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C O L O R and the human response to light. Idit Haran. Contents. Introduction: The nature of light The physiology of human vision Color Spaces: Linear (RGB, CMYK) Artistic View (Munsell, HSV, HLS) Standard (CIE-XYZ) Perceptual (Luv, Lab) Opponent (YIQ, YUV) – used in TV.

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contents
Contents
  • Introduction:
    • The nature of light
    • The physiology of human vision
  • Color Spaces:
    • Linear (RGB, CMYK)
    • Artistic View (Munsell, HSV, HLS)
    • Standard (CIE-XYZ)
    • Perceptual (Luv, Lab)
    • Opponent (YIQ, YUV) – used in TV
electromagnetic radiation spectrum

Short-

AC

Ultra-

wave

Gamma

X rays

Infrared

Radar

FM

TV

AM

electricity

violet

-12

-8

-4

4

8

10

10

10

1

10

10

Visible light

400 nm

500 nm

700 nm

Wavelength in meters (m)

Wavelength in nanometers (nm)

600 nm

Electromagnetic Radiation - Spectrum
spectral power distribution

1

Relative Power

0.5

0

400

500

600

700

Wavelength (l)

Spectral Power Distribution
  • The Spectral Power Distribution (SPD) of a light is a function P() which defines the power in the light at each wavelength
the interaction of light and matter
The Interaction of Light and Matter
  • Some or all of the light may be absorbed depending on the pigmentation of the object.
the human retina

cones

rods

horizontal

bipolar

amacrine

ganglion

light

The Human Retina
cones
Cones
  • High illumination levels (Photopic vision)
  • Less sensitive than rods.
  • 5 million cones in each eye.
  • Density decreases with distance from fovea.
3 types of cones
3 Types of Cones
  • L-cones, most sensitive to red light (610 nm)
  • M-cones, most sensitive to green light (560 nm)
  • S-cones, most sensitive to blue light (430 nm)
metamers
Metamers
  • Two lights that appear the same visually. They might have different SPDs (spectral power distributions)
history
History
  • Tomas Young (1773-1829)

“A few different retinal receptors operating with different wavelength sensitivities will allow humans to perceive the number of colors that they do. “

  • James Clerk Maxwell (1872)

“We are capable of feeling three different color sensations. Light of different kinds excites three sensations in different proportions, and it is by the different combinations of these three primary sensations that all the varieties of visible color are produced. “

  • Trichromatic: “Tri”=three “chroma”=color
3d color spaces

G

B

R

Cubic Color Spaces

Polar Color Spaces

Opponent Color Spaces

Brightness

Hue

black-white

blue-yellow

red-green

3D Color Spaces
  • Three types of cones suggests color is a 3D quantity. How to define 3D color space?
contents19
Contents
  • Introduction:
    • The nature of light
    • The physiology of human vision
  • Color Spaces:
    • Linear (RGB, CMYK)
    • Artistic View (Munsell, HSV, HLS)
    • Standard (CIE-XYZ)
    • Perceptual (Luv, Lab)
    • Opponent (YIQ, YUV) – used in TV
linear color spaces
Linear Color Spaces

Colors in 3D color space can be described as linear combinations of 3 basis colors, called primaries

=

+ c·

+ b·

The representation of :

(a, b, c)

is then given by:

rgb color model

3

Primary Intensity

2

1

0

400

500

600

700

Wavelength (nm)

RGB Color Model
  • RGB = Red, Green, Blue
  • Choose 3 primaries as the basis SPDs(Spectral Power Distribution.)
color matching experiment

-

+

test

match

-

+

-

+

Test light

Match light

1

1

~

0.75

0.75

=

0.5

0.5

0.25

0.25

0

0

400

500

600

700

400

500

600

700

Color Matching Experiment
  • Three primary lights are set to match a test light
cie rgb
CIE-RGB
  • Stiles & Burch (1959) Color matching Experiment.
  • Primaries are: 444.4 525.3 645.2
  • Given the 3 primaries, we can describe any light with 3 values (CIE-RGB):

(85, 38, 10)

(21, 45, 72)

(65, 54, 73)

rgb image

36

111

14

126

12

36

36

36

111

12

17

111

200

36

12

36

14

36

36

111

200

36

12

17

200

111

14

126

17

111

14

12

36

36

14

36

10

128

126

200

12

111

36

36

111

36

14

36

17

111

14

126

17

111

17

36

36

14

36

72

17

126

72

126

17

111

12

36

12

126

200

36

12

17

126

17

111

200

200

36

12

36

12

126

14

200

36

12

126

17

72

12

17

111

14

36

126

200

111

14

36

72

128

126

200

12

111

10

36

12

17

72

106

155

200

36

12

36

14

36

36

111

14

126

12

36

17

36

36

14

36

72

200

111

14

126

17

111

36

36

111

12

17

111

12

17

126

17

111

200

36

36

111

36

14

36

36

111

200

36

12

17

14

200

36

12

126

17

17

126

72

126

17

111

14

12

36

36

14

36

126

200

111

14

36

72

200

36

12

36

12

126

17

111

14

126

17

111

36

12

17

72

106

155

72

12

17

111

14

36

12

36

126

200

36

12

RGB Image
cmyk color model

Cyan – removes Red

transmit

B

G

R

Magenta – removes Green

B

G

R

Yellow – removes Blue

B

G

R

CMYK Color Model

CMYK = Cyan, Magenta, Yellow, blacK

Black – removes all

combining colors
Combining Colors

Additive (RGB)

Subtractive (CMYK)

example red magenta yellow

B

G

R

magenta

B

G

R

+

yellow

B

G

R

=

red

R

B

G

R

Example: red = magenta + yellow
cmy black

50

CMY + Black
  • Using three inks for black is expensive
  • C+M+Y = dark brown not black
  • Black instead of C+M+Y is crisper with more contrast

C + M + Y = K (black)

=

+

100

50

70

50

0

20

C

M

Y

K

C

M

Y

color spaces
Color Spaces
  • Linear (RGB, CMYK)
  • Artistic View (Munsell, HSV, HLS)
  • Standard (CIE-XYZ)
  • Perceptual (LUV, Lab)
  • Opponent (YIQ, YUV) – used in TV
the artist point of view

white

The Artist Point of View
  • Hue - The color we see (red, green, purple)
  • Saturation - How far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue)
  • Brightness/Lightness (Luminance) - How bright is the color
munsell color system
Munsell Color System

Equal perceptual steps in Hue Saturation Value.

Hue: R, YR, Y, GY, G, BG, B, PB, P, RP

(each subdivided into 10)

Value: 0 ... 10 (dark ... pure white)

Chroma: 0 ... 20 (neutral ... saturated)

Example:

5YR 8/4

hsv hsb color space
HSV/HSB Color Space

HSV = Hue Saturation Value

HSB = Hue Saturation Brightness

Saturation Scale

Brightness Scale

slide41
HSV

Value

Saturation

Hue

hls color space

V

green

120°

yellow

cyan

0.5

red

Blue

240°

magenta

H

0.0

S

black

HLS Color Space

HLS = Hue Lightness Saturation

color spaces43
Color Spaces
  • Linear (RGB, CMYK)
  • Artistic View (Munsell, HSV, HLS)
  • Standard (CIE-XYZ)
  • Perceptual (Luv, Lab)
  • Opponent (YIQ, YUV) – used in TV
cie color standard
CIE Color Standard
  • Why do we need a standard ?
    • RGB differ from one device to another
cie color standard45
CIE Color Standard
  • Why do we need a standard ?
    • RGB differ from one device to another
    • RGB cannot represent all colors

RGB Color Matching Functions

cie color standard 1931

X

Y

Z

CIE Color Standard - 1931
  • CIE - Commision Internationale d’Eclairage
  • 1931 - defined a standard system for color representation.
  • XYZ tristimulus coordinate system.
xyz spectral power distribution

1.8

z(l)

1.4

1

Tristimulus values

x(l)

0.6

0.2

400

500

600

700

Wavelength (nm)

XYZ Spectral Power Distribution
  • Non negative over the visible wavelengths.
  • The 3 primaries associated with x y z spectral power distribution are unrealizable (negative power in some of the wavelengths).
  • y was chosen to equal luminance of monochromatic lights.

y(l)

rgb to xyz
RGB to XYZ
  • RGB to XYZ is a linear transformation

X

R

0.490 0.310 0.200

0.177 0.813 0.011

0.000 0.010 0.990

=

Y

G

Z

B

cie chromaticity diagram

X

= x

X

X+Y+Z

Y

= y

Y

X+Y+Z

Z

0.9

= z

Z

520

X+Y+Z

530

540

510

550

y

505

560

570

500

0.5

580

590

495

600

610

490

650

485

480

470

0.0

450

0.0

0.5

1.0

CIE Chromaticity Diagram

x+y+z = 1

x

color naming

0.9

520

530

540

510

550

505

560

green

570

yellow-

green

500

580

0.5

yellow

590

495

orange

600

610

white

cyan

490

red

650

pink

485

magenta

blue

480

purple

470

450

0.0

1.0

0.5

x

Color Naming

y

blackbody radiators and cie standard illuminants
Blackbody Radiators and CIE Standard Illuminants

CIE Standard Illuminants:

2500 - tungsten light (A)

4800 - Sunset

10K - blue sky

6500 - Average daylight (D65)

chromaticity defined in polar coordinates
Chromaticity Defined in Polar Coordinates

0.8

Given a reference white.

Dominant Wavelength –

wavelength of the spectral

color which added to the

reference white, producesthe given color.

0.6

0.4

reference white

0.2

0

0

0.2

0.4

0.6

0.8

chromaticity defined in polar coordinates53
Chromaticity Defined in Polar Coordinates

0.8

Given a reference white.

Dominant Wavelength

0.6

Complementary Wavelength - wavelength of the spectral color which added to the given color, produces the reference white.

0.4

reference white

0.2

0

0

0.2

0.4

0.6

0.8

chromaticity defined in polar coordinates54
Chromaticity Defined in Polar Coordinates

0.8

Given a reference white.

Excitation Purity – the ratio of the lengths between the given color and reference white and between the dominant wavelength light and reference white.

Ranges between 0 .. 1.

Dominant Wavelength

Complementary Wavelength

0.6

purity

0.4

reference white

0.2

0

0

0.2

0.4

0.6

0.8

device color gamut
Device Color Gamut
  • We can use the CIE chromaticity diagram to compare the gamut of various devices:
  • Note, for example, that a color printercannot reproduceall shades availableon a color monitor
color spaces56
Color Spaces
  • Linear (RGB, CMYK)
  • Artistic View (Munsell, HSV, HLS)
  • Standard (CIE-XYZ)
  • Perceptual (Luv, Lab)
  • Opponent (YIQ, YUV) – used in TV
luminance v s brightness

DI2

I2

Luminance

DI1

I1

Luminance v.s. Brightness

Luminance Brightness

(intensity) vs (Lightness)

Y in XYZ V in HSV

Equal intensity steps:

Equal brightness steps:

I1 < I2, DI1 = DI2

weber s law

DI

= constant

I

Weber’s Law

In general, DI needed for just noticeable difference

(JND) over background I was found to satisfy:

(I is intensity, DI is change in intensity)

Perceived Brightness

Weber’s Law:

Perceived Brightness = log (I)

Intensity

munsell lines of constant hue and chroma
Munsell lines of constant Hue and Chroma

0.5

0.4

0.3

y

0.2

0.1

Value =1/

0

0

0.1

0.2

0.3

0.4

0.5

0.6

x

macadam ellipses of jnd just noticeable difference

0.8

0.6

y

0.4

0.2

0

0

0.2

0.4

0.6

MacAdam Ellipses of JND (Just Noticeable Difference

(Ellipses scaled by 10)

x

perceptual color spaces
Perceptual Color Spaces
  • An improvement over CIE-XYZ that represents better uniform color spaces
  • The transformation from XYZ space to perceptual space is Non Linear.
  • Two standard adopted by CIE areL*u’v’ and L*a*b*
  • The L* line in both spaces is a replacement of the Y lightness scale in the XYZ model, but it is more indicative of the actual visual differences.
munsell lines and macadam ellipses plotted in cie l u v coordinates

100

Value =5/

50

0

v*

-50

-100

-150

-150

-100

-50

0

50

100

150

200

u*

100

50

0

v*

-50

-100

-150

-150

-100

-50

0

50

100

150

200

u*

Munsell Lines and MacAdam Ellipses plotted in CIE-L*u’v’ coordinates
color spaces64
Color Spaces
  • Linear (RGB, CMYK)
  • Artistic View (Munsell, HSV, HLS)
  • Standard (CIE-XYZ)
  • Perceptual (Luv, Lab)
  • Opponent (YIQ, YUV) – used in TV
opponent color spaces
Opponent Color Spaces

+

black-white

+

blue-yellow

-

+

red-green

-

-

yiq color model
YIQ Color Model
  • YIQis the color model used for color TV in America (NTSC= National Television Systems Committee)
  • Y is luminance, I & Qare color (I=red/green,Q=blue/yellow)
    • Note: Y is the same as CIE’s Y
    • Result: backwards compatibility with B/W TV!
  • Convert from RGB to YIQ:
  • The YIQ model exploits properties of our visual system, which allows to assign different bandwidth for each of the primaries (4 MHz to Y, 1.5 to I and 0.6 to Q)
yuv color model
YUV Color Model
  • YUVis the color model used for color TV in Israel (PAL), and in video. Also called YCbCr.
  • Y is luminance as in YIQ.
  • U and V are blue and red (Cb and Cr).
  • The YUV uses the same benefits as YIQ,(5.5 MHz for Y, 1.3 for U and V).
  • Converting from RGB to YUV:
    • Y = 0.299R + 0.587G + 0.114B
    • U = 0.492(B – Y)
    • V = 0.877(R – Y)
summary
Summary
  • Light  Eye (Cones,Rods)  [l,m,s]  Color
  • Many 3D color models:
    • Reproducing Metamers to Colors
    • Different reproduction Gamut
    • More / Less intuitive
    • CIE standards
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