
		We use cues: sources of information about depth.
	Monocular or one eye cues


Binocular or two eye cues
	Vergence (Only cue to give absolute depth information):
		muscular feedback from effort to converge or diverge gives information about depth.
		works only for relatively near objects: <20’
	Stereopsis
		Binocular Disparity: measure of difference of position of an object on the two retinas
		DEFINITION: ability to use binocular disparity to see depth.
		Basis of 3-D movies
	Accuracy of Depth Judgments:
		In general, more cues more accurate.


Size Constancy
	DEFINITION: seeing objects as a relatively constant size despite change in retinal image size.
		S_{retinal image}a 1/distance to object (a mean proportional to)
		S_perceived = Constant
	Can be quite useful in object recognition
A Variation is Emmert’s Law for after effects
		S_{retinal image}= Constant
		S_perceived a distance
	Also applies to depth generated by stereopsis


	To experience Emmert’s Law fixate the center of the dot below for about 45 seconds. Then quickly view the next slide and note the size. Then look at surfaces of different distances, also noting the size.