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Capabilities and Limitations of
Color Measurement with an
RGB Camera
John Seymour
Principal Engineer – Research
QuadTech
CMM 2009
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Summary
Web inspection systems
(with RGB cameras)
offer CIELAB measurements.
They are not very
accurate.
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Summary
There is a distinction between
measuring CIELAB
and measuring DE
RGB cameras can
measure DE
“accurately enough”
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Ok, so what’s a CIELAB?
• CIELAB (or L*a*b) measures color
• CIELAB sees color the way people do
Well… kinda
• A measurement of color is three numbers
• The numbers uniquely define the color
• If L*a*b* values are the same,
the colors match
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Ok, so what’s a CIELAB?
• CIELAB (or L*a*b) measures color
• CIELAB sees color the way people do
Well… kinda
• A measurement of color is three numbers
• The numbers uniquely define the color
• If L*a*b* values are the same,
the colors match
R B G
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Ok, so what’s a CIELAB? R B G
Now that we have CIELAB
Matching by the eye is
a thing of the past!!
Did I mention that I am in research?
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What’s a DE?
• A DE (pronounced delta-E) is a measure
of color difference
• Useful for setting tolerances
• 1 DE is pretty small
• 10 DE is moderate
• 100 DE is way big
100 DE is the difference
between black and white
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What’s a DE?
Ideally
But…
1 DE should have been
a “just noticeable difference”
in color.
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What’s a DE?
5 DE difference in the saturation of
yellow is just noticeable.
0.5 DE difference around gray is just noticeable.
But…
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What’s a DE?
So they revised the formula:
The original: DE, or DE76, or DEab
First revision: DECMC (1984)
Second revision: DE94
Third revision: DE00
Fourth revision: DE99, or DIN 99 DE
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The wonderful thing
about standards is that
there are
so many to choose from!
We will use the original DE.
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from the Summary...
CIELAB from an RGB camera
is not accurate enough
DE from an RGB camera
is accurate enough
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What is “accurate enough”? R B G
First thought
the human visual system can
discern down to 1 DE (kind of)
so a color measurement system
has to do the same
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What is “accurate enough”? R B G
Second thought
press variation is larger than 1 DE
how close do we need to hold
color anyway?
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ISO 12647-6:2006(E)
Graphic technology –
Process control for the production of
half-tone colour separations, proof
and production prints –
Part 6: Flexographic printing,
Section 4.3.2.3
The ISO standard R B G
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12647-6 requirements L* a* b*
C 55 -36 -44
M 46 70 -3
Y 84 -5 88
K 20 0 0
Proof
Patches on the proof
must match the
numbers
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This is not a concern
for an online
measurement device
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12647-6 requirements L* a* b*
C 55 -36 -44
M 46 70 -3
Y 84 -5 88
K 20 0 0
Proof
OK sheet
Patches on the OK sheet
must match the proof
to within 8 DE
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12647-6 requirements L* a* b*
C 55 -36 -44
M 46 70 -3
Y 84 -5 88
K 20 0 0
Proof
OK sheet
Production
68% of patches
in production
must match the OK
sheet to within 2.5 DE
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What is “accurate enough”?
Accuracy tolerances of 8 DE
in measuring color ok sheet
The SPC folks say we need
accuracy of at the very least
2.4 DE and preferably 0.8 DE
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What is “accurate enough”?
Accuracy tolerances of 2.5 DE
in measuring production sheets
The SPC folks say we need
accuracy of at the very least
0.75 DE and preferably 0.25 DE
R B G
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Vendor claims
• “Colour monitoring with deviations calculated and expressed in delta E (@E) value.”
• “Based on the CIE-Lab colour”
• “on-line accurate L*a*b* color measurement & management”
• “accurate in-line color measurement”
• “Today, online color monitoring accuracy has reached levels previously attainable only with the use of handheld spectrophotometers.”
• “…independent tests were undertaken to study the correlation of color results between [the web inspection system] and a spectrodensitometer. The results… found almost a perfect correlation…”
• “Accuracy: CIELab colour value: 1 DE”
• “Today’s online technology allows color deviation to be measured as accurately as 0.2 ∆e.”
R B G
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That’s what marketing has to say
What does the scientific literature say?
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Review of literature Author(s) Data Method Accuracy
Kang & Anderson 236 patches (QC60) 3X6 Matrix 8 - 12
Viggiano & Wang 236 patches (QC60) Principal Component Analysis
(PCA) 4.1
Wandell & Farrell 214 CMYK 3X3 Matrix 6.0
“ “ 3X3 Matrix + correction 2.4
Sodergard et al. 236 patches Newsprint 13 term polynomial 5.8
Seymour (1997) 995 CMYK 3X9 Matrix 2.0
“ 24 Pantone “ 7.0
Seymour (2009) 11,664 CMYK 3X9 Matrix 1.5 – 2.4
Brydges, et al. 24 Newsprint 3X9 Matrix & PCA 1.2 -1.4
Ben Chouikha et al. 93 NCS set 3X3 Matrix 11.6
“ “ 3X3 Matrix + 3rd order
CIELAB regression 2.4
“ CRISATEL acrylic
chart
3X3 Matrix + 3rd order
CIELAB regression 5.3
Urban, et al. 1269 Munsell + Vrhel
database Metamer analysis 4.1 - 8.2
R B G
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Review of literature R B G
The most optimistic results
are just barely good enough for color ok…
and no where near good enough
for the production run.
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12647-6 requirements
Proof
OK sheet
Patches on the OK sheet
must match the proof
to within 8 DE
R B G
Different
measurement
devices
We need
CIELAB
accuracy
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12647-6 requirements
OK sheet
Production
68% of patches
in production
must match the OK
sheet to within 2.5 DE
Here we need
DE accuracy
Same camera and stock
OK and Production
sheets will have wrong
CIELAB values
Hopefully the errors
will be similar
R B G
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12647-6 requirements R B G
For color ok CIELAB values must be accurate
For production DE values must be accurate
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Accuracy of DE
Summary of results from
my TAGA 2009 paper
R B G
on CMYK colors
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Accuracy of DE
Correct DE values
RG
B d
eri
ved
DE
valu
es
R B G
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Accuracy of DE
False determination rate
Camera 1 3.2%
Camera 2 3.3%
Camera 3 2.6%
Status E 2.6%
An RGB camera and a spectro
will agree 97% of the time
R B G
When assessing pass/fail
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Conclusions from TAGA 2009
RGB camera can’t
measure CIELAB
“accurately enough”.
RGB camera can
measure DE
“accurately enough”.
R B G
You can’t use an
RGB camera for
color OK.
You can use an
RGB camera for
production run.
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What about specialty colors? R B G
The TAGA paper
tested CMYK
Will Pantone work
the same?
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What about specialty colors? R B G
I started with
the spectra of 1,124 Pantone colors
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What about specialty colors? R B G
I developed an
RGB CIELAB transform
for these inks
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Specialty colors - CIELAB R B G
CMYK Set Pantone Set
Mean DE 95th % Mean DE 95 %
Camera 1 1.8 4.5 3.6 8.9
Camera 2 2.3 5.4 4.8 14.0
Camera 3 1.8 4.3 4.4 10.0
Status E 1.5 3.2 4.2 10.2
CIELAB accuracy –
not so good
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What about specialty colors? R B G
I repeated the
CMYK DE test
Only with Pantone
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What about specialty colors? R B G
CMYK Set Pantone Set
Mean DE 95th % Mean DE 95 %
Camera 1 0.16 0.52 0.21 0.90
Camera 2 0.22 0.72 0.49 2.29
Camera 3 0.20 0.61 0.22 0.91
Status E 0.17 0.52 0.19 0.83
DE accuracy –
not too bad
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Conclusions for Pantone Inks
RGB camera can’t
measure CIELAB
“accurately enough”.
RGB camera can
measure DE
“accurately enough”.
R B G
You can’t use an
RGB camera for
color OK.
You can use an
RGB camera for
production run.
From CMM 2009 paper
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Conclusions for CMYK inks
RGB camera can’t
measure CIELAB
“accurately enough”.
RGB camera can
measure DE
“accurately enough”.
R B G
You can’t use an
RGB camera for
color OK.
You can use an
RGB camera for
production run.
From TAGA 2009 paper
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Conclusion
RGB cameras can be used
to measure color
… but you have to understand
the limitations.
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Thank you!
John Seymour
Principal Engineer - Research
QuadTech
R B G