Color Engineering: Colorimetry

Dr. Wayne Cheng http://color.di.nctu.edu.tw

Reference Books

R. G. Kuehni, Kuehni, Color: An Introduction to Practice and Principles, 2nd Ed., WileyWiley-Interscience, Interscience, 2005. M. D. Fairchild, Color Appearance Models, WileyWileyIS&T, 2005 G. Wyszecki and W. S. Stiles, Color Science, John Wiley & Sons, 1982. R. G. W. Hunt, The Reproduction of Colour, Colour, 5th Ed., England: Fountain Press, 1995. Billmeyer and Saltzman, Saltzman, Principles of Color Technology, 3rd Ed., WileyWiley-Interscience, Interscience, 2000. Fundamentals of Color and Appearance, GretagMacbeth. GretagMacbeth.

Available online

Color appearance phenomena and models

Comprehensive data

Outline
Colorimetry
Chromaticity Photometry Color

diagram

spaces Color difference

Color appearance
Color

appearance terminology Color appearance phenomena

Chromaticity Diagram

CIE

International Commission on Illlumination Commision Internationale de IEclairage

CIEXYZ

CIEXYZ Chromaticity Diagram

(x,y) x,y) in CIEXYZ Visible spectrum Nominal scales Spectral locus Purple line White point Dominant wavelength Complement Mixture rules Black body curve Color temperature

Spectral Locus

Purple Line

Purples and magentas Mixture of red and blue Cannot be mixed from white and a single spectral color Non-spectral color Complement of green 550C

Dominant Wavelength

Line passing C, white, and spectral locus Polar coordinates (geometrical coordinates) Purple line: complement

Purity

Ratio of CW and CL How to mixture

Imaginary Color Stimuli

CIEXYZ Chromaticit y Diagram

(x,y) x,y) in CIEXYZ Visible spectrum Nominal scales Spectral locus Purple line White point Dominant wavelength Complement Mixture rules Black body curve Color temperature

Luminous efficiency functions

Photometry

Radiometry

Energy Power Radiant Intensity Irradiance Radiance

Color Science, Wyszechi & Stiles

Steradian: solid angle

Color Science, Wyszechi & Stiles

Photometry

Luminous Intensity

Photometry Example

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Luminous efficiency functions

CIE 1924

System of Photometry

A spectral weighting function that could be used to describe the perception of brightness matches Convert radiometry into photometry
Irradiance ~ illuminance Radiance ~ luminance Reflectance factor ~ luminance factor

Photopic: Photopic: cones Scotopic: Scotopic: rods

How about colors?

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Colorimetry

Basic Colorimetry

A tool used to make a prediction on whether two lights of different spectral power distributions will match in color for certain given conditions of observation.

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Three Parties in Colorimetry

Objects
Viewing geometries

Observers
Standard observers

Illuminants
Light sources
BlackBlack-body

radiators CIE illuminants

Viewing Geometry

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CIE Viewing geometry: D/0, 0/D

Diffuse/Normal
The sample is illuminated from all angles Viewed at an angle near the normal

Normal/Diffuse

Sphere at ITRI

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CIE Viewing geometry: 45/0, 0/45

Normal/45 45/Normal

CIE Standard Illuminants

D65
Daylight at 6500K

D50
Daylight at 5000K

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CIE Standard Illuminants

A: A gas-filled coiled-tungsten filament lamp operating at a correlated color temperature of 2856K B, C: filtered A E: uniform distribution F2: fluorescent source

Popular Light Sources

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Standard Observer

Luminous efficiency functions From radiometry to photometry

CIE 1924

System of Photometry

A spectral weighting function that could be used to describe the perception of brightness matches Convert radiometry into photometry
Irradiance ~ illuminance Radiance ~ luminance Reflectance factor ~ luminance factor

CIE 1988

Photopic: Photopic: cones Scotopic: Scotopic: rods

Spectral luminous efficiency function VM()

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Photopic, Mesopic, and Scotopic

Rod and Cone

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Color Vision

Prof. Lance Williams @ cs.unm.edu

Color Spaces

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Color Spaces

CIERGB CIEXYZ NTSC YIQ CIELab CIELuv

Newtons Hue Circle

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Color Matching Functions

Two CIE standard observers

CIE 1931 2 deg RGB CIE 1931 2 deg XYZ CIE 1964 10 deg

Two individuals with different visions

Color Matching

C: color to match W: reference white P1, P2, P3: R, G, B

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Axioms of Color Matching

Any color can be matched by a mixture of no more than three colored lights A color match at one radiance level holds over a wide range of levels Components of a mixture of colored lights cannot be resolved by the human eye The luminance of a color mixture is equal to the sum of the luminance of its components

Trichromatic Generalization

Grassmans three laws of additive color mixture Binary relation of color matching = ::= color matching Symmetry law Transitivity law
a=b => b=a a=b; b=c => a=c a=b => s*a=s*b s*a=s*b

Proportionality law Additivity law

a=b; c=d => a+c= a+c=b+d

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CIERGB 1931

R = ( ) r ( ) d

G = ( ) g ( )d

B = ( )b ( )d

Plane of x+y+z=1

x=X/(X+Y+Z) y=Y/(X+Y+Z) z=Z/(X+Y+Z) x+y+z=1

cs.rit.edu

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White point?

x = y = z = 1/3 CIE Illuminant C

CIERGB chromaticity diagram

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CIEXYZ 1931

X = k ( ) x ( ) d

Y = k ( ) y ( ) d

Z = k ( ) z ( ) d

CIERGB vs. CIEXYZ

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CIEXYZ 1931 Standard Observer

Using a visual field of 2 deg 2 deg color matching functions (2 deg observer) Not representative
Fewer than 20 observers

CIE 1964 Standard Observer

Using a larger visual field of 10 deg 49 observers 10 deg color matching functions (10 deg observer) Excluded central fovea
macular absorption

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Why So Many Color Spaces?

From CIERGB to CIEXYZ

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CIERGB and CIEXYZ

YIQ NTSC

National Television Systems Committee Y=luminance; for black-and-white TV sets (I,Q)=(hue, saturation)

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NTSC YIQ decomposition

Just Noticeable Color Difference (JNCD)

Why different color spaces? A unit of sensation difference Any given luminance x = 0 Increase x until x is just-noticeably brighter than x; x=1 x = 2

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JNCD in XYZ

MacAdams ellipses Just noticeable color difference in XYZ

CIEUCS UVW 1976

Uniform Chromaticity Scale (UCS)

Equal change in chromaticity coordinates results in equal just noticeable changes in the perceived hue and saturation of a color UVW: V=luminance

u = 4x / ((-2x+12y+3) v = 6y / ((-2x+12y+3) x = 3u / (2u(2u-8v8v-4) y = 2v / (2u(2u-8v8v-4)

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UVW decomposition

JNCD in UVW

Just noticeable color difference in UVW

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Nonlinear Transform in XYZ

Opponent Colors

Red Green Yellow - Blue

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CIELAB 1976

For computational simple measure of color L* = brightness

0-100

a* = redness-greenness
+/-

L * = 116 (

Y 13 ) 16 Yn

b* = yellowness-blueness
+/-

X 13 Y 13 ( ) a * = 500 ( X ) Y n n
Y 13 Z 13 b * = 200 ) ( (Y ) Zn n

Xn,Yn,Zn: reference white

CIELab decomposition

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CIELUV 1976

From CIELab and U*V*W* Cartesian

L*: lightness u*: rednessredness-greenness v*: yellownessyellowness-blueness
Y 13 ) 16 Yn u * = 13 L* (u 'u 'n ) L* = 116(

Cylindrical
L*: lightness C*uv: chroma Huv: Huv: hue

v* = 13L* (v 'v 'n )

* Cuv =

(u

*2

+ v*2

CIELCh

Polar coordinates of CIELab L*: lightness

0 100

C*: chroma
Distance from the neutral axis

h: hue
0 - 360

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CMC (l:c)

Color Measurement Committee

Of the Society of Dyers and Colorists

Intensity, Hue, and Saturation

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Munsell Color Space

Color Difference

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Color Difference

Goal: color differences be perceptually uniform throughout the space One unit difference in red is perceived to be equal in magnitude to a unit difference in gray Based on CIELAB Eab*, LCabHab

CIELAB Eab*

Based on CIELAB Euclidean distance E * = (L* 2 + a * 2 + b* 2 ) ab between two coordinates In terms of lightness, red-green, yellow-blue In terms of lightness, chroma, and hue differences

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CIE94 LCabHab

CIE94 Eab*=LCabHab kL, kC, kH,: adjust relative weighting

Other Color Differences

Delta E DRG
Delta Red/Green

DYB
Delta Yellow/Blue

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Black-body Radiators

Black-body radiators (Planckian radiators) Plancks Law

Wiens Law

Color Temperature

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