Lecture26

CSE486, Penn StateRobert Collins

Lecture 26:

Color and Light

not in textbook (sad but true)


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


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.


Light Transport





Illumination


Color of Light Source

Spectral Power Distribution:

UV IRVisible

Wavelength λ

Amplitude

Relative amount of lightenergy at each wavelength


Some Light Source SPDs


Light Transport





SurfaceReflection


(Ir)radiance

Incominglight

surface

Outgoinglight

IRRADIANCE RADIANCE


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.


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).


More General Surfaces

Have both a specular and diffuse reflections.

embedded colorante.g. paint


Spectral albedo for several different leaves

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


Spectral Radiance

to a

SpectralIrradiance

SpectralRadiance

Spectral Albedo


Light Transport




Sensor Response


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


Human Eye: Rods and Cones

S cones (blue)

M cones (green)

L cones (red)

rods (overall intensity)

near fovea

near periphery


Putting it all Together = Color

3 cones

COLOR!


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

.*

R

elat

ive

pow

er

Spectral Radiance

0

1


continued


Rel

ativ

e po

wer

Spectral Radiance

0

1


Simple Example (continued)relative numericresponse (area)

=100

= 0

= 0

Rel

ativ

e re

spon

seGreen Cones

0

1

Wavelength (nm)600 700400 500

Rel

ativ

e re

spon

se

Red Cones

0

1


Rel

ativ

e re

spon

se

Blue Cones

0

1


.*

.*

.*

re

spon

se

r

espo

nse

re

spon

se

Green Cones

0

1


Red Cones

0

1


Blue Cones

0

1


(no response)

(no response)

looks “red”


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


.*

R

elat

ive

pow

er

Spectral Radiance

0

1

Wavelength (nm)

400 700continued

500

(no radiance)


Rel

ativ

e po

wer

Spectral Radiance

0

1


Simple Example (continued)relative numericresponse (area)

= 0

= 0

= 0

Rel

ativ

e re

spon

seGreen Cones

0

1


Rel

ativ

e re

spon

se

Red Cones

0

1


Rel

ativ

e re

spon

se

Blue Cones

0

1


.*

.*

.*

re

spon

se

r

espo

nse

re

spon

se

Green Cones

0

1


Red Cones

0

1


Blue Cones

0

1


(no response)

(no response)

looks “black”

(no radiance)

(no response)


The Abyss Clip

“One-way ticket” clip from DVD


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…


Black/Yellow under Yellow Light

0

.*

.*

yellow black

black

yellow yellow

yellow

Irradiance Spectral Albedo�

Radiance


Radiance


Blue/White under Yellow Light

0

.*

.*

yellow blue

black

yellow white

yellow


Radiance


Radiance


Lesson Learned

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


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.


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


Metamers

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


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.


Why is Color Constancy Hard?

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


Color BlindnessNormal color perception

Red/Green color blindness

Lecture26

Documents

Transcript of Lecture26