Minimizing Profiling Error

Ragy IsaacQuality Director, Goss

ICC Color Symposium

Organizer Implementing Agent Co-organizers

13/11/2013 · Hong Kong

John Seymour Applied Math., QuadTech

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 FormsTVI FormBlocks Form

Paper Motion

Direction

Magnified Cyan TVI Form

Data scatter - Variability

40% dot AverageXXXXX

XXXXXXXXXXXXXXX

XXXXXXXXXX

X_

LCL

UCL

• The same 40% CCD measurements will not be exactly the same.– Data scatter about the average

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

40%XXXXX

XXXXXXXXXXXXXXX

XXXXXXXXXX

X_

LCL

UCLUpper Control Limit = 40.7

Lower Control Limit = 39.3

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.

• Needed result: very light EVEN print

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 user-friendly

• 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

These patches might have the same density

Pure yellow ink

Yellow withcyan

Yellow withmagenta

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 characterizedin terms of

– Chroma– Hue

Quick review of CIELAB

• Or in terms of

– a*– b*

a*

b*

Quick review of CIELAB

• All colors of aparticular hue canbe characterizedin terms of

– Lightness– Chroma

Quick review of CIELAB

• Color is threedimensional

– Lightness– Hue– Chroma

Quick review of CIELAB

• Color is threedimensional

– 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?

Target a*b*

0.73D

1.00D1.26D

1.48D

Different view of the same data

What density should I run to?

ISO 12647 control

What density should I run to?

Keeping in this range

ISO 12647 control

What density should I run to?

Keeping in this range will keep DE < 4

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

Conventional, ideal hard dots

Stochastic

Conventional, real dots

Comparison of halftone dots

Conventional, ideal hard dots

Stochastic,gravure, and

ink jet

Conventional, real dots

True continuous

tone

Increasing TVI

Cyan ink – comparison of different screening

50%

50%

ConventionalStochastic

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

Murray-Davies

Beer’slaw

Equation for perfect hard dots

Murray-Daviesequation

Equation for perfect continuous tone

Beer’s lawequation

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.

Minimizing Profiling Error

Ragy IsaacQuality Director, Goss

ICC Color Symposium

Organizer Implementing Agent Co-organizers

13/11/2013 · Hong Kong

John Seymour Applied Math., QuadTech