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Colorimetry and Its Applications 1

Colorimetry and Its Applications R. Chung, Professor

Colorimetry has become more and more useful in graphic arts imaging for color specifications, process control, quality assurance and conformity assessment.!

(Rev._2015b)!

Copyright 2015 RIT – May not be reproduced without permission

Topics

  Light, object, and color vision

  Tristimulus integration is the process of computing color based on spectral data. –  1931 CIEXYZ color space –  1976 CIELAB color space

  Color matching and color difference –  Invariant match –  Metameric match

  Colorimetry applications

2



Light and Color

  Light exists as a form of energy.

  Color is a visual sensation.

  Visual sensation requires –  Light –  Object –  Observer

Brain

3


Human Color Vision

  Color perception is complex. –  Even if we standardize the lighting condition, the color of the

same object may be perceived differently by two people. -  Perception is influenced by age, sex, cultural experiences

  CIE colorimetry standardizes how we measure color. –  Color measurement may be able to predict color perception.

4



Color Vision Tests

  Ishihara�s test for color blindness

  Farnsworth-Munsell 100 Hue test

5


Color Vision Deficiency

  Anomalous trichromats

  Statistics –  7% of male population have color vision defects –  Less than 0.5% of female population have color vision defects

normal deutan" tritan"

6



Standard Observer

  In 1931, CIE defined a set of mathematical functions which describe the sensitivity of the eye. –  Based on two independent color matching experiments by

Wright and Guild -  To quantify how spectrum colors are matched rather than to specify

a color sensation

  Three imaginary primaries (R, G, B) were used in the color matching experiments.!

!

7


1931 2o Standard Observer

  The amount of tristimulus values required to match the equal-energy spectrum

600400 500 700

trist

imul

us v

alue

s

0

0.5

1.0

1.5

2.0

2.5

x-

z-

y-

wavelength (λ)

Mathematics is used to transform color matching functions (x-bar, y-bar, z-bar) without negative portion.!

8



Rods & Cones Distribution

  Color vision is centered around 2-degree fovea where cones are heavily concentrated.

Source: Leo Hurvich!Professor of Psychology!

Univ. of Pennsylvania !

9


Supplementary Observer

  A CIE 10o standard observer was established in 1964. –  Larger visual field was used for color matching.

-  Suitable for paint, textile, and plastics applications

Arm length

Thumb up4 degrees

2"

28"

10



Spectral Power Distribution

  SPD is measured by a spectroradiometer. –  Color temperature describes the hue of the light source.

-  Lamps with D50 color temperature can be different in spectral energy distribution.

–  A potential problem of metamerism

–  CRI describes its color rendering capability. -  How complete spectral energies are across the visible spectrum

0

50

100

150

200

250

390

410

430

450

470

490

510

530

550

570

590

610

630

650

670

690

Wavelenght (nm)

P70

0

Illuminant A

0

20

40

60

80

100

120

390

410

430

450

470

490

510

530

550

570

590

610

630

650

670

690

Wavelenght (nm)

P

Illuminant D50

700

11


CIE Standard Illuminants

  SPD of F2 at 5 nm intervals (blue) shows spikes on top of the continuous emission. –  SPD of F2 at 10 nm intervals (red line) does not have sufficient

resolution to show the spikes.

12



Object Color

  Measured by a spectrophotometer –  A spectrophotometer measures the reflectance of a sample at

many points across the visible spectrum.

13


Spectrophotometer

  Instrument design parameters –  Illumination source

-  M0, M1, M2, M3 –  Geometry

-  0 / 45 –  Measuring diffuse reflectance

-  Integrating sphere –  Specular component included or excluded

–  Monochrometer -  Grating -  Interference filters

–  Measurement spot size

14



Tristimulus Integration

  ISO 13655 (2009) –  R(λ) is the reflectance factor at λ.

–  WX(λ) is the weighting factor at λ for tristimulus value X.

–  WY(λ) is the weighting factor at λ for tristimulus value Y.

–  WZ(λ) is the weighting factor at λ for tristimulus value Z.

15


Measuring Tristimulus Values

  Spectro-colorimeter –  Computationally derived with spectral data

-  ANSI CGATS.5 - 1993 -  Suitable for critical color communication

  Filter-colorimeter –  Derived by means of optical integration with the light-filter-

detector combination -  Spectral data are not used

  Learn to use the Excel spreadsheet to compute color based on spectral data.

16



Densitometer vs. Colorimeter Densitometer Spectro-colorimeter

Unit density XYZ, Yxy, CIELABDifference none ∆E(Single number)

Spectral match to peak absorp- match the 1931 CIEresponse tion of process inks standard observerGeometry 0/45 0/45, integrating Color anomalous color-normalperception trichromat observerDark shade good okaydiscrimin.Light shade poor gooddiscrimin.Standardization ANSI status T CIEApplications process control for color specification

CMYK printing only process controlquality assurance

17


Color Space Models

  CIE Chromaticity coordinates, Yxy –  Y is the lightness of the color –  x represents fractional redness –  y represents fractional greenness

-  z or (1-x-y) is the fractional blueness

• x = XX + Y + Z

• y = YX + Y + Z

x

y

18



Chromaticity Diagram

  Spectrum colors form a horse shoe shaped locus. –  Dominant wavelength –  Purity

x

y

Y

Ill. D65C1

Dominatewavelength

700 nm

525 nm

400 nm

575 nm

o500 nm

19


CIE Color Naming

  Different wavelength region represents color names of different hues.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8x

y

CIE spectrum locus

700600

575

550

525

500

475

450400

Green

Yellow

Yellowishgreen

Greenishyellow

PinkWhite

OrangeReddishorange

Red

Blue

Bluishgreen

Greenishblue

Purple

Purplishred

Reddishpurple

20



Color Gamut Comparison

  A printer�s CMYK color gamut is different from a monitor�s RGB color gamut.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8x

y G

C

B

M

R

Y

CIE spectrum locus

700600

575

550

525

500

475

450400

Monitor

Printer

21


Uniform Color Space

  MacAdam ellipses –  Equal visual difference should be plotted with equal distance.

-  Chromaticity diagram -  did not do well.

22



1976 CIELAB Color Space

  Opponent color theory –  L* is lightness –  a* is redness or greenness

-  Redness (if a* is positive) -  Greenness (if a* is negative)

–  b* is yellowness or blueness -  Yellowness (if b* is positive) -  Blueness (if b* is negative)

-a* +a*+b*

-b*

Black

WhiteYellow

Blue

RedGreen

L*

23


XYZ-to-Lab Formulas

  ISO 13655

L* = 116[f(Y/Yn) – 16!!a* = 500[f(X/Xn) – f(Y/Yn)]!!b* = 200[f(Y/Yn) – f(Z/Zn)]!!X, Y, Z are the tristimulus values!Xn, Yn, Zn are the white point !

24



Process Inks

  Real inks have incomplete reflection and absorption. –  CIELAB is used to specify the ink color.

-  Sample preparation -  Measurement conditions

600400 500 700

20

40

60

80

100

% R

efle

ctan

ce

cyan

yellow

magenta

wavelength (λ)

Ink CIELAB (D50/2)Patch L* a* b*Cyan 60.5 -44 -41.1Magenta 55.7 63.2 0.3Yellow 88.7 3.2 96.3Black 10 0.4 3.8

25


CIE LCh Color Space

  The vector equivalent of the CIELAB color space –  Metric chroma

–  Hue angle

+a*-a*

+b*

-b*

C*oh

•

C* = a*2 + b*2

h = tan-1 b*a*

26



Hue Angle Calculations

  Given a* (cell D17) and b* (cell E17) –  Hue angle in radian (cell H17)

-  =IF(ATAN2(D17, E17)>=0,ATAN2(D17,E17), -  2*PI()+ATAN2(D17,E17))

–  Hue angle in degrees=DEGREES(H17)

27


Munsell Color Space

  Based on visual judgment –  Hue –  Value –  Chroma

Notation: 5R 5/10

Value ChromaHue

28



Device Dependent Color

  Computer graphics color space –  RGB

  Designers color space –  Swatchbook

  Printer color space –  CMYK

29


Device Independent Color

  Act as a reference color (profile connection) space in color management systems

Trumatch

Focoltone

CIEReference

Color Space

RGB1

RGB2Pantone

SWOP

CMYK

HSL

30



Color Matching

  Invariant (or spectral) match –  When two colors have the same spectrophotometric curves

(SPC), they will have the same tristimulus values. -  Their match is invariant

-  If (SPC)1 = (SPC)2, then (XYZ)1 = (XYZ)2

–  Can you think of examples of spectral match?

31


Color Matching

  Metameric (conditional) match –  When two colors have different spectrophotometric curves, but

have the same tristimulus values -  The color match is conditional. -  The two objects are metamers.

-  (XYZ)1 = (XYZ)2, but (SPC)1 ≠ (SPC)2

  Can you think of examples of metameric match?

32



Color Difference

  For any two colors, –  C 1: L 1*, a 1*, b 1* –  C 2: L 2*, a 2*, b 2* –  ∆E*ab is the total color difference

–  ∆E00 is based on more Complicated formulas.

ΔE = L1* - L2

* 2 + a1

* - a2* 2

+ b1* - b2

* 2

a*

L*

b*C2

C1

•

•∆E

33


Color Difference

  ∆L*, ∆a* and ∆b* expressed as human visual sensation –  Between a sample and a reference –  (sample-reference)

When ∆L* is '+' Lighter ∆L* is '-' Darker ∆a* is '+' Redder or less green ∆a* is '-' Less red or greener ∆b* is '+' Yellower or less bluer ∆b* is '-' Less yellow or bluer

The Sample is

34



Color Difference as a Number

  Spot colors and trademark colors need to be matched closely. –  Small color difference is allowed

∆E Perception Interpretation< 1 No difference Excellent match1~2 Just noticeable Good match4~6 Noticeable Fair match> 9 Strong difference Poor match

35

  ∆E Magnitude, Perceptibility, and Acceptability

Printing tolerances -- process inks

0

2

4

6

8

10

∆ E

Strong difference

Noticeable

Just noticeableNo difference

12

14

16

Instrument error

Ink verification

Printing tolerances -- single ink

Color imagereproduction analysis -- original vs. repro

18

As interpreted as a simple field

Perceptibility!Acceptability!

36



Color Tolerance as a Volume

  Three color tolerancing models –  ∆L*, ∆a*, and ∆b* form a rectangular box. –  ∆L*, ∆C*, and ∆hab* form a wedge. –  CMC takes on an ellipsoidal-shaped volume.

37


CMC Tolerancing

  The eye is better at detecting hue differences in the orange than in the green.

  The eye is more sensitive to low chroma difference than high chroma difference.

  The eye has greater acceptance for shifts in the lightness (l) than in the chromaticity (c) dimension.

38



Colorimetry Application

  Material specifications –  How inks are specified and compared

  Process characterization –  How color gamut, color variations, and RGB-to-CMYK

conversion are specified

  Color matching and image reproduction –  ∆E matters to color matching. –  ∆E does not matter to pleasing pictorial color image

reproduction.

39


Material Specifications

  Colorimetric properties of halftone tints –  Cyan screen tints

  Colorimetric differences –  Rubine ink & rhodamine ink

  Colorimetric specifications –  Standardized printing inks for offset lithography

-  ISO 2846

40



Colorimetric Properties

  Cyan halftone tints –  Four levels of dot area coverage

-  25%, 50%, 75%, & solid

0

10

20

30

40

50

60

70

400 450 500 550 600 650 700

Wavelength

C25C50C75C100

% R

efle

ctan

ce

max. absorption!— Their SPCs are non-crossing!— The wavelength of the max. ! absorption is at 630 nm!

•  The SPC of the solid patch is situated in the bottom.!

41


Colorimetric Properties

  Cyan halftone tints

— Tints have similar hue angle! in the a* b* diagram as the" solid, but differ in metric! chroma (C*)!− When solid IFT increases," hue shift may result – the" mass tone effect.!

b*!

-50!-40!-30!-20!-10!

0!10!20!30!40!50!

-50! -40! -30! -20! -10! 0! 10! 20! 30! 40! 50!

a*!•!

•!•!•!

50%!

100%!

42



Colorimetric Differences

  Two magenta inks –  Rubine is a reddish magenta ink.

-  Cost cheaper –  Rhodamine a bluish magenta ink.

-  More expensive

0

10

20

30

40

50

60

70

80

90

100

400 450 500 550 600 650 700

Wavelength (nm)

RubineRhodamine

% R

efle

ctan

ce

43


Colorimetric Differences

  Rubine & rhodamine –  Spectral reflectance in the blue region of SPC –  Hue angle of the a* b* diagram

0

10

20

30

40

50

60

70

80

90

100

400 450 500 550 600 650 700

Wavelength (nm)

RubineRhodamine

% R

efle

ctan

ce

b*

-80

-60

-40

-20

0

20

40

60

80

-80 -60 -40 -20 0 20 40 60 80

a*•

•Rubine

Rhodamine

(Yellowish)!

(Bluish)!

44



Colorimetric Specifications

  ISO 2846—Standardized printing inks for offset lithography –  C, M, Y, K –  A single set of color coordinates could adequately represent

standard inks around the world. -  Endorsed by ink associations from Europe, America, and Japan

45


ISO 2846-1 (2006)

  Sample preparation –  Should be made on the reference substrate

-  APCO II/II –  The nominal ink film thickness for web heat-set ink is 1 mm.

-  0.7-1.3 mm for cyan, magenta, and yellow -  0.9-1.3 mm for black

46



ISO 2846-1 (2006)

  Specification for color and transparency of printing inks –  Sheet-fed and heat-set web offset

  Colorimetric conditions –  D50 illuminant / 2o observer

47


ISO 2846-1 (2006)

  Example of conformance –  Ink 1

  Example of non-conformance –  Ink 2—Deviation of ink color –  Ink 3 —correct in ink color, not in

pigment concentration

1.0 1.50.5

2

4

6

8

10

0

∆E

Ink film thickness (µm)

1

2

3

tolerancerange

48



Process Characterization

  Color gamut comparison –  Monitor color gamut –  Printer color gamut

  Color variations –  Spatial uniformity within the sheet –  Temporal consistency from sheet-to-sheet –  XYZ-based TVI assessment

49


Monitor Color gamut

  Ambient lighting influences monitor color gamut.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8x

y G

C

B

M

R

Y

W

CIE spectrum locus

Dark

Light Dim

700600

575

550

525

500

475

450400

50



Monitor Color Gamut

  Ambient lighting also influences lightness range of the monitor. –  High ambient lighting

-  Adds flare in the black point -  Reduces lightness range

0 10 20 30 40 50 60 70 80 90 100Y

DarkDim

LightHard copy

(black) (white)

Hard copy has brighter white point!

51


Printer Color Gamut

  Chromaticity / a*b* diagram can be deceiving. –  Expressed in two dimensions without lightness

y

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0x

G

Y

R

M

B

C

CIE Spectrum Locus

700600

575

550

525

500

475

450400

Increaseddensity

Decreaseddensity

CIELAB a*b* -100

-50

0

50

100

-100 -50 0 50 100

a*

b*

Increased density

decreased density

52



Printer Color Gamut Slice

  Chromaticity at constant lightness plane requires special test targets.

-100

-80

-60

-40

-20

0

20

40

60

80

100

-100 -80 -60 -40 -20 0 20 40 60 80 100a*

53


Color Variation Rules

  All imaging processes have color variations. –  Some processes have more variations than other processes.

  Sources of variability –  Uniformity within the sheet –  Sheet-to-sheet –  Run-to-run

  Assignable variations can be corrected for; random variations cannot.

54



Uniformity within the Sheet

  Spot color by flexo (30 measurements across) Sample - Average Sample - Average

Location # L * a * b * ∆L* ∆a* ∆b* ∆ E Location # L * a * b * ∆L* ∆a* ∆b* ∆ E1 31.5 11.7 7.9 0.2 -0.1 -0.2 0.3 1 52.4 61.8 39.0 0.2 -0.9 -1.1 1.42 31.7 11.8 8.0 0.3 0.0 -0.1 0.4 2 52.3 62.2 39.6 0.0 -0.5 -0.6 0.83 31.5 11.8 8.1 0.2 0.0 0.0 0.2 3 52.3 62.1 39.6 0.0 -0.6 -0.6 0.84 31.6 11.7 8.0 0.3 -0.2 0.0 0.3 4 52.2 62.6 40.2 0.0 -0.1 0.0 0.15 31.6 11.8 8.0 0.3 0.0 0.0 0.3 5 51.8 62.7 40.4 -0.5 0.0 0.2 0.56 31.6 11.9 8.0 0.2 0.0 0.0 0.3 6 52.1 63.1 40.7 -0.1 0.4 0.5 0.77 30.8 11.8 8.0 -0.5 -0.1 -0.1 0.6 7 52.1 63.2 40.6 -0.2 0.5 0.5 0.78 30.9 11.8 8.2 -0.4 0.0 0.1 0.4 8 52.5 62.9 40.1 0.2 0.2 -0.1 0.39 31.1 12.0 8.1 -0.2 0.1 0.0 0.2 9 52.8 63.1 40.4 0.5 0.5 0.2 0.7

10 31.2 12.1 8.1 -0.1 0.3 0.0 0.3 10 52.1 61.5 39.1 -0.2 -1.2 -1.1 1.611 31.7 12.3 8.2 0.4 0.5 0.2 0.6 11 52.2 63.0 40.6 0.0 0.3 0.4 0.512 31.5 11.9 8.0 0.2 0.0 0.0 0.2 12 52.8 64.0 41.1 0.5 1.4 1.0 1.713 31.4 11.8 8.0 0.1 -0.1 0.0 0.1 13 51.8 62.6 39.9 -0.4 -0.1 -0.2 0.514 31.6 11.9 8.1 0.3 0.0 0.0 0.3 14 52.2 62.6 40.0 -0.1 0.0 -0.2 0.215 31.0 11.6 8.0 -0.4 -0.2 0.0 0.4 15 52.1 62.5 39.7 -0.2 -0.1 -0.4 0.516 31.4 11.7 8.0 0.0 -0.1 -0.1 0.1 16 52.6 62.7 40.0 0.3 0.0 -0.1 0.317 31.1 11.7 8.0 -0.3 -0.1 0.0 0.3 17 52.3 62.9 40.2 0.1 0.2 0.1 0.218 31.3 11.9 8.0 -0.1 0.0 0.0 0.1 18 52.4 62.8 40.0 0.1 0.1 -0.1 0.219 31.3 11.9 8.1 0.0 0.1 0.1 0.1 19 52.3 62.2 39.7 0.0 -0.5 -0.5 0.720 31.4 12.2 8.3 0.1 0.3 0.3 0.4 20 51.8 61.9 39.8 -0.5 -0.8 -0.4 1.021 31.9 12.3 8.3 0.6 0.4 0.3 0.8 21 52.3 62.8 40.2 0.1 0.2 0.0 0.222 31.4 12.0 8.2 0.1 0.2 0.2 0.3 22 52.2 62.8 40.2 0.0 0.1 0.0 0.123 30.7 11.8 7.8 -0.6 0.0 -0.2 0.6 23 52.0 62.5 40.3 -0.2 -0.2 0.1 0.324 31.1 11.7 8.0 -0.2 -0.1 0.0 0.2 24 52.0 62.3 40.3 -0.2 -0.4 0.1 0.525 31.1 11.8 8.1 -0.3 0.0 0.0 0.3 25 52.2 62.6 40.2 -0.1 -0.1 0.0 0.126 31.1 11.8 8.0 -0.3 -0.1 0.0 0.3 26 52.5 63.5 40.9 0.2 0.8 0.8 1.127 30.8 11.6 7.9 -0.5 -0.3 -0.1 0.6 27 52.4 63.0 40.6 0.2 0.3 0.4 0.628 31.4 11.8 8.0 0.1 -0.1 0.0 0.1 28 52.6 63.0 40.5 0.3 0.4 0.3 0.629 31.3 11.7 8.0 -0.1 -0.2 -0.1 0.2 29 52.9 63.5 40.9 0.6 0.8 0.8 1.330 31.9 11.7 7.9 0.6 -0.2 -0.1 0.6 30 52.0 62.1 39.8 -0.2 -0.6 -0.3 0.7

A v e r a g e 31 .3 11 .8 8 . 0 ∆E Ave. 0 . 3 A v e r a g e 52 .3 62 .7 40 .1 ∆E Ave. 0 . 6∆E Max. 0.8 ∆E Max. 1.7∆E Min. 0.1 ∆E Min. 0.1∆E Range 0.7 ∆E Range 1.7

52_B rown 75_Red

55


Consistency from Sheet-to-sheet

  Spot color by flexo (30 consecutive sheets) Sample - Average Sample - Average

Sample # L * a * b * ∆L* ∆a* ∆b* ∆ E Sample # L * a * b * ∆L* ∆a* ∆b* ∆ E1 31.6 11.8 8.2 0.1 0.0 0.0 0.1 1 52.7 63.2 40.5 0.3 0.2 0.0 0.42 31.1 11.4 8.0 -0.3 -0.4 -0.3 0.6 2 52.4 63.5 40.9 -0.1 0.5 0.5 0.73 31.6 11.8 8.3 0.1 0.0 0.1 0.1 3 52.3 63.3 41.2 -0.1 0.3 0.7 0.84 31.8 11.8 8.3 0.3 0.0 0.1 0.3 4 52.1 62.6 40.5 -0.4 -0.3 0.0 0.55 31.5 11.7 8.2 0.1 -0.1 -0.1 0.1 5 52.4 63.2 40.7 0.0 0.2 0.3 0.46 31.0 11.5 8.1 -0.4 -0.3 -0.2 0.5 6 52.3 62.4 40.2 -0.1 -0.5 -0.3 0.67 31.5 12.0 8.3 0.0 0.2 0.1 0.2 7 52.5 62.9 40.4 0.1 -0.1 -0.1 0.18 31.4 11.8 8.2 -0.1 0.0 0.0 0.1 8 52.7 63.5 40.7 0.3 0.5 0.2 0.69 31.2 11.5 8.1 -0.2 -0.3 -0.2 0.4 9 52.5 62.7 40.3 0.1 -0.2 -0.2 0.3

10 31.0 11.9 8.3 -0.5 0.1 0.1 0.5 10 52.6 63.0 40.5 0.2 0.0 0.0 0.211 32.0 12.1 8.5 0.5 0.3 0.2 0.6 11 52.4 63.4 41.0 0.0 0.5 0.5 0.712 31.5 12.1 8.4 0.1 0.3 0.1 0.3 12 52.4 62.6 40.1 -0.1 -0.4 -0.4 0.613 31.8 11.7 8.2 0.4 -0.1 0.0 0.4 13 52.0 62.4 40.0 -0.4 -0.5 -0.5 0.814 31.5 11.7 8.2 0.0 -0.1 -0.1 0.1 14 52.9 63.2 40.7 0.4 0.3 0.2 0.515 31.6 11.7 8.2 0.1 -0.1 -0.1 0.2 15 52.6 62.7 40.0 0.1 -0.3 -0.5 0.616 31.4 11.8 8.2 -0.1 0.0 0.0 0.1 16 52.8 63.4 40.5 0.4 0.4 0.1 0.617 31.3 11.9 8.4 -0.2 0.1 0.1 0.3 17 52.4 62.5 40.1 -0.1 -0.5 -0.4 0.618 31.4 11.8 8.2 -0.1 0.0 0.0 0.1 18 52.3 62.3 39.9 -0.1 -0.7 -0.6 0.919 31.7 12.1 8.3 0.2 0.2 0.1 0.3 19 52.4 62.7 40.1 0.0 -0.2 -0.4 0.520 31.4 11.9 8.3 0.0 0.1 0.1 0.1 20 52.4 62.6 40.4 0.0 -0.4 -0.1 0.421 31.4 11.8 8.3 -0.1 0.0 0.0 0.1 21 52.4 63.1 40.7 -0.1 0.1 0.2 0.322 31.9 12.1 8.4 0.4 0.2 0.1 0.5 22 52.3 62.7 40.2 -0.2 -0.2 -0.2 0.423 31.6 11.9 8.3 0.1 0.1 0.0 0.2 23 52.5 63.7 41.1 0.0 0.8 0.6 1.024 31.5 11.6 8.1 0.0 -0.2 -0.1 0.2 24 52.0 62.5 40.3 -0.4 -0.5 -0.2 0.625 31.3 11.7 8.1 -0.2 -0.1 -0.1 0.2 25 52.8 63.2 40.6 0.4 0.2 0.1 0.526 31.4 11.8 8.2 0.0 0.0 0.0 0.0 26 52.7 62.9 40.1 0.2 0.0 -0.4 0.527 31.6 11.8 8.1 0.1 0.0 -0.1 0.2 27 52.5 63.6 41.1 0.0 0.6 0.6 0.928 31.5 12.0 8.4 0.1 0.2 0.1 0.2 28 52.6 63.3 40.9 0.1 0.3 0.4 0.529 31.2 11.6 8.2 -0.3 -0.2 -0.1 0.4 29 52.4 62.6 40.2 0.0 -0.3 -0.3 0.430 31.3 11.9 8.4 -0.1 0.1 0.1 0.2 30 52.1 62.8 40.4 -0.3 -0.2 -0.1 0.4

A v e r a g e 31 .5 11 .8 8 . 2 ∆E Ave. 0 . 3 A v e r a g e 52 .4 62 .9 40 .5 ∆E Ave. 0 . 5(Production ∆E Max. 0.6 (Production ∆E Max. 1.0

center) ∆E Min. 0.0 center) ∆E Min. 0.1∆E Range 0.6 ∆E Range 0.9

52_B rown 75_Red

56



XYZ-based TVI (ISO/TS 10128)

  Magenta and black

  Yellow

  Cyan

€

TVI =100 Yp −YtYp −Ys

#

$ %

&

' ( − InputTV

€

TVI =100 Zp − ZtZp − Zs

#

$ %

&

' ( − InputTV

€

TVI =100 Xp − 0.55Zp( ) − Xt − 0.55Zt( )Xp − 0.55Zp( ) − Xs − 0.55Zs( )

#

$ % %

&

' ( ( − InputTV

Subscripts p: paper t: tint s: solid

57


Color Matching

  Spot color printing vs. CMYK printing –  Avoid out-of-gamut color –  Small ∆E matters

Pantone swatchbook is printed by dry offset using special formulated inks. Pantone-to-digital printing uses a RIP-based look-up table.

58



Pictorial Color Image

  Pleasingness of the reproduction matters. –  Minimum ∆E is not important

-  Particularly when ∆E is due to gamut clipping

59


Color Image Reproduction

  Color Research & Appl. (Pearson, 1986) –  Large ∆Es exist between the original and an acceptable

photographic print. -  Tone reproduction and gamut compression are key factors for

pleasing reproduction

All patches Std. Observer Geometry color temp.statistics in Macbeth from 1931 to 0/45 to difference

colorchecker 1964 sphere by 400oKAverage 15.9 1.4 3.2 3.1Std. dev. 6.8 1.6 3.3 2.3Min. 4.4 0 0.2 0.5Max. 31.7 4.7 12.3 8.4Range 27.3 4.7 12.1 7.9

60



Summary

  Color is a visual sensation. –  Light, object, and human vision are involved.

  Color can be measured by numbers. –  CIELAB color space is the language for color specifications.

  Colorimetry has multiple uses in graphic arts imaging practices.

  Colorimetry cannot judge pleasingness. –  Only the human color perception can –  Psychometric is a branch of statistics that studies human sensory

responses, e.g., pleasingness of pictorial color reproduction.

61


References

  ISO/DIS 13655 (2009) Graphic technology — Spectral measurement and colorimetric computation for graphic arts images

  Cheydleur and O’Connor (2011), The M Factor… What Does It Mean?

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Documents

Colorimetry and Its Applicationsrycppr.cias.rit.edu › independent_study › 2 Colorimetry_2015b...Colorimetry and Its Applications 1 Colorimetry and Its Applications R. Chung, Professor