Minimizing Profiling Error Ragy Isaac Quality 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 Forms TVI FormBlocks Form Paper Motion Direction

Magnified Cyan TVI Form

Data scatter - Variability 40% dot Average 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 systems 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% Upper 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 systems 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 with cyan Yellow with magenta

Density and CIELAB ISO specifies L*a*b* of CMYK solids ISO 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 –a* –b* a* b*

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 specifies L*a*b* of CMYK solids ISO 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 1.00D 1.26D 1.48D

Different view of the same data What density should I run to?

ISO control What density should I run to? Keeping in this range

ISO control What density should I run to? Keeping in this range will keep E < 4

Ink trajectory The green line is the estimated ink trajectory. This was computed from the spectra at 1.09D, using Beers law.

Ink trajectory Beers 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% Conventional Stochastic

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 Beers law

Equation for perfect hard dots Murray-Davies equation

Equation for perfect continuous tone Beers law equation

Curve for mushy dots

Linearization to the Rescue Need to linearize both stochastic and conventional Limitation – plate curve cant 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 Dont 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 systems TVI –Randomize the IT8 target –Dont use photos along with the IT8 taget –Increase ink coverage by using solid bars.

Minimizing Profiling Error Ragy Isaac Quality Director, Goss ICC Color Symposium Organizer Implementing Agent Co-organizers 13/11/2013 · Hong Kong John Seymour Applied Math., QuadTech