Colorimetry and Its Application 1 Colorimetry and Its Application R. Chung, Professor Colorimetry has become more and more useful in graphic arts imaging for color specification, process control, and quality assurance. (Rev._2015) Copyright 2015 RIT – May not be reproduced without permission Topics Covered Light, object, and color vision Tristimulus integration – 1931 CIEXYZ color space – 1976 CIELAB color space Color matching and color difference Colorimetry application 2
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Colorimetry and Its Application 1
Colorimetry and Its Application R. Chung, Professor
Colorimetry has become more and more useful in graphic arts imaging for color specification, process control, and quality assurance.!
(Rev._2015)!
Copyright 2015 RIT – May not be reproduced without permission
Topics Covered
Light, object, and color vision
Tristimulus integration – 1931 CIEXYZ color space – 1976 CIELAB color space
Color matching and color difference
Colorimetry application
2
Colorimetry and Its Application 2
Copyright 2015 RIT – May not be reproduced without permission
Copyright 2015 RIT – May not be reproduced without permission
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.
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Colorimetry and Its Application 3
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Color Vision Tests
Ishihara�s test for color blindness
Farnsworth-Munsell 100 Hue test
5
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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"
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Colorimetry and Its Application 4
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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
7
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Color Matching Experiment
Three imaginary primaries (R, G, B) were used in the color matching experiments.
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Colorimetry and Its Application 5
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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.!
9
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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 !
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Colorimetry and Its Application 6
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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"
11
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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
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Colorimetry and Its Application 7
Copyright 2015 RIT – May not be reproduced without permission
Copyright 2015 RIT – May not be reproduced without permission
Munsell Color Space
Based on visual judgment – Hue – Value – Chroma
Notation: 5R 5/10
Value ChromaHue
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Colorimetry and Its Application 16
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Device Dependent Color
Computer graphics color space – RGB
Designers color space – Swatchbook
Printer color space – CMYK
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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
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Colorimetry and Its Application 17
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Color Matching
Invariant (or spectral) match – When two colors have the same spectrophoto-metric curves
(SPC), they will have the same tristimulus values. - Their match is invariant
- If (SPC)1 = (SPC)2, then (XYZ)1 = (XYZ)2
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Color Matching
Metameric (conditional) match – When two colors have different spectrophoto-metric 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
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Colorimetry and Its Application 18
Copyright 2015 RIT – May not be reproduced without permission
Color Difference
For any two colors, – C 1: L 1*, a 1*, b 1* – C 2: L 2*, a 2*, b 2* – ∆E is the total color difference
ΔE = L1* - L2
* 2 + a1
* - a2* 2
+ b1* - b2
* 2
a*
L*
b*C2
C1
•
•∆E
∆E is the abbreviation for �Empfindung� in German, i.e., sensation.
35
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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
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Colorimetry and Its Application 19
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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
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∆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
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As interpreted as a simple field
Perceptibility!Acceptability!
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Colorimetry and Its Application 20
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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.
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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.
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Colorimetry and Its Application 21
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Summary
Color is a visual sensation. – Light, object, and human vision are involved.
Tristimulus integration is the basis for color measurement. – CIELAB color space is the language for color specifications.
Color matching is possible between metamers. – Color difference is useful for determining acceptability.
41
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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.
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Colorimetry and Its Application 22
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Material Specifications
Colorimetric properties of halftone tints – Cyan screen tints
Colorimetric specifications – Standardized printing inks for offset lithography
- ISO 2846
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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.!
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Colorimetry and Its Application 23
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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%!
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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
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Colorimetry and Its Application 24
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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)!
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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
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Colorimetry and Its Application 25
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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
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ISO 2846-1 (2006)
Specification for color and transparency of printing inks – Sheet-fed and heat-set web offset
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
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Colorimetry and Its Application 30
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Consistency from Sheet-to-sheet
Spot color by flexo (30 consecutive sheets) Sample - Average Sample - Average
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Summary 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
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Colorimetry and Its Application 33
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Summary
Colorimetry has multiple uses in graphic arts imaging practices. – Ink specifications and standardization – Process characterization and standardization – Spot color matching
Colorimetry cannot judge pleasingness of pictorial color images. – Only the human color perception can