Computational Evaluation of Print Unevenness According to Human Vision

Albert Sadovnikov

COMPUTATIONAL EVALUATION OF PRINT UNEVENNESS ACCORDING TO HUMAN VISION

Acta Universitatis Lappeenrantaensis 412

Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in the Auditorium of the Student Union House at Lappeenranta University of Technology, Lappeenranta, Finland on the 11th of December, 2010, at noon.

Albert Sadovnikov

COMPUTATIONAL EVALUATION OF PRINT UNEVENNESS ACCORDING TO HUMAN VISION

Acta Universitatis Lappeenrantaensis 412

Thesis for the degree of Doctor of Science (Technology) to be presented

with due permission for public examination and criticism in the Auditorium

of the Student Union House at Lappeenranta University of Technology,

Lappeenranta, Finland on the 11th of December, 2010, at noon.

MahineVisionandPatternReognitionLaboratory

DepartmentofInformationTehnology

FaultyofTehnologyManagement

LappeenrantaUniversityofTehnology

Finland

Reviewers ResearhDiretor,Doent,Dr. MarkkuHauta-Kasari

InFotonisCenter

UniversityofEasternFinland

Finland

ProfessorGöteNyman

DepartmentofPsyhology

UniversityofHelsinki

Finland

Opponents ProfessorJussiParkkinen

ComputerSiene

ShoolofComputing

UniversityofEasternFinland

Finland

ProfessorGöteNyman

DepartmentofPsyhology

UniversityofHelsinki

Finland

ISBN978-952-265-013-9

ISBN978-952-265-014-2(PDF)

ISSN1456-4491

Lappeenrannanteknillinen yliopisto

Digipaino2010

TheworkpresentedinthisthesishasbeenarriedoutattheLaboratoryofInformation

Proessingin theDepartmentofInformationTehnologyatLappeenrantaUniversityof

Tehnology,Finland,during theyears2003-2007.

For their nanial support I would like to thank the Papvision projet partners: the

European Union, Tekes, Stora Enso Oyj, UPM-Kymmene Oyj, Myllykoski Paper Oy,

MetsoPaperOyandLabvisionTehnologiesOy. Researhforthethesiswasonduted

under theTekesprojets70049/03,70056/04and40483/03.

Lappeenranta,August2010

Albert Sadovnikov

Albert Sadovnikov

ComputationalEvaluationof Print UnevennessAording to Human Vision

Lappeenranta,2010

95 p.

AtaUniversitatisLappeenrantaensis412

Diss. LappeenrantaUniversityofTehnology

ISBN 978-952-265-013-9

ISBN 978-952-265-014-2(PDF)

ISSN 1456-4491

Printquality andthe printability ofpaperareveryimportantattributeswhen modern

printingappliationsareonsidered. Inprintsontainingimages, highprintqualityisa

basirequirement. Toneunevennessandnon-uniformglossinessofprintedprodutsare

the most disturbing fators inuening overall print quality. These defets are aused

by non-idealinterations ofpaper,ink andprintingdevies in high-speedprintingpro-

esses. Sineprintqualityisapereptiveharateristi,themeasurementofunevenness

aordingtohumanvisionisasigniantproblem.

In this thesis,the mottlingphenomenon is studied. Mottling is aprintingdefet har-

aterized by a spotty, non-uniform appearane in solid printed areas. Print mottle is

usuallytheresultofuneveninklay-downornon-uniforminkabsorptionarossthepaper

surfae, espeially visible in mid-tone imagery orareasof uniformolor, suhas solids

andontinuous-tonesreenbuilds.

Byusingexisting knowledgeonvisualpereptionandknownmethodstoquantifyprint

tonevariation,anewmethodforprintunevennessevaluationisintrodued. Themethod

is ompared to previous results in the eld and is supported by psyhometri experi-

ments. Pilotstudiesaremadetoestimatetheeetofoptialpaperharateristisprior

to printing,ontheunevennessoftheprintedareaafterprinting. Instrumentalmethods

for print unevenness evaluation havebeenompared and theresultsof the omparison

indiate that the proposed method produes better results in terms of visual evalua-

tion orrespondene. The method has been suessfully implemented as an industrial

appliation andisprovedto beareliablesubstitutetovisualexpertise.

Keywords: qualityinspetion, paper industry, mottling, pereived unevenness, visual

pereption, psyhometris, automated optial inspetion, mahine vision,

image proessing and analysis, printing, print mottle, instrumental mea-

surements

UDC 004.932.2: 676.017.55

C50 yanat 50%density

C70 yanat 70%density

pd ylesperdegree

CSF ontrastsensitivityfuntion

HSWO heat-setweboset

HVS humanvisualsystem

HWC high weightoatedpapergrade

ISO InternationalOrganizationforStandardization

K50 blakat50%density

K70 blakat70%density

LGN lateralgeniulatenuleus

LWC lightweightoatedmehanialpaper

M70 magentaat70%density

MF mahinenishedpapergrade

MFC mahinenishoated

MFS unoatedmehanialspeialitypaper

MTF modulationtransferfuntion

MV mahinevision

MWC medium weightoated

RG rotogravureprinting

RGB red-green-blue

ROI regionofinterest

SC superalenderedpapergrade

SC-A superalenderedmehanialpaper

SFDA stohasti frequenydistribution analysis

SFO sheet fedoset

ULWC ultralightweightoated papergrade

WFC woodfreeoatedpaper

Y70 yellowat70%density

1 Introdution 11

1.1 Researhquestions . . . 12

1.2 Contributionsandpubliations . . . 13

1.3 Strutureofthethesis . . . 14

2 Paper and printquality 17 2.1 Paperanditsproperties . . . 18

2.2 Paperandprinting . . . 21

2.3 Printquality . . . 25

2.4 Printmottle . . . 26

2.5 Summary . . . 31

3 Human visionand psyhometris 32 3.1 Bakground . . . 32

3.2 Spatialvision . . . 35

3.2.1 Physiologialmehanisms . . . 35

3.2.2 Spatialfrequenytheory . . . 36

3.3 Contrastsensitivityfuntion. . . 37

3.4 Colorvision . . . 42

3.5 Psyhometri saling . . . 43

3.5.1 Diretintervalsaling . . . 44

3.5.2 Indiretintervalsaling . . . 45

3.6 Summary . . . 48

4 Models for mottlingevaluation 49 4.1 Overview . . . 49

4.2 Spatialdomain . . . 50

4.2.1 Speiperimeter . . . 50

4.2.2 ISO13660. . . 52

4.2.3 Stohasti frequenydistribution analysis(SFDA) . . . 53

4.2.4 Cluster . . . 54

4.3 Frequenydomain . . . 55

4.3.1 Band-pass. . . 56

4.3.2 ModiedBand-pass . . . 57

4.3.3 EnhanedBand-pass . . . 59

4.4 Colormethods . . . 60

4.4.1 Integrationmodel . . . 60

4.4.2 Pattern-olorseparablemethod . . . 62

4.5 Summary . . . 63

5 Experimentsand tests 65 5.1 Psyhometri experiments . . . 65

5.1.1 Diretsaling . . . 66

5.2.1 Summary . . . 74

5.3 Randomstimuligeneration . . . 75

5.3.1 Useoftheresults. . . 77

5.4 Preditingthemottle. . . 81

5.4.1 Papersamples . . . 81

5.4.2 Imagingandprintingsamples . . . 82

5.4.3 Experiments . . . 82

5.5 Summary . . . 86

6 Disussion 87

Bibliography 89

Introdution

Print quality is an essentialattribute when modern printing proesses are onsidered.

This is beause an inreasing proportion of data to be printed is in image form. If

the originalof aprint isassumed to be ideal,printquality depends on theprintability

of paper, printinginks, and the printing proess. Despite major improvements in the

aforementionedfatorsaetingthequality,thereareseveralundesiredeetsinprints.

One of the most severe defets is mottling, whih is the uneven appearane of asolid

printed area. It is related to the density and the gloss of print, and it is aused by

non-idealinterationsbetweenthepaperandtheinkinahigh-speedprintingproesses.

Inthesopeofthiswork,thetermsmottling,printmottleandprintunevennessreferto

thesameprintingdefet.

Reetive uniformity is what bothprinters and paper manufaturers striveto ahieve.

Whenthatgoalismissed,oneoftheresultingvisualappearanesmaybemottle. Mottle

is veryomplexinitsmanyausesanddiulttodesribefrom observationbeauseof

its manyvariations. Furthermore,thetermmottle isnotlimited touse intheprinting

and paperindustries. Furnituremanufaturersuse mottle to desribethe manygrains

of wood. Biologistshaveused mottleto desribethepatternsfound in butterywings.

Botanistsusemottletodesribeellstruturesin leaves. Mottleevenhasonedenition

for papermakersand another forprinters; however,the end usermakesno distintion

betweenthetwo.[5℄

Per-ÅkeJohanssondesribedmottleas:

...an unwanted laterally varying reetivity in homogeneous tone areas that expresses

itself asstohasti'loudiness'or'graininess'orsometimesin moreorderedpatterns. It

isnotvisibleintextareas,hardlyvisiblein'busy'imagesrihindetail,butmaybelearly

visible inalmerimageparts likeskiesorhomogeneousbakgroundtoneplates.[41℄

Mottling anbe dened asundesired unevennessin pereivedprintdensity. Figure1.1

givesavisual impression of thethesis topi. The InternationalOrganization for Stan-

dardizationhasalso denedprintunevennesusing twoterms,namelygraininess,whih

standsforaperiodiutuationsofdensityataspatialfrequenygreaterthan0.4yles

(a) (b)

(c) (d)

Figure 1.1: Samplemottling eetinprintpathes: (a)low mottling inblak

at50%;(b)highmottlinginblakat50%; ()lowmottlinginyanat50%;(d)

highmottlinginyanat50%.

per

mm

^{inalldiretions;andmottle,whihisdenedasaperiodiutuationsofdensity}

at aspatialfrequenyoflessthan0.4ylesper

mm

^{inalldiretions.[36℄}

Thisworkaimsatvisuallystohasti(aperiodi)mottleatfrequenieslessthan0.4yles

per

mm

^{, whih falls under the} International Organization for Standardization (ISO) dened term.

1.1 Researh questions

Thespei problemaddressedinthisthesisismottlingorprintunevenness. Thesetof

questions denedin thissopeareasfollows:

•

^{Whatarethepossibleausesofprint}unevenness?

•

^{Howdoesthehumanvisualsystempereivethemottling}phenomenon?

•

^{Whatisthemodelofthehumanpereptionofmottling?}

•

^{Isitpossibletopreditmottlingpriorto printing?}

Several termsfrom the dened researhquestions needto belaried. Thevisual sys-

tem (humanvisualsystemin the sopeof this work) isthe partofthe entralnervous

system whih enables organisms to see, aswell as enablingseveral non-imageforming

photoresponsefuntions. Itinterpretsinformationfromvisiblelighttobuildarepresen-

tation of thesurrounding world. Thevisual systemaomplishesanumberof omplex

tasks, inluding thereeption of lightand theformation ofmonoularrepresentations;

the onstrution of a binoular pereption from a pair of two-dimensionalprojetions;

theidentiationandategorizationofvisualobjets;theassessmentofdistanestoand

betweenobjets;andtheguidaneofbodymovementsinrelationtovisualobjets. The

psyhologialmanifestationof visual information is known asvisual pereption, a lak

ofwhihisalledblindness.

A numberof methods for mottlingevaluation by an automatimahine vision system

havebeenproposed. TheISO13660standardinludesamethodformonohromeimages.

It isbasedonalulatingthestandarddeviationofsmalltileswithin asuientlylarge

area[36℄. Inthe standard,the sizeof thetilesisset toaxed value,whih isaknown

limitation[11℄. Therstimprovementtothestandardmethodwastousetilesofvariable

sizes [101℄. A number of methods relying on lustering, statistis, and wavelets have

also been proposed [5, 6, 93℄. Other approahes to evaluate gray-sale mottling have

their basis in frequeny-domain ltering [41℄ and frequeny analysis [79℄, whih were

thoroughlystudied in[83℄.

Themainontributionofthisthesisisthemethodforautomatedprintmottleevaluation.

A number of methods developed for the purpose of unevenness evaluation have been

previously published [5, 6, 41, 42, 40, 79, 101℄. Several of these methods laked the

orrespondene with human visual system (HVS) harateristis [36℄, other methods

weremissing anappropriatepsyhometri experimenttoderiveasuitableintervalsale

to measurethemottlingpereptionaurately.

1.2 Contributions and publiations

In thisthesis, themethod forthepereptionof printunevennessis derived andtested.

Severalresultshavebeenalreadypublished[86,83,84℄. Themethodhasbeensuessfully

implemented in industrial appliations and has proved to be areliable substitute to a

visualexpertise.

Beause thisworkonsistsofresultspreviouslypublished bytheauthor inanumberof

sientimedia,itisworthgivingashort summaryofthesepubliationshere:

In [47℄, the general approah to mahine vision appliations in the paper industry is

disussed. Theo-linepaperprintabilitytests,suhasheliotest,mottlingandpikingare

onsidered andtheinitialpossibilityofanautomatedinspetionisstudied. Theauthor

ontributesapreliminaryevaluation ofheliotestand mottlingevaluationmethods.

In[86℄, thegoalwastostudy themethodspresentedinliterature,and modifythem by

are disussedandompared basedontheirorrespondene tothehumanvisualsystem

and theexpertopinion of thegiven sampleset quality. The authorpartiipated in the

implementationofthemethods,statistisomputationandwritingthepubliation.

In [83℄, the analysis initiated in [86℄ is ontinued while the best performing bandpass

methodisenhanedtoovermorefrequeniesandtobetterorrespondto humaneval-

uation. The updated method is ompared to the ones disussed in [86℄. The paper

onludes that havingasingle,ontrastsensitivityfuntion (CSF) basedlterisbetter

thanasetofbandpasslters. Theauthordevelopedtheband-passmethodenhanement,

omputedperformanestatistisand partiipatedin writingthepubliation.

Publiation[85℄extendsthemethodintroduedin[83℄toolorprints. Anewexperiment

forolletingexpertknowledgeisperformed. Thesuggestedapproah,basedonpattern-

olor pereption separability,proved to orrelatewith thehumanevaluation well. The

authordevelopedthepsyhometriexperimentframeworkandpereptionmodel,ranthe

testsand wrotethepubliation.

In [49℄, the behaviorof gloss is studied using goniometri imaging and spetral olor

information. A goniometri imaging system is proposed to apture reetion images

that reveal thespatial variationof theglossoverpapersamples. Theharateristisof

thegloss aredisussedbyomparingtheresultsofgoniometriimaging,standardgloss

measurements and spetroradiometri measurements. The author partiipated in the

systemdesign,supervisedthetestsandreviewedthepaper.

Finally, [82℄ ombines the resultsof [85℄ and[49℄ to study the orrespondene between

pre-printarea images and gloss maps, and nal prints. The methods desribed in the

publiationusethegoniometriimagingsystemtoolletpre-printdataandpattern-olor

separable methodto omputemottlingindexesofnal prints. Theauthor partiipated

in experimentdesign,method implementationandwriting.

1.3 Struture of the thesis

Thespeiproblemoffastandreliablemottlingevaluationrequiressubstantialknowl-

edge inthefollowingelds:

•

papermaking,

•

^printing,

•

^{imagequality,}

•

^{humanvisionandpereption,}

•

psyhometris,and

•

^{mahinevision.}

Theproblemisbasedonvariousfatsfrom theaforementionedelds,butisnotlimited

to those. The thesistries to giveanoverview of allthe subjetsinvolved to provide a

solid bakgroundtothe ideasstated in theauthors'publiations. Figure 1.2showsthe

ChapterIIgivesaninsightintoprintqualityfromthepointofpapermakingandprinting.

Alsotherelationshipbetweengeneralimage qualityandprintqualityisdisussed.

Chapter III desribesrelevantfats from theelds ofhumanvisionand psyhometris.

Inthispart,thegroundsfordesigningexperimentsareestablished.

ChapterIVsummarizesknowledgeofdierentmottlingevaluationmethods. Itinludes

previouslypublishedapproahes,withsomehangesandupdatesthatareintroduedin

theaforementionedauthors'publiations.

Chapter V is about the experimental part of the work. Here, the psyhometris ex-

periments are desribed and a statistial analysis is given. This hapter also overs

goniometriimagingsystemdesignandthelatterresultsevaluation.

Finally,hapter VIpresentsashortsummaryanddisussionsetion.

Introduction ( Chapter I )

Discussion ( Chapter VI ) Print

Unevenness

Unevenness Evaluation

What is print unevenness?

( Chapter II )

What causes unevenness?

( Chapter II )

Which models are used for instrumental measurements?

( Chapter IV )

How the Human Visual System perceives unevenness.

( Chapter III )

How to extract knowledge about HVS unevenness perception.

( Chapter III )

How to set up an unevenness perception experiment.

( Chapter V )

Comparing instrumental and visual measurements;

evaluating results.

( Chapter V )

Figure 1.2: Contentsandstrutureofthethesis.

Paper and print quality

Paper making tehnology has a very long history ongoing from anient ivilizations.

During thelast 150years, it has greatlydeveloped and progressedsgniantly. In the

19thentury,Kellerdisoveredwoodpulp,andlatertheellulosetreatmentproesswas

invented,whihhasallowedtehnologytodevelopuptotoday'sstate. Currently,paper

is the major polygraphimaterial and meets alltehnologial, onsumerand eonomi

requirementsofsuhmaterials.

Tolaythe groundwork for this hapter, basidenitions should be given. First of all,

thetermpaperisgenerallydesribedasasheet materialmadeprimarilyofplantbers

[34℄. Note thatthe sope ofthis thesisislimited toaso alled graphi paper,in other

words,apapertypethat issuitableforprintingandwriting.

Theverbtoprintisdenedintheditionaryas"Transfertoasurfae;tomakeamarkon

asurfaebypressingsomethingontoit. OriginallyfromLatinpremere-topress." [72℄.

The sameditionary denes quality asadegree of exellene of something[72℄. Tobe

ableto onsiderprintquality, orin awidersense pereivedprintquality,it needstobe

lookedat asaombinationofthefollowingfators:

•

^paperandinkproperties,

•

^{printingproessissues,and}

•

^{pereivedimagequality.}

Asanbeseenfrom Figure2.1,thepereivedprintqualityisdependentonanumberof

fators,startingfrombasipaperandinkpropertiesandinludingmoreomplexaspets

ofprintingproess.

Perceived image quality

Printed image quality

Printing process quality

Paper properties

Ink quality

Paper quality

Ink properties

Figure 2.1: Simpliedshemaforpereivedimagequalitydependeneonbasi

paper/inkproperties.

2.1 Paper and its properties

Papermaking today inludes,in priniple,the sameproessstepsapplied for enturies:

preparation of the ber material, sheet or web forming, pressing, drying, sizing and

smoothing[34℄. Paperisanetworkformedbybresandbrefragments. Theproperties

ofalltheomponents,inludingllersandadditives,inuenethepropertiesofthepaper

andaetthequality. Singlebrepropertiesdesribestatistialpropertiesofbressuh

astheir length. Fibresan belassiedbygivenriteria,for example,in to earlywood

and latewood bres [54℄. Theanalysis of pulp bres is an importantareaof pulp and

paperqualityinspetion.

Paper harateristisvary widely, depending on thetypeof paper. Nevertheless,there

areanumberofpropertiesthatareharateristiofallpaper. Theseareinhomogeneity,

hygrosopiity,anisotropy,and visoelastisity[34℄.

Paper is an inhomogeneous material made from homogeneous elements: bers, llers,

and air-eld pores [34℄. Thehomogeneousregions'sizes vary from several mirometers

toseveralmillimeters(inthelongitudinalberdiretion),dependingontheharateristi

dimensionsofthesepartiles.

A sheet of paperalsoexhibits other inhomogeneities, whih anhave harateristidi-

mensionsoverafewmillimetersorentimeters. Theseareknownasloudinessandresult

from undesiredberoulationinthesheetformingproess. Inthisway,variationsin

aliperand density develop,andthis leadstoorrespondingvariationsin transpareny,

the"louds". Theprodutionproessanalsogiverisetoinhomogeneities,forexample,

duetostokpulsationsintheheadboxandinthesheetformingsetion[34℄. Onlybetara-

diographi measurementsangiveexatinformationaboutloal massdierenes. The

existene ofinhomogeneities must be takeninto aountin paper testing, forinstane,

byhoosingtheappropriatesizeof sample.

The most important harateristi of paper is its hygrosopiity [34℄, in other words,

its ability to absorb or release moisture, depending on the ambient limate, until an

equilibriumisreahed. Itissigniantwhetherthestateofequilibriumisestablishedby

absorptionor disorptionofwater.

As a resultof the hygrosopiity ofpaper, physial paper properties, suh assheet di-

stiness are dependent on the the ambient onditions. With a hange in the ambient

limate, thefullattainmentofanewequilibrium moistureontentofpaperisaproess

whih takesseveralhours. However,thehange in itsphysialpropertiesstartsalmost

immediately and ours veryquikly at the beginning and slowsdown later. Forthis

reason,papershouldbeproessedinroomsinwhihthelimationditionsarefavorable

forthepartiularpaperpropertyrequired.[15℄

Paperisananisotropimaterialwithregardtomanyphysialproperties. Thisanisotropy

is duetotheanisotropipropertiesoftheindividualbers,whih resultfrom thebril-

latedmirostrutureoftheberratherthanthebershape. Asaresultofthebrillated

struture, the ber an aept, for example, high tensile fores in the diretion of the

beraxiswithlowelongation;however,evensmalltensileforesatingperpendiularto

theberaxisausehighelongations.[34℄

The bers in a paper produed on a paper mahine are not aligned randomly: the

mahine diretion is usually preferred (anisotropy of ber orientation). Furthermore,

the ber matis passedthrough thepaper mahine under tension, whih prevents free

shrinkageofthebersduringdrying,mainlyinthemahinediretion.Restrainingfores

in therossmahinediretionarelowerandnonuniformarossthewidth, beingsmaller

at theedges. Thisresultsinanonuniformrossmahine proleofshrinkage.[34℄

Both ber alignment and shrinkage restraintsare responsible forthe anisotropy of the

moistureexpansionofnishedpaper,whihisgenerallyfarlowerinthemahinediretion

thanintherossmahinediretion,thelatterbeingnonuniformarossthewidth. Both

theabovefatorsalsoaettheload-deformationproperties(strain-to-stress)ofpaper,

whiharethereforealsoanisotropi.[34℄

Many papers exhibit an anisotropy with respet to their omposition in the normal

diretion (

z

-diretion). Theproesstehnologyof produtionon thefourdrinierwireis responsibleforthisphenomenon. Onthewireside,substanes(llersandnes)thatan

passthroughthewirearewashedout,while ontheotherside, theyare retainedbythe

bermat. Thisresultsin dierentsmoothnesspropertiesonthetwosidesof thepaper,

also knownas thetwo-sidednessofpaper. Two-sidednessofpaperanbeminimized by

twinwireformers. Afurtheranisotropyinthe

z

^{-diretionresultsfromthefrozenstresses}

duringnonsymmetrialdryingofthewebatthetopandbottomsides. Asaresultofthe

in-plain anisotropy ofthesheet, the mahine diretion mustbeonsidered andmarked

when takingsamplesfortesting anisotropiproperties,suhasstrength. Two-sidedness

is importantinthetestingof printability,for example. Thereforetopand bottom sides

havetobemarkedaswell.[34℄

Finally,paperhasvisoelastiproperties,inother words,itanbeelastilikeasolidor

visous likeathikliquid. The visoelastiityisalso aresultof thesuperimpositionof

thepropertiesoftheindividualbersandthoseofthebernetwork. Thevisousowof

theindividualbersisausedbytheslidingofbrilsathightensileloads. Theindividual

bers exhibit elastibehaviorat lowtensile loads. In theber struture ofpaper, the

sliding of bers at ber intersetions results in a ow eet and the paper undergoes

plasti deformation. A harateristifeature ofvisoelastiityis thedependene of the

onset ofowontheloadingrate. Smallloads atingoverlongperiodsof timeresultin

ow,whereaslargeintermittentlyatingloadsauseonlyelastideformation. Therefore,

tensilestressappliedausesthepapertobreak.[89℄

Thematerialpaperhasessentiallyfourharateristifeatures: inhomogeneity,hygrosop-

iity,anisotropy,andvisoelastiity[34℄. Thesefeaturesmustbetakenintoonsideration

in thetesting of paper. Themagnitude ofthese features depends onthe typeof ber,

theberrawmaterial, thepulpingproess,andontheproesstehniquesusedin stok

preparation and papermaking. Consequently, there are alarge number of variables in

papermakingandmanydegreesoffreedomareinvolvedintheadjustmentofthedesired

paperproperties.[34℄

An analysis of paperprodutsdenes the properties that are related to theuse of pa-

per. For ertain produts and produt behavior, dierent properties are required(see

Table2.1).

Table2.1:Requiredbehaviorofpaperprodutsandrelatedpaperproperties.[54 ℄

Required behaviorof paper Measurableproperties

Suientstrength Tensilestrength

Burstingstrength

Tearingstrength

z

-diretionalstrength Suitablestruture Density

Airpermeane

Optialproperties Brightness

Opaity

Color

Suitablesurfaeproperties Smoothnessorroughness

Surfaestrength

Surfaestiness Bendingstiness

Conoramediumtest

Edgerushtest

Manyphysial propertiesdesribepaperharateristis. Thepropertiesanbegrouped

asfollows[54℄:

•

^basiproperties,

•

^strengthproperties,

•

^stinessproperties,

•

^struturalproperties,

•

^surfaeproperties,

•

hygrosopityproperties, and

•

^optialproperties.

Themostbasipropertiesofanypaperorboardinludemoistureontent,basisweight,

thikness,density,andlterontent. Paperandboardtradeisbasedonweight,therefore

basisweightlinksthepaperweighttoitssurfaearea. Thiknessanddensity,ontheother

hand, desribethe paperstruture. Paperstrength properties inlude tensilestrength,

bursting strength, internal tearingresistane, folding strength, surfaestrength and

z

^-

diretionalstrength [54℄.

Stiness relatesto amaterial's elastipropertiesand measures how muh thematerial

resists when it is deformed by an external load. Paper stiness is usually measured

astensilestinessand bending stiness. Tensilestiness ismeasuredby subjetingthe

papertoaforeparalleltothepapersurfae,andtheresultingdeformationiselongation.

Bendingstinessmeasuresthepaper'sabilitytowithstandabendingforewhenoneend

ofthepaperistieddownandaforeappliedto thefreeend[54℄.

Themostimportantsurfaepropertiesofpaperaresurfaestrength,roughnessorsmooth-

ness, fritionand gloss. Papersmoothnessorroughness desribespapersurfaetopog-

raphy. Papersmoothnessis obtainedbymeasuringairowbetweenapapersurfaeand

measuringsurfaeoredge. Informationneededforthesemeasuresarepressuredierene

usedtoreatetheairow,pressureofthemeasuringheadagainstthepapersurfae,and

the areaof the measuring head. The volume of air ow per time unit is reported as

roughnessandthetimeforaertainairvolumetostreamoutisalled smoothness. The

mostimportantpropertyintermsofthetopi ofthisthesisisgloss.[54℄

Glossmeasurestheinterationoflightandthepapersurfae. Therearefourbasiways

how light an interat with paper and usually, they an allhappen at the sametime.

Gloss measures thepaper'sabilityto speularlyreet light. High glossis desirablein

high quality paper with many images. Paper with high gloss has a wider tone range

than that ofamatte surfae. Thedownside ofhigh glossis that itusually impairs the

readabilityoftext andthereforein textbooksitis ahighlyundesirable property. Gloss

an be measured in many ways, but the paper industry adopted a 75

o

standard [94℄,

wherethemethodisintendedformeasuringthespeularglossofpaperat75

o

(15

o

from

the planeof paper). Although itsmain appliation is onoated papers,it is alsoused

foravarietyofunoatedpapers.

2.2 Paper and printing

Approximately 3000 dierent kinds of paper and board produts exist[69℄. Generally,

the produtswith aweightbelow225

g/m ²

^{isonsidered tobepaper,antthose above}

areonsidered tobeboard. Paperislassiedbasedonthefollowingriteria:

•

^{berfurnish ofthebase paper,}

•

^oating,

•

^{typeofsurfaenish,and}

•

^enduse.

Table2.2: Printingpapergrades. [69 ℄

Papergrade Desription

MF(mahinenish)News mehanialberbased,usedinnewspapersandpaper

baks

MFSpeial

SC(superalendered) unoatedwood-ontaininggravureandheat-setoset

paper;usedin magazinesandatalogs

MFC(mahinenishoated) oated (pigmented) wood-ontaining oset paper;

softalendered

LWC(lightweightoated)SC oated wood-ontaining gravure and heat-set oset

paper;superalenderedoline

LWCmatt mattalendering

MWC(mediumweightoated)SC doubleoated wood-ontainingosetpaper;

MWCmatt oatweightabovethelevelinLWC

HWC(highweightoated) doubleortripleoated wood-freeosetpaper;

HWCmatt oatweightabovethelevelinMWCgrades

WFC(woodfreeoated)

WFCmatt

basepaperisproduedfrombleahedhemialpulp,

withverylittleornomehanialpulp;

papersanbesingle,doubleortriple-oated;

thesurfaeistypiallyeithermattorgloss-alendered

unoatednepaper unoatedwood-freeosetpaper;

usedinbookprinting,opyingandeletroniprinting

ultralightweightwoodfreepaper unoated oroated wood-free"bible paper"; usedin

osetprintedbooks

The alenderingreferredto hereis anishingproess bywhih paper, plastis,rubber

andtextilesaresmoothed,glazed,polished,orgivenanembossedsurfae. Thematerial

is passed throughaseries of rollers, and the resultingsurfaedepends on thepressure

exerted bytherollers, their temperature, omposition,andsurfaedesigns, andon the

typeofoatingpreviouslyappliedtothematerialtobealendered.[34℄

Printingproessharateristisand itsresultingoutputdependnotonlyon paper,but

also on theinks used. Inksare dividedinto twolasses[81℄: printinginks and writing

inks. Printinginksarefurtherbrokendownintotwosublasses[81℄: inkforonventional

printing,inwhihamehanialplate omesinontatwithortransfersanimagetothe

paperorobjetbeingprintedon;andinkfordigitalnonimpatprinting,whihinludes

ink-jet andeletrophotographitehnologies.

Inindustrial statistis,printinginkmanufaturingislassiedasapartofthehemial

industry. Inkmakingsharesmanyfeatures withpaintmaking, but thereare alsosome

majordierenes. Onthelevelofpriniples,amajordierenearisesfromthefatthat

theolorofapaintedsurfaeisgeneratedwithonepaint,whereastheolorofaprinted

surfaeisgeneratedwiththreeormoreinks.[81℄

Atthemostgenerallevel,theompositionofprintinginksisguidedbythemainfuntion

ontrolled way. More speially, the hoie of raw materials and their quantitative

proportionsareinuenedbytherequirementsoftheprintingmethod andtheprodut,

inludingeologialrequirements.[81℄

The main omponents of printing inks are the pigment, the binder, and the arrier

phase[81℄. Inkmakingoperationsonsistprimarilyofhemialrawmaterialonversion,

andsurfaehemialandmehanialproessing.

Inkpigmentsaremainlysynthetiorganisubstanes,althoughsomemayontainmetal-

li elements. Blak pigments absorblight spetrally unseletively and olored inks se-

letively. Inkpigmentsshould not,when dispersed in the ontinuousphase of theink,

satter light[81℄.

A widerange of binders is used in inks. This is in partdue to the fat that available

binders originate from several soures inluding plants (drying oils), wood proessing

(pith, resins), rude oil distillation (hydroarbon resins), and hemial synthesis [81℄.

In part, this is due to variationsin therequirementsset onthe binders bythe arrier

phase of the ink and printing substrate. Thebinder and the arrierphase have to be

mutuallyompatible. Generalized, this meansthat the binderhasto besoluble in the

arrierphasein aontrolled manner. Thebinderandtheprintingsubstrateshouldalso

bemutually ompatible; thebindershould suientlyadhere and bind thepigmentto

the substrate. The overall quality level of printedmatter from a physial viewpointis

assoiated with the quality of the paper used. This being the ase the paper largely

determines the requirementson the binder. Asa rule of thumb, the binderontentin

inks is roughlythe sameasthe pigment ontent; typially"high quality" inks ontain

relativelymorebinder[81℄.

Aordingtothetypeofthearrierphase,inksaredividedinto thefollowinggroups:

•

^{liquidinks(gravure,exo),}

•

^{oilbasedinks(oset,} letterpress,silksreen),and

•

^{waterbasedinks(exo).}

Inoset,theombinationofthebinderandtheoilisalledavarnish,whereasthegeneral

termusedis"vehile". Thearrierphasebeomessuperuousaftertransferringtheink

to the paper and it is either removed by vaporation (gravure, exo, heat-set oset),

onvertedintosolidformbyoxidation(sheetfedoset(SFO))orpolymerization(SFO)

or it beomes absorbed into the paper (newspaperoset). In heatset oset, asilione

solutionisappliedafterdryingasathinlayer. Itfuntionsbyreduingthesurfaeenergy

oftheprintedlayer.[34℄

An inherentharateristiof mehanialprintingistheuseofthe impatprinipleand

mehanial pressure to transfer ink to the plate and from the plate to the paper. In

eletroni printing, ink transferto thelatent image, alled development, is aeted by

other typesof interativeforesbetweenthe inkand thelatent imagethan mehanial

fores. Theink'stransfertothepapermay,however,utilizemehanialfores.[34℄

In eletroni printing, no single-useplate for eah job is needed, and thelatent image

imageisgeneratedatall. Inthosemethodswherealatentimageisgenerated,itisbased

on interation of energy emanating from aprinting head with areeptor surfae. The

reeptor surfaemayalsobepaper.[81℄

Inthiswork,mottlingsampleswereprintedusingsheetfedoset(SFO)tehnology(see

Figure 2.2). SFO,heat-setweboset(HSWO) and rotogravureprinting(RG)printing

belongs to thelass of mehanialprinting. Mehanialprinting refersto themethods

of impat printing, whilst eletroni, often alled digital, printing refers to nonimpat

printingwhere datafromtheomputerisput ontothesheetofpaper[69℄.

Unprinted sheets

Inking system

Ink reservoirs

Cyan Magenta Yellow Black

Plate cylinder

Blanket cylinder

Impression roller

Dampening system

Figure 2.2: Sheetfedoset(SFO)pressshema.

Sensitivity to mottling is tested onoated papersusing HSWOand SFO printingand

bothsuperalenderedpapergrade(SC)andlightweightoatedmehanialpaper(LWC)

papersforRG. Sensitivityismeasuredinonnetionwiththepropertiesofpaperswhih

haveonnetionsto ink, oil and water interation. Mottling is measuredfrom printed

samples andink absorptionsamplesusing"wipe-out"tehnology. Coatedpapergrades

(woodfreeoated paper (WFC), medium weightoated (MWC), LWC, mahine nish

oated(MFC))forHSWO(SFO)andSCpapersandoatedpapers(WFC,MWC,LWC)

for RG are tested for mottling. Coatedgrades for HSWO (SFO) are both glossy and

matt,andonlyglossyforRG.

Figure2.3presentsageneralviewofanymehanialprintingmethod. Mehanialmeth-

odsin generalusetherotarypriniple,meaningthat theinkistransferredonthepaper

in a nip between tworotating ylinders. Ink transfersto the paper from theprinting

ylinder orfrom aseparatetransferylinder. Themethod thatusesaseparatetransfer

ylinderisalledanosetmethod. IntheRGmethod, anegativereliefisengravedinto

aopperlayerontheprintingylinder. Heat-setintheHSWOrefersto thesettingand

drying mehanismsin oset printing. Setting refers to a type of ink used in printing.

Heat-set inks ontainveryvolatileoils ompared to old-set inks. In theheat-set pro-

ess, theweb(rollsratherthantheseparatesheetsofpaperinSFO)isfed intoanoven

weight oils. A web typially travels throughfour printing units (see Figure 2.2), eah

printingadierent olor,and dryingours after these printingunits. RGprintingon

theother hand,integratesthedryinginto theunits,dryingtheink aftereaholorhas

beentransferredonto thepaper[69℄.

Inking unit Ink transportation and metering

Latent image on printing press

Ink application to printing plate

Data as ink distribution on plate

Ink transfer to paper

Drying

Printed paper Paper transportation

and registering Paper feeding

devices

Dryer

Paper Ink

Figure 2.3: Theimagingstepsinapress(toptobottom),supportingfuntions

(left),andrawmaterials(right)inprinting.[69 ℄

InFigure 2.3,theparametersthat anbealteredto ahievedierentprintqualitiesare

ink properties, ink feed, fount feed in the dampening proess, printing speed, drying

parameters,varyingrubberblankets,andprintingplates. Dierentlevelsofmottlingin

test printing an be reahed by hangingink transfer, watertransfer, ink setting, ink

absorption, water absorption, ink trapping, and drying parameters. Most hanges an

bemadebyadjusting theprintingproessparameters.

2.3 Print quality

Printedpaper in itsdierent manifestations isfamiliar to and usedby allpeople. The

timespenteahdaybrowsingprintedmatterforwork,eduation,informationandleisure

isonsiderable. Theneedstobefullled arediverse,asarealsotheusersituations.

Printprodutiontehnologieshavehangedimmenselyduringthelastfortyyears,more

sothanduringthepreedingfourhundredyears. Thisismainlyduetothedigitalization

of the basi rawmaterials of prints,namely information. Previousprogress was made

largely through mehanization. However, the appearane of printed matter has not

Fortraditionalgraphiprintedproduts,printabliltyisdenedasaombinationof:

•

runnabilityofthepress(lakofweb-breaks,aumulationproblems,fold-raking, blistering),and

•

^{adequate print (produt) quality (olor} reprodution, uniformity of print, print gloss,missingdots,printthrough).

Despite all endeavourson the part of the paper industry, it is not possibleto reliably

predit either printability, runnability orprint quality. In other words, paper quality

annot be adequately haraterised as far asthese two aforementioned properties are

onerned.

Printability of paper is a paper property that indiates how paper behaves during a

printingproess. Printability depends on interations between paper and printing ink

and printingproessvariables. Good printability ofpapermeans that thepaperis not

very sensitive to variations in dierent proess variables and easily gives good print

quality.

Printquality desribes thenal resultof printing orthe quality of the printed image.

Printqualitydenition doesnot haveabsolute terms. It dependson theprintdensity,

resolution, and evennessof theprintedimage. Manyotherpropertiesare also ofinter-

est.[69℄

Printabilitytestsdependontheprintingproessinvolved. Certaintypesoftestmethods

aim at generalprintquality. These tests areusefulfor alltypesof papersregardlessof

the printing methods used. Some testmethods are only suitablefor aertainprinting

proess.

Printability and print quality depend on many dierent fators. The paper and its

propertiesareimportantomponentsasis theprintingproess withitsmanyvariables.

Due totheomplexnatureoftheprintingproess anddierentinterationsinuening

it, printabiliyandprintqualitytestinganonsistofdierenttehniques.

Papermakersandprintershavesearhedforafast,simple,andeetivetestmethodsto

predit paperbehaviorin printing withoutatuallyprintingthepaper. Everyprinting

proesshas its own features that often requireaseparate testing devie and a spei

test. In many ases, measuring similar properties after dierent proesses is possible.

Table 2.3 lists the laboratory printing tests ommonly used. Some tests may be used

with all the listed printingmethods, and some are more spei to a ertain printing

proess[69℄. AsitanbeseenfromTable2.3,testingformottlingisperformedonlyfor

HSWOand SFOprinting.

2.4 Print mottle

Printunevenness,oftenreferredtoasprintmottle,graininessorloudiness,or,generally

speaking, noise, redues the quality of the printed image. Mottling an be dened as

the unwantedunevennessin thepereivedprintolor,printdensityorprintgloss. The

seen in diuse but not speular illumination. If an image appears uneven in speular

illumination, it is due to gloss mottle. Topographi variations in the substrate, and

thereforealsoin theimage, analsobeareasonbehindmottledappearane.[41℄

Table 2.3: Laboratory test methods for prediting the printability and print

qualityindierentprintingmethods.[69 ℄

Oset Digital Rotogravure Flexo

Printing Printing Printing Printing

Coldset Heatset Sheet Ink-jet Laser

Printdensity x x x x x x x

Inkrequirement x x x x x

Inkset-o x x x x

Rub-o x x

Printthrough x x x

InkGloss x x x x x

Drypik x x

Wetpik x x

Blaktrappik x x

Mottle x x

Fiberroughening x x x

Toneradhesion x

Missingdots x

Residualsolvent x x

Dotgain x x x x x x

Dotsgeometry x x x x x

Unevennessan appearinmanyformsandatdierentsales. Typially,theunevenness

is stohasti, with lighter and darkerareas, with dierent sizes randomly spreadover

theprintedarea. Smallsale randomvariationissometimes alledgraininessand large

sale variation is alled mottling orloudiness. Aording to ISO 13360:2001[36℄, the

periodiutuationsofdensityat aspatialfrequenygreaterthan0.4ylesper

mm

ⁱⁿ

alldiretionsarealledgraininess(sparselyreferredtoasgranularity)andutuationsat

lowerfrequeniesare alled mottling. Sometimesthe unevenness inludes asystemati

omponent, when streaks, bands or other resular patterns are added to the random

unevenness[70℄. Typialprintunevennessdefets arepresentedinFigure2.4.

Thetypesofprintunevennessare:

•

one-dimensionalperiodinoise: banding(Figure2.4d),

•

one-dimensionalrandomnoise: streaking(Figure2.4),and

•

two-dimensionalrandomnoise: graininess/granularity(Figure2.4b),mottling(Fig- ure2.4a).

Bandingisthepreseneofextraneouslinesonaprintedpage. Bandinggenerallyours

when aolor printer needs to pass the print headover apage multiple times to print

(a) (b) (c) (d)

Figure 2.4: Typesofprintunevenness: (a)mottle;(b)granularity;()streaks;

(d)banding.[93 ℄

Streaks are primarily one-dimensional visible defets in the image that run parallel to

theproessdiretion,alsoreferredto astheslow-sandiretion. Inauniformgraylevel

path, streaksmayappear asavariationin thetone level, whether the toneris blak,

yan,magenta,yellow,orsomeotherolor. Streaksinsingleolorseparationsthatmay

beunobjetionable,anauseanundesirablevisibleolorshiftforoverlaidolors.[63℄

There arethree main typesof mottling known in printing: absorption mottling, bak-

trapmottlingandwaterrepellanemottling. Thelastoneisseeninosetprintingwhere

thefountainsolutionwhihhasbeenemulsiedinto inkannot betransferred properly

on paper (paper is too dense or hydrophobiompared to the amount of fount fed in

printing). The original ause of mottling (absorption, baktrap or water repellane)

annot be deteted straightforwardfrom the printed samples - that an only be seen

whilewathingtheprintinganddoingsomehangesintheproess(inkfeed,fountfeed,

printingspeedet.) Thedierentmehanismsofmottlingourringindierentprinting

methodsforporousmaterials(notlms)aresummarizedin Table2.4.

Table 2.4: Mottlingourene mehanisms.

Printingmethod Mottlingmehanism

HeatsetWebOset(HSWO), Absorption,Baktrap

SheetFedOset(SFO) Waterrepellane,Piling,Contamination

ColdsetWebOset, Absorption,PrintThrough

Flexo,Rotogravure(RG) Piling,Contamination

Inkjet Absorption,PrintThrough

Paper formation and surfae harateristis ause the most basi set of mottle types.

Variations in thedensity, surfaesmoothness,berontent, nes, llerontent, sizing,

surfaepH,andspeienergyofthepaperanhaveadramatiinueneontheability

of anink to transferontothe surfae,itsabsorbeny,wiking, penetration,and drying

harateristis. Unevenberdistributionwithinthesheet(formation)isoftenattheroot

oftheseeetssinetheformationalsoinuenestheretentionofnesandadditives. A

poormassdistributionalsohasanegativeinueneonthelightsatteringoeientof

Mottling is usually the result of non-uniform ink absorption aross the paper surfae

and/oruneveninklay. Printmottleispartiularlyvisibleinmid-toneimagesofuniformly

oloredareas,suh assolidsand ontinuoustone sreenbuilds. Thisvisibleunevenness

anhappenasaresultofnon-uniformink gloss,densityorolorsofprintedinklm,or

it maybeaomplexfuntionof randomlyonnetedhalftonedots. Mottlingisusually

desribedmorespeially in termsof apartiularrootause. Insomesoures[5,24℄,

mottlingmehanismsareextendedinto awidersetoftypes,whiharelistedasfollows:

•

^{baktrapmottle,}

•

^waterinterferenemottle,

•

^{wet inktrapmottle,}

•

^{densitymottle,}

•

^{glossmottle,and}

•

^{drytrapmottle.}

Baktrapmottleisausedbytheinksinabilityto transferfromtheprintblanket tothe

paper. Typiallyaused whenthe ink is unstableorthesolvent/oil islost tooquikly,

anynon-uniformityin theink transferrateanreate amottledappearane. Baktrap

mottle usually appears in rst down inks, and is sometimes a result of improper ink

pH or visosity for the paper being used. Low visosity inks sometimes produe an

appearanealled'orangepeel',named forthepattern thattheinkreates[88℄. As the

sheet travelsfrom unit to unit,the ink lm non-uniformly traps bakonto subsequent

blankets resulting in uneven ink transfer to and absorption on the paper. The usual

reasons of bak trapmottling are paper uniformityand setting harateristis, ink set

rates,poorinktrap,andblankettype.

Waterinterferenemottleisthenon-uniformaeptaneorrejetionofthefountainso-

lutionarossthesheetsurfae. Typially,thismottlingmehanismisausedbyexessive

water, improperinkformulation,inadequately mixed solutions,orexessivealohol. If

thesheetdoesnotabsorbthefountainsolutionuniformly,theinkappliedinsubsequent

unitsmaynottransferandlayuniformly. Inkandwaterimbalaneanaettheunifor-

mityofinktransfer,resultinginanon-sharp,hollow,orweakdotstrutureinrespetive

units ofprint.[88℄

Ink trapmottle is aresult of aninorret ink takor pH sequene. Usually, aseond

downinkappearsmottledwhenitisprintedoverarstdownink,butappearsunmottled

whenprintedaloneonthesubstrate. Inktrapmottleisdesribedaspoororinonsistent

unit-to-unit ink trap whih transfers non-uniformly to the paper and/or previous ink

lms. Traprequiresonewetinklmtoaptureor"trap"subsequentinklms. Inorret

inktakgrading,wronginksequene,sreensoversolids,andpaperabsorbenyaremost

oftentheauseofinktrapmottling.[88℄

Density mottle is aused by uneven pressuresbetween metaltype, gravureylinder or

osetblanketandpaper. Itisnotommoninlithography,butanbeseenasaresultof

Glossmottleisanuneveninksurfaeharateristinotimmediatelyevidentatprinting.

It is aused by a paper's non-uniform absorption, resulting in portions of the ink to

appearglossyandothersmatte. Inkthatisimproperlyformulatedforapartiularpaper

isalsoaontributorto glossmottle.[98℄

Drytrapmottleisalsoalledrystallization: thisoursbeauseofanexessivedrytime

betweenruns ofamultiolorjob onasingleolorpressortheimproperwax ontentin

asetofink. Asinkdries,waxmigratestothetopsurfae. Exessivewaxmigrationtoa

dried inksurfaeanmaketheadhesionofsubsequentinksdiultbeausethesurfae

beomesslippery,hardandsmooth[98℄.

Therelationshipbetweenprintmottleandotherinhomogeneitiesinpapers,bothprinted

andunprinted,isworthtakingintoonsideration,andaredenedasfollows[27℄:

•

^Mehanialproperties: Mehanialpropertiesofunprintedandprintedpaperrefer tothe strengthandstiness ofthepaper,andits apabilitytohandle stressesby

strainandompression.

•

^Mehanialinhomogeneities: Mehanialinhomogeneitiesare spatialvariationsin mehanialproperties.

•

^Optialproperties: The paper'sand print'soptialpropertiesreferto theirmode ofinterationswithlight,inotherwords,howthelightisreeted,sattered,and

absorbedin thepaperorprint.

•

^Optialinhomogeneities: Optialinhomogeneitiesarespatialvariationsoftheopti- alpropertiesintheprintedorunprintedpaper. Themostobviousformofoptial

inhomogeneitystemsfrom variations in light absorptionovertheprintedsurfae.

Thisisseenasprintmottlein diuseordiretedilluminationorasglossvariation

inspeularviewing[59℄.

•

Printability: Theprintabilityofpaperis theombinationof paper-relatedfators thatontributetotheahievementofadesiredqualitylevel. Printabilityparame-

tersaredividedintoasetofoptial,surfae,struturalandmehanialproperties,

whih are then measured. Some of the printability parameters refer to inhomo-

geneitiesin optialandmehanialproperties.

All of these properties and inhomogeneities, as well as their ombinations, aet the

resulting print mottle. Printingis an interation betweenthree main omponents: pa-

per, ink, and press. The auses of mottle anberelated to the propertiesof all three

omponents,ortotheinterationbetweenthem[27℄:

1. interationbetweenpressandink,

2. interationbetweenpressandpaper,

3. interationbetweenpaperandink,and

Mottlemehanismsvarydependingonthetypeofpaperandtheexisteneoftheoating.

In general, all printing methods using inks with solvents need mottling testing. The

main ausefor mottlein printingis theunevenabsorptionof inkand fountainsolution

(in oset) into paper,and eventhroughthepaper,ausing mottling. Other auses are

onneted to the wetting, trapping and setting of ink onto the surfaeof theprinting

substrate (normally paper). In oset printing, the ontamination of rubber blankets

and/orprintingplates ausemottledprintingaswell. This ontaminationantypially

beinkpilingonrubberblanketsoronprintingplatesineahprintingunit,pilingtherst

printed inks onto rubber blankets of latter printing units (arry-overpiling, bak-trap

piling) or ontamination on water areas of printing plates or rubber blankets in eah

printingunit.

Mottlingisnormallyevaluatedfromoverlappingoloursbetweenyan50%magenta50%

-yan70%magenta70%tones(C50M50-C70M70). Thereasonforthisistheproblem

onnetedtoredandblue shadesin printing(skin,sky). Tonesof50%strengtharethe

most sensitivefor pikingand ifitis theause ofontamination,this ouldbeseenas

mottledprinting. If pikingourss in allolors,it anbebestseenwith 50%tones in

eaholourofblak,yan,magentaandyellow. Absorptionandprintthroughmottling

of eaholour: yan,magenta,yellow,andblak(C,M,Y,K), isseenbest withsolidink

areas(100%)ofeahprimaryolor.

2.5 Summary

Inthishapter,thebasiaspetsofprintingandvariousonstituentsofthisproessare

onsidered. The main harateristisof thepaper, ink, and printmethods are dened.

Theommondefetsofsolidtone areareprodutionandtheironnetiontoparameters

oftheprintproessaredesribed. Themaindenitionsonnetedwiththeprintquality

arealsogiven.

The maintypesofprintmottle andthemehanismsof itsourenearepresentedand

desribed. Printingis aninteration betweenthree main omponents: paper, ink and

press. Paper,ink,andprintingproessproperties,andinhomogeneities,aswellastheir

ombinations,aettheresultingprintmottle.

Human vision and psyhometris

This hapter desribesthe human visual system and the aspets of the psyhometris

used in thisthesis. It startswiththephysiologialaspetsofthehumanvisualsystem,

ontinuingwiththespeisofthespatialandolorvision. Suhphenomenaashromati

adaptation,lightanddarkadaptation,andolor-patterseparabilityareonsidered. The

goalofthishapter istolaythegroundworkfortheexperimentalpartofthisthesis.

3.1 Bakground

In orderto understandthebasioneptsofhumanvision, thestrutureand thefun-

tionalityofthehumaneyeshouldbeonsideredrst. Figure3.1presentsarosssetion

of the human eye. The outer layerof the eyeis alled theornea. It is atransparent

tissue protetingthe eye, behind whih isthe aqueous humor, alear liquidlling the

avitybehindtheornea. Theorneaisaprimaryrefrativeinstrumentoftheeye,thus

serving astheforemostimageformingelementin thehumaneye[80℄.

The next part of the eye is the pupil, a dilating opening in the iris of the eye. The

iris expands orontrats, ontrolling the amount of light passinginto the eye [13℄. A

rystallinelensisloatedrightbehindtheiris. Theshapeofthelensisontrolledbythe

iliarymusles. Depending onthe shapeof thelens, apersonansee objetsloseror

fartheraway. Thehamberbehindthelensis lledwith learvitreoushumor,whihis

alearvisouswater-likeliquid. Finally,thebaksurfaeoftheeyeisalledtheretina.

Theretina has,infat,over100millionlight-sensitivephotoreeptorells,andit isthe

plae where thelightoming intothe eyeis onverted into neurohemialativitysent

intothebrain [100℄.

Photoreeptorsinthehumaneyeanbedividedintotwodistintlasses: rodsandones.

Theyare namedfor theirdistintshapes(seeFigure3.2). Eah ofthese photoreeptor

ellsperformsadierentfuntion. Notethat theamountof rods issigniantlyhigher

thanthatofones(approximately120millionvs. 8million). therods'primaryfuntion

is visionunder verylowlighting onditions,also alled sotopivision. Rods thus are

Figure3.1: Humaneyeinrosssetion[100 ℄.

extremelysensitivetolightandloatedthroughouttheretina. Conesontheotherhand

areonentratedintheenteroftheretinaandarenotassensitivetolightasrods. Cones

are responsible forpereption under thedaylight orsimilaronditions, alled photopi

vision. Theasewhenbothrodsandonesareemployedis alledmesopivision.

Figure3.2: Rodsandones[78 ℄.

Animportantfuntionoftheonesisthepereptionofolor. Thehighestonentration

of onesis loatedin the fovea,asmall regionin the enter oftheretina. Thisareaof

theretina aountsforthebestspatial andolorvision,thefovea, inturn, isproteted

byayellowlteralledthemaula,intendedtoprotetthefoveafromshortwavelength

Bothtypesofphotoreeptorshavesimilarstruture(seeFigure3.2). Theyonsistofan

outer segment, inner segment and a synaptiterminal. The outer segmentonsists of

billionsofpigmentmoleules sensitiveto light,whihin turnareproteinsalledopsins,

whihdeterminethewavelengthoflighttheyabsorb,andhromophores,apturinglight

protons. Eahphotoreeptoranontainoneofthefourpossibletypesofvisualpigments,

or opsins. Rods ontaina pigmentalled rhodopsin,while ones ontainthe pigments

responsible for responding to short, medium, and long light wavelengths. The inner

segmentofthephotoreeptorsonsistsofanuleusandotherellularmahinery[71℄.

Howanopsinatsingeneral,isitabsorbsaphotonoflightomingintoit,andtransmits

asignaltothephotoreeptorell,resultinginhyperpolarization. Figure3.3showshowa

humanretinaisonstruted. Asitanbeseen,photoreeptorssynapseintobipolarells,

whihthensynapseintotheganglionells. Theaxonsoftheganglionells,inturn,lead

to the opti hiasm viathe opti nerve. From the optihiasm there aretwoseparate

paths into thethe brain. The rstpart goes to the superior olliulus, that primarily

ontrolseyemovements. Theseond pathwayleadstolateralgeniulatenuleus(LGN)

andthenintotheprimaryvisualortex[71℄.

Light

Optic nerve fibers

Ganglion cells

Amacrine cells Inner synaptic layer

Horizontal cells

Receptor nuclei Bipolar cells

Receptors pigmented layer

Figure 3.3: Thehumanretina[71 ℄.

An important onept that needs to be onsidered here is the visual auity. What is

usually meantbythevisual auityisautenessorthedegreeof learnessofthevision,

whih depends on thesharpnessof theretinal fous. It anbeviewed asa measureof

thespatial resolutionof thehumanvisualsystem. Normalvisionisoftenreferredtoas

a20/20vision, whih if youput intonumbersmeansthat aperson withnormal vision

shouldbeabletodistinguish1arminuteatadistaneof6meters,andanresolvelines

with aspaingof1.75

mm

^[17℄.

3.2 Spatial vision

In this setion an integral view of human spatial vision is disussed, both from the

physiologialandpereptualpointsofview. Thevisualproessingandthefuntionality

of theprimary visual ortexare onsidered herein moredetail,aswellasthe ontrast

sensitivity funtion (CSF) of the human eye. In this hapter, CSFs for isohromati,

luminane-varying,one-dimensionalgratingsareonsidered.

3.2.1 Physiologialmehanisms

Asithasbeenpreviouslydisussed,retinalganglionellsonnettheretinaviatheopti

nerve to the brain. One of the most important harateristisof ganglion ellsis the

reeptive eld. What is meant by this term is the area in the retina that exites the

ganglionell. Itwasshownthatganglionellsrespondto spotsof lights,however,they

arealsoresponsibleforpereivingstripesoflightanddarkareasorsoalledgratings[52℄.

Ganglion ellsin theretinaareloatedinonentri irles. Partof theellsloatedin

airular areareattoan inreaseoflightomingin;these are so alledON-enteror

simplyONganglionells,theyaresurroundedbyellsthatgivethestrongestresponseto

thedereaseinlightintensity,thesoalled OFFells. Suhanorganizationofganglion

ells allows the retina to be sensitive to the dierene in the intensity of light in the

enterandtheperiphery,and berelativelyindierenttoaverageoveralllighting. Thus,

the retina atsas a lterthat inreases therepresentation ofontrast in thepereived

imageandltersoutambientlighting. itisassumedherethatontrastisadierenein

illuminationbetweenthemain objetandthebakgroundorbetweenlighteranddarker

areasintheobservedsene [100℄.

Figure3.4showsanONretinalganglionell'sresponsetogratingsofdierentfrequenies,

orinotherwordstodierentwidthsoflightanddarkareas. Whenthefrequenyistoo

lowor too high (see onsequentlya and), the response of the ganglion ellsis weak.

However,themedium frequenygratinggivesarelativelyhighresponse,leadingto the

onlusion that ganglion ellsare adapted to ertainspatial frequenies,meaning that

eah ellrespondsbest to a spei spatial frequeny, and as seenin Figure 3.4 worse

to higherandlowervalues[100℄. Anotheraspetof ganglionellsisthat theirresponse

depends onthe phaseofthe grating,whih isthe position within the reeptive eldof

theell[22℄.

TheaxonsoftheretinalganglionellssynapseontotwoLGN, whihatasarelayfrom

the retina to the primary visual ortex. They exhibit the same onentri positioning

asganglionells,andhavesimilarsensitivitiesto spotsoflightandgratingsofdierent

frequenies. LGN is, in turn, onneted to the erebralortex and other areas in the

brain thatontrolthefeedbaktothevisualinformationreeived[100℄.

TheareaoftheerebralortexthatreeivesdiretinputsfromtheLGNandisresponsible

forvisualinformationproessingisalledtheprimaryvisualortex,V1orstriateortex.

Theortexisdividedintotwohalves,allederebralhemispheres,that aresymmetrial.

The two sides are onneted via the orpus allosum, a large ber trat that allows

ommuniationbetweenthetwohemispheres. Thestriate ellshaveseveral important

(a)

(b)

(c)

Figure 3.4: Response of ganglion ell to gratings of dierent spatial frequen-

ies [71 ℄: (a) low frequeny yields weak response, (b) mediumfrequeny yields

strongresponseand()highfrequenyyieldsweakresponse.

frequenies. There are twoimportant featuresof V1 [29℄. The rstis the topographi

mapping,whihmeanstheimagefromtherighteyeismappedontotheleftstriateortex

and vie versa. The seond feature is alled ortial magniation. To put it simple,

it is the size of the ortial area dediated to a spei region of the visual eld, for

exampletheareadediatedtofovealvisionisgreaterthanthatoftheperipheralvision.

An importantonsequeneofortial magniationisthat visualauityissigniantly

reduedin proportiontothedistanetothefovea[53℄.

3.2.2 Spatial frequenytheory

Spatialfrequenytheoryisbaseduponaso-alledatomistiassumption,forexamplean

image ofanyomplexityanberepresentedwithprimitivespatialatoms. Intheframe-

workofthespatialfrequenytheory,thesearesinusoidalgratings. Sinusoidalgratingsare

two-dimensionalpatternsofwhihtheluminanevaries aordingtoasinewaveinone

ofthespatialdimensions,andremainsonstantintheother(perpendiular)dimension.

Eahofthesesinusoidalgratingsisompletelyharaterizedbythefollowingparameters:

spatial frequeny,orientation,amplitude,andphase[71℄.

Spatialfrequenysimplyisthewidthofthedarkand lightareasin thegrating,usually

expressed in the number of these yles per degree of visual angle. Orientation refers

Amplitude, often alled ontrast, in turn, an be desribed as the dierene between

the lightest andthedarkestareaswithin thegrating. Contrastisusually speiedasa

perentageof the maximal possible amplitudedierene, forexample 0% isequivalent

to auniform greyeld, while 100%is a blak andwhite eld. Finally, phase refersto

the position of the sinusoid relativeto aertain referene point. Phaseis speied in

degrees. Forinstane,a

0 ^◦

^{sinusoidisalled asinephase,a}

90 ^◦

^{,aosine phaseand, an}

180 ^◦

^{,ananti-sinephase[71℄.}

3.3 Contrast sensitivity funtion

ACSFisonsideredtobeameasureofhowsensitiveanobserveristogratingsofdierent

frequenies. Note that in this hapter,CSFs forisohromati, luminane-varying,one-

dimensional gratings are onsidered. Typially, an imaging system in general, be it

a amera, or an optial system, is haraterized by the modulation transfer funtion

(MTF). In general, the modulation of a sinusoidal luminane pattern an be dened

as the amplitude of the sinusoidal grating divided by the average luminane [7℄. To

onstrut theMTF givento an inputof aertainamplitude, theoutputamplitude for

eahspatialfrequenyismeasured. Oneanmeasurethespatialresponseofasingleell

in thehumanvisualsysteminasimilarmanner. However,whendealingwithaomplex

system,suhasthehumanvisualsystem,reeivinganobjetive,aquantitativeresponse

isnotpossible[20℄.

Figure3.5: Frequenydependeneofontrastsensitivity,illustratedbythevary-

ing visibilityof atestpattern. Thefrequenyofthepatterninreasesgradually

fromlefttoright,andtheamplitudefromtoptobottom.[76 ℄

AsolutiontotheproblemofdeterminingtheshapeoftheCSFistorstndthelowest

detetable value ofontrast, whih isthe lowest valueof ontrastat whih anobserver

andetermine thedierenebetweenasinusoidalgratingand auniformgrey eld(see

Figure 3.5). Suh measurementsaredoneat variousspatial frequenies. CSFwouldbe

thereiproaloftheontrastthresholdrequiredforthedetetionof aertainfrequeny

grating. AnexampleoftheCSFshapeisgiveninFigure3.6. ThisisaCSFforanormal

human observer, obtained under photopi onditions, for a stati luminane varying

grating. Contrast sensitivity (y-axis) is found by taking the reiproal of the ontrast

threshold. Looking at the plot, itis lear that thehighest sensitivity is loatedin the

middle spatialfrequenylength,however,withtheinreaseof frequeny,thesensitivity

experienesasharpdrop.

and that whih falls above would non-visible. The inrease in ontrast sensitivity is

due to one spaing and the optis of the eye; the derease, however,is due to neural

fators[100℄.

Figure 3.6: Theontrastsensitivity funtion: windowofvisibility. Anyobjet

whosespatialfrequeniesandontrastsfallwithinthelightgrayregionwillbevis-

ible. Thoseoutsidethelightgrayregionareoutsidethewindowofvisibility.[100 ℄

Theeye'sontrastsensitivityvariesdepending onluminaneaswellasretinal loation.

Forhigh photopiluminane,thepeakofCSF liesathigherfrequenyvalues. Another

important harateristiof the shape of the CSF is that a sharp drop in the ontrast

sensitivityis exhibited foranyluminane, however,thedegreeof derease in thelower

frequenyrangedepends onthegeneralluminaneonditions[20℄.

CSF is often used in image quality researh. Suh measures as modulation transfer

area (MTFA) [16℄, integrated ontrast sensitivity (ICS) [96℄, subjetive quality fa-

tor (SQF) [31℄, disriminabledierene diagram (DDD) [14℄, and square-root integral

(SQRI)[9℄aredesribedin literature.

Contrastsensitivity funtion formulationsare derived using variousexperimental data,

the omplexity of whih depends on the tted data size and the intended generality.

There is anumberofformulationsavailable in literature[61, 99, 66,19, 7, 8℄. Someof

theformulationsarepresentedin Figure3.7.

One ofthe simplest ontrastsensitivity funtion formulationsis aparameterizedexpo-

nentialfuntion,rstdesribedinMovshonetal[66℄. Theformoftheequationisshown

in Equation3.1.

CSF (ω) = k s (ωk ω ) ^α e ^{− βωk ω} ,

^(3.1)

where

ω

^{isthespatialfrequeny. Thefourfreeparametersprimarilyaetthesteepness}

of thelowfrequeny(

α

^{)andhigh frequeny}

β

^{setionsoftheurve,lateralshiftsalong}

thefrequenyaxis(

k ω

^{),andvertialshiftsalongthe}sensitivityaxis(

k s

^{). Thisfuntion}

10 ^-1 10 ⁰ 10 ¹ 10 ² 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Movshon Mannos Daly Barten

Frequency [cpd]

S en si ti v it y

Figure 3.7: Dierentontrastsensitivityfuntions.

CSF (u) = au ^c e ^{− bu} ,

^(3.2)

where

u

^{isthespatialfrequeny,}

a

^,

b

^,and

c

^{arefree,adjustable}parameters. Thisequation anbettedto existingluminane ontrastsensitivity dataifdesired. Forgeneraluse,

the values of 75, 0.2, and 0.8 an used for

a

^,

b

^{, and}

c

respetively [43℄. This three parameterequationformsthegeneralband-passshapedesiredfortheahromationtrast

sensitivityfuntion. Foruseinaolorimagedierenemetri,thetwo-dimensionalform

of the equation is used and the result is an isotropi funtion. The relatively simple

form of this model has both strengths and weaknesses. This funtion is the same for

all viewing onditions, unless the parameters are speially t to existing data. It

is generally assumed that viewing onditionsan greatly alter the ontrast sensitivity

funtion. This is espeially thease forluminane level,whih is known to atten the

shapeoftheontrastsensitivityfuntion. Tobetterpredithangesinviewingondition,

amoreompliatedfuntion isneessary.[43℄

Another relatively simple formulation (Mannos et al [61℄) also presents parametrized

exponentialandisgiveninEquation3.3.

CSF (u) = 2.6 (0.0192 + 0.114u) e ^{− (0.114u) 1.1} .

^(3.3)

This funtion has apeak valueat afrequeny of8 ylesperdegree(pd) and azero-

frequeny interept of 0.5. A disadvantageof this CSF is that it does not aount for

theadaptivenatureofthevisualsystem[62℄.

DalydesribedanotherformofCSFforuseintheVisibleDierenePreditor[19℄. This

modelisafuntion ofmanyparameters,inludingradialspatialfrequeny,orientation,