Slides on color vision for ee299 lecture

Prof. M. R. GuptaJanuary 2008

Color is an event

light source

object has absorptionspectra and reflectancespectra

human cones respond:

???

humanperceives color

w

1

(oversimplified, linear model) L = long wave = redM = medium wave = greenS = short wave = blue

Visible spectrum

Are all the colors we see in the rainbow?

Cone spectral sensitivities

L = long wave = redM = medium wave = greenS = short wave = blueimage from: www.omatrix.com/uscolors.html

What does it mean to see black?

light source

human cones respond

???humanperceives color

L = long wave = redM = medium wave = greenS = short wave = blue

What does it mean to see white?

light source

human cones respond

???humanperceives color

L = long wave = redM = medium wave = greenS = short wave = blue

Blackbody illuminants

Ex: tungstenincandescent light bulb:

3000 K

set of illuminants:sun,candles,hot stuff incandescents

Example blackbody curves

Illuminantsoften describedby theircorrelated colortemperature

Ex: tungstenincandescent light bulb:

3000 K

What does it mean to see white? images from: www.omatrix.com/uscolors.html

You can see “white” given light made up of 2-spectra

What does it mean to see white? images from: www.omatrix.com/uscolors.html

You can see “white” given light made up of 2-spectra

(spectra that cause the samecolor sensation called metamers)

What does it mean to see white? images from: www.omatrix.com/uscolors.html

If the sun produced this sort of light,what would the world look like?

What does it mean to see white? images from: www.omatrix.com/uscolors.html

If the sun produced this sort of light,what would the world look like?no reds or greens, lots of yellow/brown

fundus photo

Red eye and glowing cats

reflective layerbehind photoreceptors

reflectionoff red blood vessels.Only noticeif lights are low.

Cone and rod absorption spectraNote: these are relative absorbance over different wavelengths.In actuality, the rods are much more sensitive than the cones.

Theories

full spectrum

HumansensorsL,M,S

Opponentdecompositionby neurons

???

Young first proposed early 1800’s. Sometimes called Young-Helmholtz, but Helmholtzcouldn’t believe only three different receptors.

Also known as “trichromancy

theory”

Early experimental evidence is that any test colorcan be ‘matched’

by a combination of only three primary colors.

Theories

full spectrum

HumansensorsL,M,S

Opponentdecompositionby neurons

???

Young first proposed early 1800’s. Sometimes called Young-Helmholtz, but Helmholtzcouldn’t believe only three different receptors.

Also known as “trichromancy

theory”

mid-late 1800’s: Ewald

Hering

noted trichromancycan’t explain afterimages. Proposed “opponency

theory”

Theories

full spectrum

HumansensorsL,M,S

Opponentdecompositionby neurons

???

trichromancy

theory

opponency

theory

Fierce scientific debates!

Theories

full spectrum

HumansensorsL,M,S

Opponentdecompositionby neurons

???

Trichromancy

theory

Opponency

theory

Zone theory

Directions:Stare at one of the squares for about 20 sec.Then look at the white.Should see color-opposites.

Explanation of Afterimages:Helmholtz explanation:

Your photoreceptors have bleached out, and they need timeto re-set. For example, if your green cones can’t respond to the green part of the white light, then you only get the blue + red response, and so you see the white as magenta.

Explanation of Afterimages:Helmholtz explanation:

Your photoreceptors have bleached out, and they need timeto re-set. For example, if your green cones can’t respond to the green part of the white light, then you only get the blue + red response, and so you see the white as magenta.

Hering argued: that theory andtrichromancy can’t explain the black/whiteafterimage. Because the black area isn’tbleaching anything, and yet causes a brighter-than-white afterimage.

Explanation of Afterimages:Helmholtz explanation:

Your photoreceptors have bleached out, and they need timeto re-set. For example, if your green cones can’t respond to the green part of the white light, then you only get the blue + red response, and so you see the white as magenta.

Hering argued: that theory andtrichromancy can’t explain the black/whiteafterimage. Because the black area isn’tbleaching anything, and yet causes a brighter-than-white afterimage.

Source: Steven Turner’s “In the Eye’s Mind”

Trichromancy theory: the world is a black slate, and light-colors write upon it.

Hering opponency theory: the resting stateis a neutral gray. Black is a color sensation.

Evidence: when you close your eyes, notas black as black.

Grays: are mixtures of black and white.

Source: Steven Turner’s “In the Eye’s Mind”

Trichromancy theory: the world is a black slate, and light-colors write upon it.

Hering opponency theory: the resting stateis a neutral gray. Black is a color sensation.

Evidence: when you close your eyes, notas black as black.

Grays: are mixtures of black and white.

How does his idea explain B/W afterimages?:Afterimage: the black response is exhausted,the white-background is really a “gray”, but since your “black responders” are tired, you see the gray background as “whiter.”

Source: Steven Turner’s “In the Eye’s Mind”

Opponent color theoryHerring proposedthree opposing channels: K to W

R to GB to Y

Only in 1960’s did evidencereally surface. Now his basic ideas (but not exactly) widely accepted.

Felt that yellow was asprimary as R, G, B.

Zone theory: both are right.Thanks to G. E. Muller in 1930’s.Incorporated into CIE standards and models by 1950.

Most people are happy with the zone theory but it’s notreally all that settled.

Some people think three channels not six.

Some experts think there’s only red-green and blue-yellow channels, and luminance info is carried within.

Opponent color theoryHerring proposedthree opposing channels: K to W

R to GB to Y

IMAGE SOURCEhandprint.com

Modern Theory: Six separate outputchannels. Rods “feed” into luminosity channel.

Ganglion neural cells:

Receive input from many rods/cones.

We can map their response spatially: +-

excited by signalin the center

inhibited by signal in the periphery

-

-

-

KEY: Human visual system responds to differences.

Ganglion neural cells

Receive input from many rods/cones.

We can map their response spatially: +-

-

-

-

QUIZ: If white is signal, which of these is the best stimulus?

quizzes due to www psych.hanover.edu/Krantz

Past the Ganglions

retina dLGN V1other visual cortical areas

highercortical areas

correlated spatial responseof a V1 neuron (Hubel and Weisel, ’62)

Past the Ganglions

correlated spatial responseof a V1 neuron (Hubel and Weisel, ’62)

Quiz: Which of these is the best stimuli for the V1 neuronwith response shown on the left?

Past the Ganglions

correlated spatial responseof a V1 neuron (Hubel and Weisel, ’62)

Quiz: Which of these is the best stimuli for the V1 neuronwith response shown on the left?

Blackbody illuminants

Ex: tungstenincandescent light bulb:

3000 K

set of illuminants:sun,candles,hot stuff incandescents

Example blackbody curves

Illuminantsoften describedby theircorrelated colortemperature

Ex: tungstenincandescent light bulb:

3000 K

Luminescents:1) electric charges excite gas molecules2) gas molecule electron’s energy level is raised3) gas molecules emit the energy as photons at specific wavelengths

What is it?

Fluorescent lights:Fluorescent lamp (electric discharge lamps in general):

Well, that’s not the whole story.

1) electric charges excite gas molecules2) gas molecule electron’s energy level is raised3) gas molecules emit the energy as photons at specific wavelengths

Luminescents:1) electric charges excite gas molecules2) gas molecule electron’s energy level is raised3) gas molecules emit the energy as photons at specific wavelengths

What is it?

Fluorescent lights:Fluorescent lamp (electric discharge lamps in general):

Well, that’s not the whole story.

1) electric charges excite gas molecules2) gas molecule electron’s energy level is raised3) gas molecules emit the energy as photons at specific wavelengths

Fluorescent lightsFluorescent lamp:

Well, that’s not the whole story. Common sodium vapor gas emits UV light. Gas tube is coated inside with phosphors.Phosphors absorb UV and re-emit visible.

1) electric charges excite gas molecules2) gas molecule electron’s energy level is raised3) gas molecules emit the energy as photons at specific wavelengths

electric discharge tube

gas molecule

electricfieldaccelerateselectrons electron collides with gas molecule

kinetic energy transferred to molecular excitation

Fluorescence and Phosphorescence

BOTH: energy in, light out

Fluorescence and Phosphorescence

BOTH: energy in, light out

Fluorescent: energy in, light out fast

Phosphorescent: energy in, excited electron relaxes slowlyto lower energy state, releasing more energy, eventually light out.

Gets “stuck” in triplet statewhere it is improbable to get out.

Fluorescence and Phosphorescence

BOTH: energy in, light out

Fluorescent: energy in, light out fast

Phosphorescent: energy in, excited electron relaxes slowlyto lower energy state, releasing more energy, eventually light out.

Gets “stuck” in triplet statewhere it is improbable to get out.

Luminescents emit light without being heated:fluroescence, phosphorescence,also: triboluminescence, bioluminescence, chemoluminescence

Differences between illuminants

Fluorescence only lasts 10-8 of a secondPhosphorescence lasts longer: up to hours

Differences between illuminants

Fluorescence only lasts 10-8 of a secondPhosphorescence lasts longer: up to hours

Fluorescence unchanged by heat.Hot phosphorent objects glow more brightly for shorter times than

cold objects.

What fluorescent material do you have in your house?

Differences between illuminantsFluorescence only lasts 10-8 of a secondPhosphorescence lasts longer: up to hours

Fluorescence unchanged by heat.Hot phosphorent objects glow more brightly for shorter times than

cold objects.

Fluorescents and phosphorescents are by nature more spectrally spiky than thermal radiators and their correlated color temperaturesare ‘less correlated’

Acquarium fluorescent lamp spectrum in white.

What is in plotted in green?

Differences between illuminantsFluorescence only lasts 10-8 of a secondPhosphorescence can last tens of seconds or even hours

Fluorescence unchanged by heat.Hot phosphorent objects glow more brightly for shorter times than

cold objects.

Fluorescents and phosphorescents are by nature more spectrally spiky than thermal radiators and their correlated color temperaturesare ‘less correlated’

Acquarium fluorescent lamp spectrum in white.

What is in plotted in green?Chlorophyll absorption

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Electron gun produces thin electron beam by thermionic emission:Cathode (electrode) is heated, emits electrons.Anode attracts electrons (voltage diff between cathode and anode).

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Electron beam bent by electromagnets to hit screen where desired.Electron gun produces thin electron beam by thermionic emission:

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Electron beam bent by electromagnets to hit screen where desired.Electrons hit phosphors, excite molecules, relax and emit light.

Electron gun produces thin electron beam by thermionic emission:

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Why do CRT’s take a moment to turn on?

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Why do CRT’s take a moment to turn on?Heating element has to warm up.

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Why is CRT glass made of leaded glass?

Phosphors in Action: the CRT (first invented by Germans, 1897)

phosphorescent

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Why is CRT glass made of leaded glass?High-energy electrons can hit the CRT screen and create x-rays.Leaded glass needed to keep your tv from giving you cancer.

CRT phosphor design issues:

Similar spectra from Phosphor Technology atwww.bu.edu/smec/lite/spectroscopy/spectra.html

CRT phosphor design issues:1) phosphor spectra2) emission time

too fast? Flicker.

too slow? Motion blur.

CRT phosphor design issues:1) phosphor spectra2) emission time

too fast?

too slow?

Similar spectra from Phosphor Technology atwww.bu.edu/smec/lite/spectroscopy/spectra.html

Why is this an exciting time for color?

Color production/repro is technologically difficult and costly.

Why is this an exciting time for color

Color production/repro is technologically difficult and costly.

Color repro waking up to a digital world.

Why is this an exciting time for color

Color production/repro is technologically difficult and costly.

Color repro waking up to a digital world.

Increasing color abilities in consumer market creates pressure.

Why is this an exciting time for color?

Color production/repro is technologically difficult and costly.

Color repro waking up to a digital world.

Increasing color abilities in consumer market creates pressure.

Color sells.

Why is this an exciting time for color

Color production/repro is technologically difficult and costly.

Color repro waking up to a digital world.

Increasing color abilities in consumer market creates pressure.

Color sells.

Immense challenges for controlling color over Internet and enabling telecommunication.

Why is this an exciting time for color?

Color production/repro is technologically difficult and costly.

Color repro waking up to a digital world.

Increasing color abilities in consumer market creates pressure.

Color sells.

Immense challenges for controlling color over Internet and enabling telecommunication.

Postmodernism: simulations augment life experience.