Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486,...

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1 CSE486, Penn State Robert Collins Lecture 26: Color and Light not in textbook (sad but true) CSE486, Penn State Robert Collins Physics of Light and Color Light is electromagnetic radiation Different colors correspond to different wavelengths λ Intensity of each wavelength specified by amplitude Visible light: 400-700nm. range R O Y G B I V (ROYGBIV) CSE486, Penn State Robert Collins What is Color? Objects don’t have a “color” Color is a perception; what we “see” It is a function of light source power at different wavelengths proportion of light at each wavelength reflected off object surface sensor response to different wavelengths CSE486, Penn State Robert Collins Sketch: Light Transport Source emits photons Photons travel in a straight line They hit an object. Some are absorbed, some bounce off in a new direction. And then some reach an eye/camera and are measured. CSE486, Penn State Robert Collins Light Transport Source emits photons Photons travel in a straight line They hit an object. Some are absorbed, some bounce off in a new direction. And then some reach an eye/camera and are measured. Illumination CSE486, Penn State Robert Collins Color of Light Source Spectral Power Distribution: UV IR Visible Wavelength λ Amplitude Relative amount of light energy at each wavelength

Transcript of Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486,...

Page 1: Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486, Penn State Robert Collins Lecture 26: Color and Light not in textbook (sad but true)

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CSE486, Penn StateRobert Collins

Lecture 26:

Color and Light

not in textbook (sad but true)

CSE486, Penn StateRobert Collins

Physics of Light and Color• Light is electromagnetic radiation

– Different colors correspond to different wavelengths λ

– Intensity of each wavelength specified by amplitude

• Visible light: 400-700nm. range

ROYGBIV

(ROYGBIV)

CSE486, Penn StateRobert Collins What is Color?

• Objects don’t have a “color”

• Color is a perception; what we “see”

• It is a function of– light source power at different wavelengths– proportion of light at each wavelength reflected off object surface– sensor response to different wavelengths

CSE486, Penn StateRobert Collins

Sketch: Light Transport

Source emits photons

Photons travel in astraight line

They hit an object. Some areabsorbed, some bounce offin a new direction.

And then some reachan eye/camera andare measured.

CSE486, Penn StateRobert Collins

Light Transport

Source emits photons

Photons travel in astraight line

They hit an object. Some areabsorbed, some bounce offin a new direction.

And then some reachan eye/camera andare measured.

Illumination

CSE486, Penn StateRobert Collins

Color of Light Source

Spectral Power Distribution:

UV IRVisible

Wavelength λ

Amplitude

Relative amount of lightenergy at each wavelength

Page 2: Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486, Penn State Robert Collins Lecture 26: Color and Light not in textbook (sad but true)

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Some Light Source SPDs CSE486, Penn StateRobert Collins

CSE486, Penn StateRobert Collins

Light Transport

Source emits photons

Photons travel in astraight line

They hit an object. Some areabsorbed, some bounce offin a new direction.

And then some reachan eye/camera andare measured.

SurfaceReflection

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(Ir)radiance

Incominglight

surface

Outgoinglight

IRRADIANCE RADIANCE

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Specular Surfaces

Light rays purely reflect via Snell’slaw (angle of reflection = angle ofincidence)

Properties:

Outgoing light has same SPD(“color”) as incoming light.

If you stand in the right place yousee a little picture of the lightsource reflected off the surface.

CSE486, Penn StateRobert Collins

Lambertian Surfaces

Purely “matte” surface.

Properties:

Apparent brightness is proportionalto cosine of angle betweenobserver’s line of sight and thesurface normal (Lambert’s Law)

Outgoing light has SPD thatdepends on spectral albedo ofsurface (what wavelengths getabsorbed vs transmitted).

Page 3: Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486, Penn State Robert Collins Lecture 26: Color and Light not in textbook (sad but true)

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More General Surfaces

Have both a specular and diffuse reflections.

embedded colorante.g. paint

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Spectral albedo for several different leaves

Spectral AlbedoRatio of outgoing to incoming radiation at different wavelengths.(proportion of light reflected)

CSE486, Penn StateRobert Collins

Spectral Radiance

to a

SpectralIrradiance

SpectralRadiance

Spectral Albedo

CSE486, Penn StateRobert Collins

Light Transport

Source emits photons

They hit an object. Some areabsorbed, some bounce offin a new direction.

And then some reachan eye/camera andare measured.

Sensor Response

CSE486, Penn StateRobert Collins

Human Vision

• Human eyes have 2 types of sensors:– CONES

• Sensitive to colored light, but not very sensitive todim light

– RODS• (very) Sensitive to achromatic light

CSE486, Penn StateRobert Collins

Human Eye: Rods and Cones

S cones (blue)

M cones (green)

L cones (red)

rods (overall intensity)

near fovea

near periphery

Page 4: Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486, Penn State Robert Collins Lecture 26: Color and Light not in textbook (sad but true)

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Putting it all Together = Color

3 cones

COLOR!

CSE486, Penn StateRobert Collins

Simple Example

600 700Wavelength (nm)

0

1

albe

do

Spectral albedoof apple (red)

Rel

ativ

e po

wer

Relative Spectral PowerDistribution of White Light

0

1

Wavelength (nm)400 700

.*

Rel

ativ

e po

wer

Spectral Radiance

0

1

Wavelength (nm)600 700

continued

CSE486, Penn StateRobert Collins

Rel

ativ

e po

wer

Spectral Radiance

0

1

Wavelength (nm)600 700

Simple Example (continued)relative numericresponse (area)

=100

= 0

= 0

Rel

ativ

e re

spon

se

Green Cones

0

1

Wavelength (nm)600 700400 500

Rel

ativ

e re

spon

se Red Cones

0

1

Wavelength (nm)600 700400 500

Rel

ativ

e re

spon

se Blue Cones

0

1

Wavelength (nm)600 700400 500

.*

.*

.*

resp

onse

res

pons

e

r

espo

nse

Green Cones

0

1

Wavelength (nm)600 700400 500

Red Cones

0

1

Wavelength (nm)600 700400 500

Blue Cones

0

1

Wavelength (nm)600 700400 500

(no response)

(no response)

looks “red”

CSE486, Penn StateRobert Collins

Simple Example

600 700Wavelength (nm)

0

1

albe

do

Spectral albedoof apple (red)

Rel

ativ

e po

wer

Relative Spectral PowerDistribution of Blue Light

0

1

Wavelength (nm)400 700

.*

R

elat

ive

pow

er

Spectral Radiance

0

1

Wavelength (nm)

400 700continued

500

(no radiance)

CSE486, Penn StateRobert Collins

Rel

ativ

e po

wer

Spectral Radiance

0

1

Wavelength (nm)600 700

Simple Example (continued)relative numericresponse (area)

= 0

= 0

= 0

Rel

ativ

e re

spon

se

Green Cones

0

1

Wavelength (nm)600 700400 500

Rel

ativ

e re

spon

se Red Cones

0

1

Wavelength (nm)600 700400 500

Rel

ativ

e re

spon

se Blue Cones

0

1

Wavelength (nm)600 700400 500

.*

.*

.*

resp

onse

res

pons

e

r

espo

nse

Green Cones

0

1

Wavelength (nm)600 700400 500

Red Cones

0

1

Wavelength (nm)600 700400 500

Blue Cones

0

1

Wavelength (nm)600 700400 500

(no response)

(no response)

looks “black”

(no radiance)

(no response)

CSE486, Penn StateRobert Collins

The Abyss Clip

“One-way ticket” clip from DVD

Page 5: Physics of Light and Color Lecture 26: Color and Lightrtc12/CSE486/lecture26_6pp.pdf · 1 CSE486, Penn State Robert Collins Lecture 26: Color and Light not in textbook (sad but true)

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What is Going On in This Clip?

Under yellowish green light,both the blue/white wire and theblack/yellow wire look identical.

Now for the spectral explanationof why this happens…

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Black/Yellow under Yellow Light

0

.*

.*

yellow black

black

yellow yellow

yellow

Irradiance Spectral Albedo�

Radiance

Irradiance Spectral Albedo�

Radiance

CSE486, Penn StateRobert Collins

Blue/White under Yellow Light

0

.*

.*

yellow blue

black

yellow white

yellow

Irradiance Spectral Albedo�

Radiance

Irradiance Spectral Albedo�

Radiance

CSE486, Penn StateRobert Collins

Lesson Learned

Surfaces materials that look different under whitelight can appear identical under colored light.

CSE486, Penn StateRobert Collins

Metamers

Definition: two different spectral reflectances that appearindistinguishable to a given observer under givenillumination conditions.

Illumination metamerism: two color distributions look thesame under a given illumination

Observer metamerism: two color distributions look thesame to a given observer.

CSE486, Penn StateRobert Collins

Sample Metamers

From http://www.iitp.ru/projects/posters/meta/

Metameric curves for human eye under daylight Test pattern viewed under daylight

Overcoming metamerism by viewingunder different illumination

Overcoming metamerism by having a different observer

Viewed underincandescent lighting

Viewed by camerawith more sensitivered response thanhuman eye

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Metamers

To further explore observer metamerism, see theinteractive metamer applet at: http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html

CSE486, Penn StateRobert Collins

Perception: Color Constancy

Humans are very good at recognizing the same material colors under different illumination. Not clear how thisis achieved in the general case.

CSE486, Penn StateRobert Collins

Why is Color Constancy Hard?

Want to factor this signalinto these two components.Need prior knowledge!

CSE486, Penn StateRobert Collins

Color BlindnessNormal color perception

Red/Green color blindness