Transcript Minimize Profiling Error

ICC Color Symposium
13/11/2013 · Hong Kong
Minimizing
Profiling Error
John Seymour
Applied Math., QuadTech
Organizer
Implementing Agent
Ragy Isaac
Quality Director, Goss
Co-organizers
Agenda
• Image-setter linearization.
• Press calibration and TVI determination.
• Image-setter / Press topics:
– Goal
– Test, measure, and analyze
– System check
– Choosing system components
– Summary
Main Theme
Same input
Produces
Same result
Within a variability
Blocks/TVI Test Forms
Blocks Form
Paper
Motion
Direction
TVI Form
Magnified Cyan TVI Form
Data scatter - Variability
• The same 40% CCD measurements will
not be exactly the same.
– Data scatter about the average
UCL
X
XX
XXX
XXXX
XXXXX
40% dot Average
XXXXX
XXXX
XXX
XX
X
LCL
_
X
Image-Setter System: Goal
• Computer file dot = plate dot
• Provide a methodology to allow:
– Linearization:
• Force image-setter to produce intended dot on plate
– Understanding system’s natural variability
– System Diagnosis if off target
– Choice of a winning system
Error = File Dot – Plate Dot
Image-Setter: Test & Measure
•
•
•
Set the image-setter to mfg specs.
Create a plate with the TVI test form.
With a CCD measuring device:
– Measure and input in spreadsheet every
plate spot.
TVI Form
Image-Setter: Analyze
•
•
Graph the data in Excel
Calculate average & variability (standard
deviation) for every dot%.
1. Analyze data per color and per dot%
2. Combine colors & re-analyze per dot%
3. Similar average values in step 1 & 2 ??
• Pre-press treats CMYK the same.
Cyan Dot = Magenta Dot = Yellow Dot = Black Dot
Image-Setter: RIP Input
•
If CMYK %dots are equal for same %dots
– Combine all CMYK dots
– Set the RIP linearization curve to the average
dot values.
•
If CMYK %dots are NOT equal:
– Create separate linearization curves for each
color.
•
Determine variability for every dot %
Daily System Check
• Daily check: should include 4 strips
• Average all 4 strips for each dot %
• If within linearized values ± variability
– Do nothing. Do not re-adjust your RIP.
– Adjusting the RIP is tampering
• Tampering pushes the system off target
Daily System Check: Example
Upper Control Limit = 40.7
UCL
40%
LCL
Lower Control Limit = 39.3
X
XX
XXX
XXXX
XXXXX
XXXXX
XXXX
XXX
XX
X
_
X
Daily System Check: Off Range?
• If system correctly linearized:
– Do not re-linearize!
– Identify root causes
• New consumable batch,
• Room temperature,
• Maintenance, …etc
– Rectify the issue.
How to Choose Best Components?
• The best system should be:
– Digital values = Average dots on plate
– The lowest variability across the plate for
every dot %
• Applies to:
– Image-setter
– Aluminum plate
– Chemistries
Prepress system Summary
•
We have a methodology to:
– Linearize the system
– Quantify & control performance
– Choose the perfect system
Press System: Goal
• Same image/key value = Same density
for:
– All presses, press keys, press speeds, solids
& dot%
• Understanding system’s natural variability
• System Diagnosis if off target
• Choice of a winning system
Press: Testing
• Three press tests:
– Zeroing test with no image and no water
– Blocks Test: uniform solid (100%) image
– TVI Test: uniform variable dot image
light EVEN print
Press: Testing – All Tests
• To reduce variables & speed-up testing:
– Set all press ink keys the same
– Use the same image across the entire press,
– Set all press keys the same
– Evaluate inking with & without dampening
light EVEN print
Press: Zeroing Test
• Test conditions:
– Set press to mfg specs.
– Blank plate, no image
– No dampener, one color at a time
– Full ink fountain
– All press keys set to zero
– Use any paper
– Low press speed to conserve paper.
light EVEN print
• Needed result: very light EVEN print
Press: Block Test
• Test conditions:
– Uniform image plate
– With calibrated dampener
– Full ink fountain
– All press keys set the same
– Use any paper
– Production press speed.
• Needed result: EVEN density
Press: Measure Densities
• Measure solid density inside each square
• Record data in a spreadsheet
Press: Analyze Densities
• Graph densities in Excel and view pattern
• Density variation has no pattern??
– Yes: press is set-up correctly
– No: systematic press issue
• Correct to eliminate pattern
• Calculate average & variability (standard
deviation.)
Solid Density
How to Choose Best Components?
• The best system should be:
– Higher average solid densities for the same
ink rheology
– The lowest variability across the press
• Applies to:
– Ink, dampening solution, blankets, … etc
Press: TVI Test
• Test conditions:
– Uniform image plate
– With calibrated dampener
– Full ink fountain
– Set Solids to density specs
– Use frequently used paper
– Production press speed
• Needed result: required solid density
Density versus CIELAB
• Density is a single number, so it is userfriendly
• It is useful because
– Density relates (loosely) to ink film thickness
– Density relates (loosely) to the color that we
see
Density does not predict color
Pure yellow ink
Yellow with
cyan
Yellow with
magenta
These patches might have
the same density
Density and CIELAB
• ISO 12647 specifies L*a*b* of CMYK
solids
• ISO 12647 does not specify density
Quick review of CIELAB
• Colors can be
characterized
in terms of
– Chroma
– Hue
Quick review of CIELAB
• Or in terms of
b*
– a*
– b*
a*
Quick review of CIELAB
• All colors of a
particular hue can
be characterized
in terms of
– Lightness
– Chroma
Quick review of CIELAB
• Color is three
dimensional
– Lightness
– Hue
– Chroma
Quick review of CIELAB
• Color is three
dimensional
– Lightness
– Hue
– Chroma
(L*a*b*)
Density and CIELAB
• ISO 12647 specifies L*a*b* of CMYK
solids
• ISO 12647 does not specify density
• How do you find the correct density?
• We need to find the relationship between
the two
Cyan color as density changes
What density
should I run to?
0.73D
1.00D
1.26D
1.48D
Target a*b*
Different view of the same data
What density
should I run to?
ISO 12647 control
Keeping in
this range
What density
should I run to?
ISO 12647 control
Keeping in
this range
will keep
DE < 4
What density
should I run to?
Ink trajectory
The green line is the
estimated ink trajectory.
This was computed from
the spectra at 1.09D,
using Beer’s law.
Ink trajectory
Beer’s law can be used
to estimate the
“perifarbe” density
from a single
measurement.
Press: Measure Densities
• Measure solid & dot densities
• Record data in a spreadsheet
Press: Analyze Densities
•
•
•
•
•
•
•
•
Graph densities & view pattern per dot%
Calculate average & variability
Exclude columns if solid density is off-spec
Calculate TVI for on-spec columns
Input TVI values in prepress
FORCE TVI to match ISO Specs
Reprint with new TVI settings
Revaluate TVI
How to Choose Best Components?
• The best system should be:
– Provides lower average TVI
– Lower TVI variability for each dot%
– Higher average solid densities for
the same ink rheology
• Applies to:
– Ink, dampening solution, blankets, … etc
Press system Summary
•
We have a methodology to:
–
–
–
–
Use simple and effective test forms
Calibrate a press with and without water
Evaluate TVI & match ISO spec
Choose the perfect system
Comparison of halftone dots
Stochastic
Conventional,
ideal hard dots
Conventional,
real dots
Comparison of halftone dots
Conventional,
ideal hard dots
Conventional,
real dots
Stochastic,
gravure, and
ink jet
Increasing TVI
True
continuous
tone
Cyan ink – comparison of different screening
Stochastic
Conventional
50%
50%
Cyan ink – comparison of different screening
• Stochastic has much higher TVI (tone
value increase)
• Stochastic is greener; conventional is
bluer
Comparison to two popular equations
MurrayDavies
Beer’s
law
Equation for perfect hard dots
Murray-Davies
equation
Equation for perfect continuous tone
Beer’s law
equation
Curve for mushy dots
Linearization to the Rescue
• Need to linearize both stochastic and
conventional
• Limitation – plate curve can’t change hue
of halftones
• Set-up a workflow for each
Color Profile: Reduce Press Errors
Why randomize?
Column Number
Variation within an ink key zone
• It is well known that there is lateral
variation in print density due to ink key
openings.
• There is also a fair amount of
circumferential variation (that is, within a
single ink key zone).
Variation within an ink key zone
Are they all the same color?
From presentation by Franz Sigg at TAGA 2007
Variation within an ink key zone
• The maximum variations of the solids were
close to 0.08D.
Variation within an ink key zone
• The variation has a pattern.
• The variation is consistent from one sheet
to the next.
• Even with the ink keys set perfectly, there
will still be substantial circumferential
variation in density.
Color Profile: Reduce Press Errors
Randomization gives us an
undistorted view of TVI
Profiling: Prerequisites
• Linearized image-setter
• Calibrated press
• Repeatable & predictable TVI
Color Profile: Advice
• Increase total image coverage:
– Use solid bars across the press
• Don’t use photos in-line with IT8 target
• Avoid creases and mechanical
imperfections
• Use reliable measuring device
IT8 Target with Bars
Summary
• To minimize profiling error:
– Linearize the image setter
• You may need multiple linearization workflo
– Correctly set the ink fountain zero
– Ensure accurate dampening
– Determine the system’s TVI
– Randomize the IT8 target
– Don’t use photos along with the IT8 taget
– Increase ink coverage by using solid bars.
ICC Color Symposium
13/11/2013 · Hong Kong
Minimizing
Profiling Error
John Seymour
Applied Math., QuadTech
Organizer
Implementing Agent
Ragy Isaac
Quality Director, Goss
Co-organizers