Analyzing Metallic Gloss Using a Micro-Goniophotometer

Rochester Institute of Technology

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2007

Analyzing Metallic Gloss Using a

Micro-Goniophotometer

Saara Suni

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Recommended Citation

Analyzing

Metallic Gloss

Using

aMicro-Goniophotometer

by

Saara Suni

Athesissubmittedinpartialfulfillmentoftherequirements forthedegreeofMasterofScience

intheSchoolofPrint Media

intheCollegeof

Imaging

ArtsandSciences oftheRochester Instituteof

Technology

May

2007

School of Print Media

Rochester Institute of Technology

Rochester, NY

Certificate of Approval

Master's Thesis

This is to certify, that the Master's Thesis of

Saara E. Suni

has been approved by the Thesis Committee as

satisfactory

for the thesis requirement for the Master of Science degree

at the convocation of

May 2007

Thesis Committee:

Franziska Frey

Primary Thesis Advisor

Franz Sigg

Secondary Thesis Advisor

Illegible Signature

Technical Consultant

Illegible Signature

Graduate Thesis Coordinator

Illegible Signature

Graduate Program Coordinator

Illegible Signature

Permission to reproduce thesis

Analyzing Metallic Gloss using a Micro-Goniophotometer

I,

Saara Suni, hereby grant permission to the Wallace Memorial

Library of RI.T, to reproduce my thesis in whole or in part. Any

reproduction will not be for commercial use or profit.

Acknowledgments

Aspecialthankyoutomembers of_mythesiscommitteeforalltheassistance.Iwould also

liketothank_myfriendsand

family

for

being

thereforme and

helping

methroughthis

TableofContents

ListofFigures.

.iv ListofTables

vii

Chapter 1

Introduction _\

Chapter2

TheoreticalBasis ₃

Measuring

ReflectedLight 3

Bi-directionalReflectionDistribution Function. 7

Polarization 8

Chapter 3

ReviewoftheLiterature 10

MetallicPigments 10

Materials 10

Technology

and

Manufacturing

11

Particle SizeandShape ₁₃

Pigment Classifications 14

UseofMetallic Inks 14

Visual Effects 16

Metallics in

Packaging

17

ProcessControl 17

Color Management 17

Controlling

thePress 18

RealLifeApplications ₂₀

Conclusionforliteraturereview. ₂₁

Chapter4

Research Questions ₂₃

Experiment1:

Detecting

Difference

_{between Sampleswith aLarge Visual Difference. 23} Experiment2:

Detecting

_{Difference between SampleswithaSmall Visual Difference. 23} Chapter5

Methodology. ₂₄

The Device

Setup

₂₅

TheCameraandthePolarizers ₂₇

Measurement Procedure ₃₀

Exposure Control ....31

Image DataAnalysis ₃₂

Calibrating

theInstrument

Using

aKnown Reference ₃₃

ImageAnalysis Procedure ₃₄

Chapter 6

Results ₃₈

DeviceCalibration

Using

aKnown Reference 38

Presenting

theResults 43

Detecting

Difference between Sampleswith aLarge Visual Difference 44

CompositeImage 44

Red Channel. 47

Green Channel 50

Blue Channel 53

Detecting

Difference between Sampleswith aSmall Visual Difference 56

Composite Image 56

GreenChannel 62

BlueChannel ₆₅

Chapter7

Summary

andConclusions ₆₉

CapabilitiesoftheDevice 69

Detecting

Difference between Sampleswith aLarge Visual Difference. 69

Detecting

Difference betweenSampleswith aSmall Visual Difference 69

Issueswiththe

Methodology

70

Polarizer

Setup

70

Exposure Time 71

Aperture

Setting

71

Device

Geometry

71

Suggestions for Further

Study

72

Repeated Measurements 72

Exposure Time Optimization for Each Channel 73

Blue Channel BRDF Shape 73

Analyzing

Bulk Reflectance 74

Polarization

by

Reflection 74

Neglected Feature Vectorelements 75

Reflectance Factors 75

ResolutionofParticles 76

ListofFigures

Figure 1.Specularreflectedlight 4

Figure 2. Bulkreflectedlight fora single_ray 4

Figure 3. Bothspecular andbulkreflectiontakeplace 4

Figure 4. Reflection_geometryandterms 5

Figure 5. Sample BRDFcurve 7

Figure 6. Basicprinciple of_{polarizing filters} 8

Figure 7. Comparisonof conventional andVMPpigments 12

Figure 8. Comparisonofinkwith conventional andVMPpigments 12

Figure 9. The differenceof

leafing

and_non-leafingpigmentsinanink layer 14

Figure 10. The setupofthemicro-goniophotometer 25

Figure 11. The geometryofthemicro-goniophotometer 25

Figure 12. Bronzedsample mounted onthevertical cylinder 26

Figure 13.Themicro-goniophotometersetup,as seenfromabove 27

Figure 14. Thepolarizer mountedin frontofthecameralens 28

Figure 15. Nikon D50 Camera Controls 29

Figure 16.Thepolarizerscale 30

Figure 17. ScreenshotofImageJ 31

Figure 18. Image histogram in ImageJ 32

Figure 19. Mathcadscreenshot 35

Figure20. TheangularlimitsforthisspecificBRDFcurve 37

Figure21. Referencesamplecompositeimagegloss 38

Figure23.Referencesamplegreenchannel gloss 39

Figure 24. Referencesamplebluechannel gloss 39

Figure25.Referencesample compositeimage BRDF. 40

Figure 26. ReferencesampleredchannelBRDF. 40

Figure 27. ReferencesamplegreenchannelBRDF. 41

Figure 28.ReferencesamplebluechannelBRDF. 41

Figure 29. Areabetween-30and30

degrees,

comparedtotheentire curve area. 42

Figure 30. Silversamplecomposite glossimage 45

Figure 31. Bronzedsamplecompositeglossimage. 45

Figure 32. BRDFcomparison ofthesilver andbronzed_sample,compositeimage 46

Figure 33. Normalized BRDFcurves,silver andbronzedsample,compositeimage 46

Figure 34. Silversample red channel gloss 48

Figure 35. Bronzedsample red channel gloss 48

Figure 36. BRDFcomparison ofthesilver andbronzedsample red channel 49

Figure 37. Normalized BRDFcurves,silver andbronzedsample redchannel. 49

Figure 38. Silversample green channel gloss 51

Figure 39. Bronzedsamplegreen channel gloss 51

Figure 40. BRDFcomparisonofthesilver andbronzedsample greenchannel 52

Figure 41. Normalized BRDFcurves,silver andbronzedsample green channel 52

Figure 42. Silversamplebluechannel gloss 54

Figure 43. Bronzedsamplebluechannel gloss 54

Figure44. BRDFcomparisonsofthesilver andbronzedsample,bluechannel 55

Figure 45. Coatedsample compositeimagegloss 57

Figure 46. Uncoatedsample compositeimagegloss 57

Figure 47. BRDFcomparisonsofthecoated and uncoatedsamples,compositeimage 58

Figure 48. Normalized BRDFcurvesforthecoated anduncoatedsamples,composite

Figure 49. Coatedsampleredchannel gloss 60

Figure 50. Uncoatedsampleredchannel gloss 60

Figure 51.BRDFcomparisonofthecoated anduncoated_samples,redchannel 61

Figure 52.Normalized BRDFcurvesforthecoated and uncoatedsamples,redchanneL...61

Figure 53.Coatedsamplegreenchannelgloss 63

Figure 54.Uncoatedsample greenchannel gloss 63

Figure 55.BRDFcomparisonofthecoated and uncoatedsample,green channel 64

Figure 56. Normalized BRDFcurvesforthecoated and uncoatedsamples,green channel64

Figure 57. Coatedsamplebluechannel gloss 66

Figure 58. Uncoatedsamplebluechannel gloss 66

Figure 59. BRDFcomparison ofthecoated and uncoatedsamples,bluechannel 67

ListofTables

Table 1.Feature Vector Components 36

Table 2.Referenceareasmeasuredand corrected 43

Table 3. Feature Vector

Comparison,

compositeimage 47

Table 4. Feature Vector

Comparison,

red channel 50

Table 5. Feature Vector

Comparison,

green channel 53

Table 6. Feature Vector

Comparison,

bluechannel 55

Table 7. Feature Vector

Comparison,

compositeimage 59

Table 8. Feature Vector

Comparison,

red channel 62

Table 9. Feature Vector

Comparison,

green channel 65

Abstract

Metallicinksareinwidespreaduseinthegraphic arts

industry,

butthere_currentiy

isno_wayto"puta

number"

on metallic gloss. Process-controlmethods,such as

densitometry,

whichanalyzeshow light isabsorbedintotheink

layer,

are not usablefor

metallics,since

hardly

_{any light}gets absorbed.Inaddition,it isverydifficulttosee visual

differences whentheink film thicknesscrosses a certainthreshold.

Dr. Jonathan

Arney

attheChester F. Carlson Center for

Imaging

Scienceat

Rochester Instituteof

Technology

(RTT)

has been_usinga micro-goniophotometerfor

analyzingpaper gloss on a microstructure

level,

andin_{this exploratory}research, thedevice

was usedtoanalyze metallic prints.Twosets of measurements were madetodeterminethe

device'stoleranceand range.Inthefirstexperiment, twovery differentsamples_visually

were measuredtodetermine ifthedevicecandetectthedifference. Thesecond experiment

consisted of_measuringtwo_visuallysimilar samplestodetermine ifthedevicecandetect

thissmall visualdifference.

Theresults showedthat thedevicecandetectthelargevisualdifference inthefirst

experiment, thedifferencewas observableinthegloss

images,

as well asinthenumerical

data. Inthesecondexperiment,asmalldifferencewas

detected; however,

without repeated

measurements,therewas no_waytorule outthedifference

being

caused

by

experimental

factorsand noise.

Overall,

theresultsshowedthatthemicro-gomophotometer canbeused

Chapter

1

Introduction

Theuse ofmetallicinks introduces a challengetoprint processcontrol,since

"traditional"

processcontrol_methods, such as

densitometry,

which analyzeshowtheink

filmabsorbs

light

are not useful.This is becausethepigment particlesinmetallicinkare

like

tiny

mirrorsthatreflect

tight;

_thus,

hardly

_anylight isabsorbed.

Visually,

it ispossibletoseedifferences inmetallic prints when_varyingpigment

size andink film thickness, butthereisno_wayto"puta onthemetallic

properties.Even forthemost skilled press_operator,visualjudgmentaloneisnot good

enoughforproper process control.Changes in inksettings at_verylow ink film levelsare

easytodetectvisually,butonce enough metallic particles aretransferredtocoverthe

wholesurface,_addingmoreink

hardly

changestheappearance oftheprint.

Having

a_way

todetenninethis thresholdwould

help

prevent_usingmoreinkthanwhatisneeded and

wouldincreasecost-efficiency,and prevent problems caused

by

thickink film layers.

Inadditionto

finding

a measure ofthis_threshold,another area ofinterest istofind

a_wayto_numericallycharacterizethe

"metallicness"

of metallic prints.Metallicprints are

often coated with alacquertoprotectthemetalparticlesfromrub-off andtoaddadditional

gloss; thisadditional gloss makesitevenmoredifficulttodetectwhat glossismetallicin

natureand whatis _onlyspecularsurfaceglossfromthecoating._{The coating}thus takes

mixedintothelacqueritselfaswell

(Lenz,

2005).

This,

in turn,makesiteven more

difficulttoanalyzethemetallic glossoftheactualink layer beneaththecoating.

Rosenberg

(2001)

hasdevelopeda_{methodology foranalyzing}metallic gloss_using

a_{spectro-goniometer, usingthedevicetoanalyzethecolor of gloss.}Rosenberg'smethod

canbeusedtodifferentiate betweencopper and gold_gloss,becausetheirglosshasa color

componenttoit Itcannotbeusedtoanalyze silverbecausesilverhasnoapparent_color,

thus

having

differentspectral propertiesthancopper and goldRosenberg'smethod uses

twodimensionstocharacterize metallicness: thespectral component andthegloss.

Amicro-goniophotometercanbeusedtoapproach metallicnessinadifferentway,

analyzingthegloss component atthemicrostructurallevel.Theresearcher was unaware of

anyother researchdoneon_analyzingmetallic gloss_usinga_{micro-goniophotometer;}thus,

theresults ofthisresearch were unpredictable.Theprospectof

doing

new research was a

big

motivator.Ifa micro-goniophotometer couldbeusedtocharacterize metallicness and

metallic gloss at a particle

level,

itmight

help

inthedevelopmentofnewtools forprocess

control.

Theresearcherbecame interested inthis topicin

May

2006afterdiscussionswith

Professor Franz Sigg. Theresearcherdidnothavethatmuch priorknowledgeofthe_topic,

buttheideaof

doing

fundamentalexperimental research was_veryappealing.

Also,

workingwith metallic printsis_visuallymore_pleasingand

interesting

than_workingwith

Chapter

2

TheoreticalBasis

Understanding

themathematicalbasisandtheoptics ofhow tightreactswith

substratesisimportantwhen_attemptingtointerpretthedata fromthe

micro-goniophotometer.

Measuring

Reflected Light

Therearetwokindsof reflectedlight: specular andbulk. Specular light isthelightthat

reflects at aninterface betweenmaterials ofdifferentrefractiveindexes. Bulk tight

penetratesthe substrate,undergoes multiple_scatteringeventsinside_{the substrate,}and

re-emerges at a randomangle. Specular light is_onlydetectedatthespecular_angle,whichis

equalto theangleofentry,as showninFigure 1. Bulkreflectedtightre-emergesthebulk

ofthematerialina randomdirectionafter

being

penetratedto thebulkofthematerial and

having

gonethroughmultiple_scatteringeventsinsidethematerial

(Arney,

2006a). Figure 2

[image:17.524.74.441.46.253.2]

Figure 1. Specularreflectedlight (Source:

Arney, 2006a)

Figure2. Bulkreflectedlight fora single_ray

(Source:

Arney,

2006a)

Formost_objects,both kindsof_reflectingtakeplace simultaneously. Iftheobject

surfaceissmooth and"shiny"it is_{very easy}totellthespecular andthebulkcomponent

apart,becausethespecular componentis_relativelymuch_stronger,as showninFigure 3.

Figure3. Bothspecularand bulkreflectiontakeplace

(Source:

Arney,

2006a)

Specularlightcanbe describedas

"gloss,"

anditcanbeexplained

by

Fresnel's

laws,

which can calculatethefractionoflightthatgets reflectedatthesurface andthe

fractionoflightthatgetstransmittedintothemedium.

(Arney,

2006a).

According

to [image:17.524.193.346.359.510.2]

transmissionofelectromagneticwavesat aninterface.That

is,

they

givethereflection and

transmissioncoefficientsforwavesparalleland perpendiculartotheplaneof

incidence."

Figure4illustratesthereflection_geometryand

terrninology

usedinFresnel's law.

Incident light

Figure 4. Reflection_geometryandterms

(Source: Georgia State

University, 2006)

In Figure 4 II standsforparallel and

-1-forperpendicular. Thereflection r and

transmission tcoefficients are calculated withthe

following

_equations,where

"pa"

stands

forparallel,

"pe"

forperpendicular and

Q

and

ft

standfortheangles ofincidenceand

transmission,respectively.

Here,

"parallel"

means parallel to theplane of

incidence,

and

"perpendicular"

means perpendicularto theplane of

incidence,

orinother_words,parallel

to the

interface,

asillustratedin Figure 4.

r =

tan

(0-0

₁₎

r =____!!<__!__

(1)

"

tan

(ft

+0,)

" sin

(0,4-0,)

2sin0.cos0f

2sin0.cos0. ...

t = _t = -

(2)

"

Thevalues obtained_usingtheseformulasarefractionalamplitudes,andthese

values mustbesquaredtogetfractional intensities.Duetoconservationof_energyand no

light

being

"lost"

inthe_process,the

following

_relationship appliestoboththe

perpendicularandtheparallelcases:

, ,n9cos0,

r+t

i=l

(3)

njcos0j

Here,

n2is therefractiveindexofthemediumthat thetightpenetrates

into,

and_njis

therefractiveindexofthemedium wherethetightcomesfrom.

By

_squaringthevalue

gotten

from,

for_example,r^theresultisthefractionoftightthatisreflectedintheparallel

direction.

Subtracting

thisresultfrom

1,

theresultisthefractionoftightthatistransmitted

intheparalleldirection (Georgia State

University,

2006).Theseequations showthattight

iseither reflected or_transmitted,andinthisresearchthebehavioroftightwith metallic

surfacesis looked into inmoredetail.

Inordertoobtaina reflectedtight_readingacross all angles ofthesample_surface,

threeapproaches canbetaken.The firstmethodistomovethetightsource acrossthe

sample surface while

keeping

the camera,ortheinstrumentthatcapturesthe

tight,

stationary.Thesecond methodistomovethecameraacross_{the surface,}

keeping

thelight

source stationary.Thethirdmethodisto

keep

thetightsource andthecamera_stationary,

andto_wrapthesample arounda roundcylinder, sothatalargerange of angles are

Bi-directional Reflection Distribution

Function

Whenthesurface radianceismeasuredat all possibleangles, theresultistheBi

directional Reflection Distribution

Function,

orBRDF.WhentheBRDFdiagram is

examined,the_{spike representsthespecular componentofthereflectedtightandthe}

remainderofthearea underthecurvethebulkreflectedtight Thetotalareaunderthe

curveisthe totalreflectedlight

(Arney,

2006a).AsampleBRDFcurve withthedifferent

parametersisshowninFigure5.

C

[image:20.524.142.394.264.536.2]

Angle

Figure 5. SampleBRDFcurve

(Source:

Arney,

2006b)

Theshape oftheBRDFcanbe interpretedas a measure ofthe topographic

properties of_{the surface, making}themeasurement similarto thatof a profilometer. The

differenceisthata profilometermeasurestheheightandvariationof_{the surface,}while a

variationintheangle ofthereflectedtight.Themetrics,such as curveheightand curvehalf

width,can alsobeanalyzedforextra significance

(Arney,

2006b).

Polarization

Light isanelectromagneticwave andtheelectricforce fieldthatmakes_upthewave

canvibrate_upanddown (inthevertical_plane),fromsideto side(inthehorizontalplane),

orinanintermediatedirectionthatis in betweenthehorizontaland vertical planes.This

kindof waveis also called a_{transverse wave,}becausethewave vibrates perpendicularto

thedirectionof propagation.

Ordinarily

a_rayoftightconsists of a mixture of waves

vibrating in different directions. Ifthe_{tight only}vibratesinone

direction,

thistightis

polarized.

(Anon,

2004;

Murphy,

Spring

&

Davidson,

2007)

A_polarizingfilter_onlylets tightwith a certain_vibrating plane passthrough.

By

placingtwofilterscrossed,i.e.at90 degreeangles, thisstops alltight from_passing

through.

(Murphy

et._al,

2007)

This is illustratedfurther in Figure.

Polarizer 1 (Vertical)

Incident Beam (Unpolarized)

Polarizer2 (Horizontal)

Vertically Polarized Light Wave

Figure6. Basicprinciple ofpolarizingfilters

Inthis _{study polarizing}filtersare usedto tellapartbulkand specular reflectance.

Specularreflectedtight(Figure

1)

maintainsitspolarizationcharacteristicswhen

being

reflected offthe_surface,whilethepolarizationofbulkreflectedlight (Figure

2)

changes

after_re-emergingfromthesubstrate.Whentwo_{polarizing filters}arecrossed,notightgets

through,unlessthetightgetsdepolarizedor changes polarization while

being

betweenthe

Chapter

3

ReviewoftheLiterature

Studying

metallicinkbehaviorrequiresknowledgeof metallicinkproperties and

manufacturing.

Knowing

_howandwheremetallicinks are usedhelpsunderstand whatkind

ofbehavior isexpected oftheinks. Processcontrol and color management arediscussed

briefly

togiverelatedbackgroundinformationto

discussing

thespecial requirements of

process control with metallicinks.

Metallic Pigments

Materials

Aluminum,

copper,and zinc arethemost common metals usedinmetallic pigment

manufacturing. Aluminum hasnoinherentcolor;it isusedforsilver_effects,as well asfor

mixingwith other colors_{to create,}forexample,metallicblue. Copper isusedtogenerate

thecolor

bronze,

and

by

_mixingcopper withzinc,various shades of gold canbeachieved.

When mixingcopperand zincintheproportion90:

10,

theresultisa reddish-yellow_gold;

themorezincis_{added, the}more yellowishthegoldbecomes. Theproportion70:30results

ingreenish-yellowgold

(Eckart,

2006b).Theraw materials usedforpigmentshave

fluctuating

pricetrends,partlydueto therisingprice of oil.Thepricesforcopper and

looking

at alongertime_span,thepriceofcopperhasgone_{up nearly 200%}

(Tonbridge,

2006).

Technology

and

Manufacturing

Conventional

Metallic Pigments.Theprocessbeginswith_meltingmetal pellets and

then_sprayingthemoltenmass underhighpressure sothat themolten metalformsafine

mist.Whenthesemistdropletscool

down,

they

areinpowderform. Theseparticles needto

be

flattened;

thisis done

by

a series of_{millingoperations,grinding}theparticles until_they

areflakes. Theseare called"corn flakepigments" inthe

industry

becausetheshape

resembles cornflakeswiththeirrough edges.These "corn flakepigments" are

manufacturedfrom

irregularly

shaped atomized powder particles.Iftheatomizedparticles

takeasymmetricalspherical

form,

the_resultingpigments are rounder and are called"silver

dollars"

(Eckart,

2006b).

Vacuum Metallized Pigments. Vacuummetallized pigment

(VMP)

_{manufacturing}

is different fromconventional pigment manufacturing.Theprocessbeginswiththe

application of a release_coatingtoa carrier_substrate,such as polyester. Aluminumisthen

appliedto the_coatingvia physical vapordeposition. Thecarrier substrate withtherelease

coatingand aluminumlayers isthentaken toa solvent

bath,

wheretherelease_{coating is}

dissolved,

releasingthealuminum_coatingto thesolvent.The final_{stepis collecting}the

metalfromthesolventand_sizingitto thedesiredsize.

Eventhesmoothestsilverdollarconventional pigmentshavesometopographical

variationintheirpigment surface.When light hitsthesurface,itwill scattermoreor

less,

surfaceis very_{smooth,makingthelightreflectwithout scattering.Electron-micrographs}

comparingconventional"cornflakes"

toVMPpigments aredisplayedin Figure

7,

where

thedifferenceinsurface

topography

isevident.

Conventional

aluminum

flakes

VMP aluminum

flakes

Figure 7. Comparisonof conventional andVMPpigments (Source:

Eckart,

2006)

VMPpigments alsoforma more uniform metal surfaceintheink

layer,

_causing

lesslightscattering,asillustrated intheelectron-micrographs and schematicillustrations in

Figure8.

VMP ink

V

. v

i~s"

*

-itiM

V/1 \\mt^BlKU^^BmmlKa\\a\\\\\m\\mmmmTWikWBa\m

[image:25.524.55.471.395.627.2]

Inkwith conventional pigments

Embossed

Vacuum

Metallized

Pigments.The_{manufacturing}of embossed vacuum

metallized_pigments,

EVMP,

_{is verysimilarto the}

manufacturingofVMPpigments.With

EVMP,

therelease_coatinglayerisembossed_{using laser technology, before}thealuminum

isapplied_usingphysical vapordistribution.Fromthere on, themanufacturingprocessis

thesame as withVMP.EVMPpigmentshavethe same characteristics asVMPpigments

whenitcomestoparticle characteristics.Themajordifference isthemicro-embossed

patterninthepigment_surface,which givesthepigmentholographiccharacteristics, giving

themetallic shine a rainbow effectinthefinishedproduct

(Eckart,

2006b).

Particle SizeandShape

The relationship betweenparticle size and various metallic propertiesis

straightforward.Asparticle size

increases,

sodoesthebrillianceeffect. This is

because,

thereare more uniform surfacestoreflecttightonlargerparticles,andthereis lessof a

chanceofthetight

hitting

theparticleedge,which causes _scatteringoflight. Whenparticle

size

increases,

thehalftone dot definitioncapabilitiesdecrease. This is

because,

withlarger

particlesit is hardertoimagethe smallestimage details. Thepigment coverage_abilityalso

decreases astheparticlesizeincreases. This is becausewithlargerparticlesit ismore

difficulttofill inthesmaller

"gaps."

There isno perfect pigmentparticlesize; theright

balancebetween brillianceanddot definition/coverageshouldbeconsideredforeach

application

(Eckart,

2006b).

Particleshape alsohas an effect.Theconventional pigmentsare not assmoothas

the"silverdollars,"which are alsolargerin diameter. The "silver

dollars"

metallic_properties,but dueto their_size,thehalftone dot definitionandcoverageis inferior

toconventional pigments. Thesameis truefor VMPpigments; theirexcellent metallic

propertiesarecounter-balancedwithdecreasedhalftone dot definitioncapabilities

(Eckart,

2006b).

Pigment Classifications

LeafingandNon-Leafing. One_wayto_classifyconventional pigmentsis between

leafing

and_non-leafingpigments.

Leafing

pigmentshavepoor_{wettingcapabilities;}thus,

they

"float"

to the

top

oftheink filmwhen

drying,

_creatingabrilliantmetallic shine.This

is becausetheflatpigments andthe_way

they

are oriented

forming

a more uniform mirror

likesurfacetoreflecttight.

Non-leafing

pigmentsdonothavethis property,and

they

are

dispersed evenlywithintheink. Thiseffectis illustrated in Figure 9. When_coatingtheink

layerwith a

lacquer,

thepigment properties also comeinto_play;lacquersadhere_poorlyto

inkswith

leafing

_pigments,whiletheadhesionisgoodto_non-leafinginks

(Eckart,

2006b).

t^-'

>"WK_:--'- --_Z:-7

--*?-.

\////////////T777TT7\

J?/?7t7Wfi??7ffm

Leafing Non-leafing

Figure9. The differenceof

leafing

and_non-leafingpigmentsinanink layer (Source:

Eckart, 2006)

UseofMetallic Inks

Metallicpigments are presenteverywherefromcar paintstometallicinks in

stand outfromthecompetitors'

products.Consumersoften makethedecisionto

buy

or not

to

buy

anitem insplit_seconds,andiftheitemstands_out,it hasmoreof a chancetobe

purchased. Consumersoftendonot evenrealizethat the_everydayproducts that

they

see at

grocerystoresareproductsthathavebeentailored toattracttheirattention.For_example,

Shamrock_{Farms has been paying}special attentionto the_packagingdesignoftheir

products,and

they

_{have been using}metallicinksas a part oftheir_strategytomake

consumersnotice and

buy

ShamrockFarms icecream,ratherthan theircompetitor'sbrand

(Petrak,

2003;

Pianoforte,

2005).

Themetallic effect canbeachievedthreewaysinprinting:foilstamping,printing

with metallic

inks,

and_usingmetallic paper. In_{addition,applying}metallic powdertoa

surface can achievethedesiredeffect.Foil_stampinggives excellent metallic effect and

coverage,but is onlycost-effectivein_relativelysmall areas.Inaddition,foilstampingis

not suitableifthemetallic area needstobeprintedon,inwhichcase,_usingmetallic papers

wouldbethebestoption.

Using

metallicinksgives mostfreedom becausethereisno

additional equipmentneeded,andmetallicinkscanbeusedin_printingpressesinline

whenever needed. Themost advancedinks

today

can createfoil-likeeffects

(Sharon,

2003,

2004; Pianoforte,

2005).

One_waytoproduce metallic effects without_usingactualmetallicink istoprint a

silverimage

first,

thenprint with process colors on

top

ofit. This

technology

is called

MetalFX,

andithas becomemoreand more popularintheUS inthepastfewyears.The

majoradvantagetothissystemisthat_virtuallyunlimited metallic colors canbeprintedin

thesamepress run withouttheneedforallkindsofdifferentshades ofmetallicink

(Anon,

Inthe_{printingworld,}metallic effectsare_{predominately}usedin packaging

printing,but_usingmetallic effectsas

finishing

touchesis _verycommoninall areas of

printing

(Pianoforte,

2005). One specialty printingarea worth_mentioningis_printing

electronic circuits_usingmetallicinks.

Currently

themost common printed electronic

applicationinuseisRFID_tags, where metallics are usedtoprint a small antenna on

variousitems.Newapplications_{for printing}with metallics are

being

_{discussed constantly}

(Gasman, 2006; Hariharan,

2006).

Visual Effects

Metallicscan create_smnningvisual effectsthatcannotbeachieved

by

normal

four-process color printing.Printersare_usingmetallicsinnumerous waystoenhancetheprinted

product's_appeal,and customers are

demanding

more andmore,

forcing

theprinterstofind

new creative waystouse metallic

inks, foils,

and papers.Thesenewwaystouse metallics

and

inks,

inparticular,havemadeinkmanufacturers such asEckartshifttheir_{focus away}

from

being

justa pigment providerto

being

theprovider ofhigh-performancemetallicinks

forthegraphicarts

industry

(Toth,

2000).

Becauseofthereflectivenature of metallic

inks, they

givebestresults when used

with a coated paper stock.

Using

uncoated paper makestheprinted surface uneven since

ink isabsorbedintothe paper,resultinginreduced metallic properties.When printing

metallics on uncoatedstock, theink filmthicknessshouldbeatthemaximumlevelandthe

ink/watercontentminimizedtoavoid

diminishing

themetallicness.Printersalso use

dry

trappinganddouble printingas waystoincreaseandtoenhancethemetallicproperties

Metallics inPackaging

Metallicinksand metallicshave beenusedin_packagingtomaketheproducts stand

outfromtheircompetitionandtocreatea certain air_surroundingtheproduct

Using

gold

gives a sense of_tradition,while_usingsilver makestheproduct seemhigh-techand

sophisticated.

Traditionally,

_{using foil stamping has been}thepreferredmethod,butwith

today'sadvancesinmetallic

inks,

_{packaging designers have begun}toembracethemas a

viablelowercostalternative

(Sharon,

2003,

2004).

Specialty

_packaging(suchasgiftwraps,

creative packagesforcosmetics, and gift package_cartons)is onemajorarea where metallic

inksare used.The increaseduse of metallicsin_packaginghasalsobeensparked

by

the

developmentof new substratestouse

-card stock paperisnotthe_onlyviablealternative

anymore

(Lustig

2004; Sharon, 2003,

2004).

Process Control

Controlling

theprint processincludesvariousactivities;afewofthemare

discussedinmoredetail below. Processcontrolisessentialifthefinalprinted pieceisto

matchtheoriginaldesignor proof.

Color Management

All devicesusedtocapture andoutputimageshavetheirowncharacteristics,and

expectingthemtointerpretcolorinformation

identically

is alostcause.

Previously,

this

problemwas solved

by

_workinginclosed-loopcolorcontrolsystems withafixed

necessaryadjustments.Butwiththenumberofdevices

increasing,

thiskindof systemisno

longerviable

(Sharma,

2004).

Open-loop

color managementis basedon a centralized connectionspace,a

"commonlanguage"

thatalldevicesunderstandThismakesitpossibleforalargenumber

ofdevicestocornmunicatewith andtounderstand each other.Each inputand outputdevice

isgivena profilethatdescribesthedevice'suniquecharacteristics,relativeto theprofile

connection_space,wheretheotherdevicescaninterpret it

(Sharma,

2004).

Colormanagementis importantwhen_printingbecausewhatthedesignersees on

thescreen_maynotbewhatis goingtocome out ofthe_printingpressintheend.

Managing

deviceprofilesisneeded when_{setting up}a_proofingsystem.Insoftproofing, themonitor

is adjusted sothatitsimulatestheprinted end_result,andin

hardcopy

proofing,a proofer or

a

desktop

printerisconfiguredtoactlikethepress

by loading

thepress profile ontothe

system

(Sharma,

2004).

ControllingthePress

When_runninga multicolor_press,it isvitaltogetthedifferentcolorstoalign on

top

of each other_{perfectly ;}thisis done

by

_controllingtheregistration.

Virtually

all printjobs

haveregistrationmarks(thincrosshairpatterns)ontheirnon-image areas neartheedges of

thesubstrate.Thereis apatternforeachsub-color,andif

they

print_exactlyon

top

of each

other, onlyone patternisobserved. Ifseveral crosshairs are seen, thepress canbeadjusted

by

moving theplate or platesthatneedadjusting.Registrationcontrolis_{the same,}no

Oncetheprintjob is inregister,thecolordensitiesshouldbeobserved.

Usually,

thereis a proof withknown

density

valuesthat thepress operators can use as atargetpoint.

Inadditionto_relyingonregistration_marks,printjobs usually featuretest targetsforcolor

accuracycontrol.Manualsystemshavetheoperatortakesamples fromthepress whileit is

ninning, thenmeasurethedensities fromthecolor patches.If

they

are not within a certain

acceptable_range,the_{corresponding ink keys}are adjusted_{accordingly, then the}

measurements are repeated until allthe

density

values

(i.e.,

all colorsthatthepress_prints)

are withinthe accepted range.Somemore sophisticated systemshave_closed-loopprocess

controlfor bothregistration and

densities,

wherethe_controllingsystem samplestheprint

run at constant

intervals,

thenself-adjuststoadefinedgoal,basedonthatinformation.

When_usingmetallic

inks,

densitometry

isnot_veryreliablefor_monitoring

inking

levels

accurately, and process_controllingthepress runbecomesmoredifficult.

Proofingmetallics

Inkjet

technology

is_commonlyusedfor_generatingproofs oftheprintjob before

runningitonthepress.

However,

metallicinkscannotbeused withink jet

technology

becausethepigment particles would_clogtheink jetnozzles. Anothermethod of_proofing

issoft_proofingon amonitor,wherethefinalizedprint productissimulated onthemonitor

usingcolor profilestoproducea color match. Butsoft-proofingmetallicshas itsown

problemsbecause simulatingmetallic shine and effects on amonitoris

difficult,

ifnot

impossible.

By

_viewinga metallic color on a monitorittakesawaytheeffect ofthesurface

proofs with metallicinksis_{thermal transfer technology.Issues} with_proofingmetallics are

further discussedinthe

following

section.

RealLife Applications

Tofindouthowtheseissueswith process_controllingmetallicsaredealtwithin

real

life,

theresearcherinterviewedMr. Calvin

Curtice,

Senior Application Engineerat

EastmanKodak (personal_{communications,}

February

23,

2007). Mr. Curtice has been

workingwiththe_{Kodak Approval proofing device} anddealswith customers_wantingto

proof_packagingprintsthathavemetallics onthem.

Proofing

metallics cannotbe donewith

ink

jet,

becausethepigments_clogtheinknozzles.Approvalusesthermal transfer

technology

andiscapable of_proofingmetallics. (Eastman

Kodak, 2007)

The Kodak Approval includessix process colors:_{cyan, magenta, yellow,}

black,

orange, green,and

blue;

toachieve metalliceffects, a silverink isalso used.Toachievethe

desiredmetallic effect andcolor, thesilverisprinted

first,

thentheprocess colors are

printed on

top

ofit. The device iscapable of_maintainingthehalftone

dot;

_thus,thefinal

resultlooks asif itwereprinted with a press.Ifthecustomer wants extra metallic

reflectance,itispossibletoprintthesilvertwice,andifthereflectance needstobetoned

down,

addingalittleblackoftenresultsinthedesiredeffect.(C.

Curtice,

personal

communications

February

23, 2007;

Eastman Kodak,

2007)

When_proofingmetallics,matchingtheproofstopresssheetsis donevisually.

According

toMr.

Curtice,

thereare nostandardguidelines inplaceforthevisual

inspection,

thus_{making matching}metallicstricky.Sometimescustomers

try

tomatchthe

metallics,thesemethodsdonot workthatwell.Visualinspectiontakesplace_{in viewing}

booths,

andthecustomers ofteninspecttheproofinmultiple_angles,dueto themetallic

shine effect.

Having

somesort ofstandardforthe_viewingconditions andinstructions for

proof_matchingwould_simplifytheprocess significandy.

Customers_payattentiontoboththemetallicshine andreflectance,andthemetallic

color.Thereflectanceisobserved when viewedat an_angle,whilethecolorisobserved and

matched when_viewingtheproof at a90 degreeangle.Mr. Curtice saidthat often, the

customers"are asking forthe

impossible",

and

they

havetocompromise with eitherthe

desiredmetallic_color,orthemetallic reflectance and shine.

Conclusionfor literaturereview

Inthegraphic arts

industry,

metallicinksare used_predominandyin_packagingdue

to thevisual effects achieved.Metallicsmaketheproduct stand outfromthemasses.

Copperand aluminum arethemost common metals usedinpigment manufacturing.Notall

metallic pigments arealike,andthe_{different manufacturing}processes givethedifferent

typesof pigments specific characteristics.Particlesize and shape alsohavean effect onthe

pigments '

characteristics.Theend use ofthemetallicink determineswhattypeof pigment

worksbest.

When printingwithmetallics process controlis not as simpleasit iswith_using

onlynormalprocess colors.Methodssuchas

densitometry

and_colorimetryare not as

Chapter

4

ResearchQuestions

Totheresearcher's

knowledge,

themicro-goniophotometerhasnotbeenusedin

analyzingmetallicsbefore. Thereare_many

interesting

topics toapproachinthis research;

however,

gettingtoknowthedevice's

limitations, behavior,

and rangewhen usedfor

measuringmetallicsisthemain area of_{focus. This study is}an_exploratorystudy,intended

toget_{preliminary insight}onhowthemicro-goniophotometer analyzes metallic print gloss.

Experiment 1:

Detecting

Difference between Sampleswith aLarge Visual Difference

Giventwosamplesthatare_{visually very different}andhave differentmetallic

properties,canthedevice detectthe

difference,

andif itcan,how doesthedifferenceshow

upinthedata?

Experiment2:

Detecting

Difference between Sampleswith aSmall Visual Difference

Giventhat themicro-goniophotometercandetectadrasticvisualdifference in

metallics,if it isusedtomeasuretwo_verysimilar_samples,howsmall adifferencewouldit

Chapter

5

Methodology

Theresearch_{methodology is based}ontheuse of a specialmicro-goniophotometer,

developed

by

Dr. Jon

Arney

fromtheChester F. Carlson Center for

Imaging

Scienceat

RTT. The devicecanbeusedtomeasure gloss of various substrates.Previousresearch

usingthisdevice has been done inmeasuringand_comparingthegloss ofdifferentcoated

substrates

(Arney

&

Heo, 2004),

butneverbeforewith metallic prints.

Simplegloss-metersmeasurejustat one angle ofincident tight Inordertoobtain a

gloss_readingat_{continuouslychanging}angles acrossthesamplesurface, threeapproaches

canbetaken.The firstmethodistomovethelightsource acrossthesample surface.The

second methodistomovethecamera acrossthesurface.The_{third method, the}one usedin

this study,isto

keep

thetightsource andthecamerastationary,andto_wrapthesample

whichistobemeasuredaround a roundcylinder, sothatalargerange of angles are

capturedina singleimage

by

thecamera.Thecameraisusedtocapturetwoimages

-one

withthepolarizers parallel andtheother onewiththepolarizerscrossed

(Arney,

2006a,

Camera Capture 2 images

Light

Source

[image:38.524.127.393.44.450.2]

Cylinder

Figure 10. The setupofthemicro-goniophotometer

(Source:

Arney

2006a)

ample

Figure 11. The_geometryofthemicro-goniophotometer

(Source:

Arney, 2006a)

The Device

Setup

Themicro-goniophotometerusedinthesemeasurementsis butit usingadigital

camera with a

long

objective

lens,

a verticalmetal cylindertomount_{the samples,}and a

lightsourcetoiUuminatethesample surface.Thesampleismountedon a verticalcylinder

(diameter=4.6_cm)

tightly

aroundthecylinder without

damaging

_{the sample,}as showninFigure 12. The

cylinderdiameterislargeenoughso_{that the samples,}often_{times coated,} are notdamaged

[image:39.524.85.448.128.390.2]

by

crackingwhen

being

wrapped around the cylinder.

Figure 12. Bronzedsample mounted onthevertical cylinder

Inthis experiment, thesample_mountingcylinderisat a5.5 inch distance fromthe

polarizers,andthelightsourceis 3 inches_awayat an_{approximately}20 degreeangle.The

[image:40.524.89.424.45.299.2]

Figure 13. Themicro-goniophotometersetup,as seenfromabove

The CameraandthePolarizers

Thecamera,Nikon

D50,

hasa

long

objective attachedto

it,

inordertoprovidethe

required magnification.Onepolarizeris mountedbetweenthelensandthesample andthe

other oneisattachedto thelightsource. Thepolarizerin frontofthetightsourceis fixed

andtheoneinfrontofthelenscanberotated360 degrees. Thepolarizerthatisattachedto

thetightsource polarizestheincident tightandbasedontheposition ofthesecond

polarizer, as wellashowthepolarizationchangedwhen

hitting

_{the sample,} all or notight

passesthrough the secondfilter. Theproblem withthisarrangementisthat thereare no

predefined positionsforthecamerapolarizer; therotationisseamless andis done

by

hand

Thereare markersthat

help

with_{the alignment,}butthe_accuracyofthe_{positioningvaries,}

depending

onwho operatesthedevice. Thepolarizerpositionedin frontofthecameralens

[image:41.524.129.402.42.443.2]

Figure 14. Thepolarizer mountedin frontofthecamera

lens

Thecamera exposuretimeiscontrolled on a computerinterface

by

_changingthe

shutter_speed, as shownin Figure 15. Thecontrolisa slider scale with pre-defined_steps,

ranging from 25secondsto1/4000second.Thepictureiscaptured_usingthe computer

interface,

aswell; thereisno needtotouch thecameraitself.Theaperture controlislocated

selectedbeforethe measurementsare_taken,andthenthat_{setting is kept}constant

throughouttheexperiments.

Nikon Capture Camera Control

File Camera Settings Tools _Help

The D50 is connected.

Imagescaptured

by

thiscamera willbypassthe_Memorycard andbe downloaded

directly

to thecomputeruntilthiswindowisclosed.

HideCameraControls DownloadOptions...

__]Enablecontrolsonthecamera

body

Exposure1 _{Exposure 2 Storage Mechanical}

Image _Processing

Exposure Mode: Manual

Shutter Speed: f!31 -i

<

__J1/15sec

Aperture: f/~

Exposure Cornp.: 4

'

OEV

Flash Comp.: <

\

OEV

Flexible Program: < r

-OStep(s)

[image:42.524.87.448.105.539.2]

AF arid..hoot Shoot

Figure 15. Nikon D50 Camera Controls

Once_captured,theimagesarestored onthecomputer atthelocationthatissetin

thecapturesettings.Thecapturedimages are renamedandlabeledto

identify

thepolarizer

system via"Download Thecustom curve givesthe_{resulting images}a gamma of

1,

makingthem_{radiometrically}as correctaspossible

by forcing

alinear_relationship

betweentheinputlight

intensity

andtheoutputpixelvalues.

Measurement Procedure

Themicro-goniophotometerisusedtocapturetwoimages. The first image is

captured withthepolarizers_{parallel,showing}thespecular andbulkreflectance.Thenthe

polarizerin frontofthecameraisturned90

degrees,

sothat thepolarizers are crossed and

theimage iscaptured_again,now_showingonlythebulkreflectance.This is sobecausethe

bulkreflectance,i.e.thelightthatpenetratesthesample andre-emerges,getsdepolarized in

theprocesswhilethespecularreflectance,i.e. thetightthatreflects off_{the surface,}remains

polarized.Toplacethepolarizers_{parallel, the}markerisalignedwiththe0mark onthe

scale,and whenthepolarizers arecrossed,it isrotated sothat themarker aligns withthe90

[image:43.524.188.348.454.650.2]

degreemark onthescale.Thepolarizer scaleis shownin Figure 16.

Exposure Control

Theexposuretimeiscontrolled

during

theparallel polarizerphasesothat the

resulting image hasnoover-exposedpixelsin it Thecapturedimageisopenedinanimage

editingsoftware,andthepixellevelsareexamined.

Checking

forover-exposedpixelsis

done separately forthe red, green,andbluechannels oftheimage. Even ifthecombined

image hasnoover-exposed_{pixels, the}differentsub-channels mighthavethem.Inthis

experimentImageJ,isusedforallimageeditingandinspectionpurposes.ImageJ isa

free,

open source_{image editing}software

by

theNational InstituteofHealthand available

for_anyplatformfrom htro://rsb.irdo.nm.gov/ii/index.html . Aviewofhowtheimagesplit

is done in ImageJ is shownin Figure 17.

Figure 17. ScreenshotofImageJ

Ifthereare morethanfivepixels withthevalue

255,

chancesarethat

they

areover [image:44.524.48.479.318.600.2]

for checkingtheimagepixel values.Onemethodis toinspecttheimage

histogram,

which

showshowtheimagepixelvalues are

distributed.

Theimage

histogram,

shownin Figure

18,

has106pixelswiththevalue

255,

soitis_severelyover-exposedand needstobe

recapturedwithanother exposuretimesetting.

Having

over-exposedpixels present causes

data lossbecausethedetail differentiationis lost inthehighlights. Oncenone oftheimage

separations contain over-exposed_{pixels, the}separatedimage filesarelabeledandsaved.

Histogram ofImg0085

300x240 pixels;8-bit; 70K

r!li_0lt3

Count: 1504000

Mean: 24.568

StdDev: 33.966

Min:0

Max: 255

Mode: 4(162505)

List Copy Log

Value: 255

[image:45.524.146.385.212.482.2]

Count: 106

Figure 18. Imagehistogram in ImageJ

Image Data Analysis

Whenthebestexposure_{setting is}

found

thecapturedimagesare enteredintoa

program_runningin

Mathcad;

thisprogramwasdeveloped

by

Dr. Jon

Arney

tobeused

withmicro-goniophotometric measurements.The basicoperation oftheprogramconsists

material measurement_{values; then}theprogram outputstheBRDFcurveandfeature

vector,

including key

dataanalysisfigures.

Before anycalculationstake_place,the referencevalueshavetobeobtained.The

measurementanalysisthattakesplaceinthe_program,comparesthemeasured valuesto the

valuesobtained_{from measuring}aknown_{reference,making}theoutputvaluesrelative,not

absolute.

Calibrating

theInstrument

Using

aKnown Reference

Inthis experiment, thereferenceisa piece of

black,

matte_vinyl,whichhas been

used as a reference materialinother research

involving

themicro-goniophotometerdone in

theChester F. Carlson Center for

Imaging

ScienceatRIT. Thematerialwas chosen

because itasa mattesurface,it hasa wide range of gloss angles.

By

_usingthisspecific

material as areference, theobtained results are comparableto_anyother measurementdone

withthe

device,

on samples of_anymaterial,as

long

asthesame reference material was

usedinthoseothermeasurements.

Measuring

thereference material requiresthesameprocedureas

taking

measurementsof_anyothermaterial.Thereferencematerialiswrapped aroundthecylinder

and

imaged;

afterthatthe checkingforover-exposed picturesis doneonImageJ forthe

red, green,andbluechannelsseparately.Theexposuretimeistweakeduntilthe_resulting

images havenoover-exposedpixels,andthefinal imagesare saved.

Thereference materialimage file locationsareenteredintotheMathcadprogram,

specifying thereference area as 1. Separatecalculationsare madeforeachimagechannel.

isrecorded and used asthereferenceareawhen_analyzingsample

images,

thus_comparing

allfurtherresultsto theresultsobtainedfrom_measuringthereference.

ImageAnalysis Procedure

Afterthereference analysisis

done,

thefile locations forthepolarizers crossed

imageandthepolarizersparallelimageareinput intotheprogram.This is done_separately

forallimagepairs

-the red,green andblue_channels,as well as thecomposite

image,

usingthereferenceinformation from_{the red, green,}

blue,

and composite

image,

respectively.

Theprogramproceeds

by

_subtractingthepolarizers crossed(bulkgloss)image

fromthepolarizers parallel(all_gloss)image fromeach_{other,resulting in just first}surface

gloss.After this,theprogram performs_{calculations,resulting}ina plot oftheBRDFcurve,

then_outputtingthe

key

dataelements_regardingtheBRDFcurveintoafeaturevector.A

*

[Hi]

*-_{J <? S_<tf *,}

MySite (*Go

GoniophotometrlcAnalysis 17 June 2006 Mba

READBMP("brot_ed26_90_redt_")

Mdb_{READBMF("bro_d26_0_d.tif") Nr}m_tow<Mb) Nr-_{1000 Nc}s_cols(Mb) Nc- ₁₅₀₄ (1) SpecifytheHorizontal Heldof viewInmillimeters: FOV=16.8 millimeters

II

d^33 9instB130 (2) Reference AreaandRl: Aia7 07

(3) ReferenceIDNumber REFa0

noa1 Default: Ar=no=1

(4) Specifyexposure constants: ta 15

3b800 Sca1 [Default: 1. 1 1]

(7) Picture Adjustment: _ya1

ctLa-20 aRs20

ado=-27.995

Show

(A) BRDF tuvsn

Vdl..

WxRITEPRN("BRDF-silvM_green0112tit"):=DAT I 5.91 8.53 20.53 0.19 6.08 5.26 37.93 0.06 0.81 16.8

(B) FeatureVector

WxRITEPRN("Vec-SP_silver_green0112txt"

):=_Vdat

0 A=_{Arearelativeto referencearea}

1 whalf=halfpeak width indegrees 2 w10=peak width ath/10indegrees 3 h=_peakheight

4 omax=_stdevas%ofheightat peak 5 2ndmom.abt mean ofBRDF. 6 Glossas%ofidealized referencepeak 7 SkewnessoftheBRDF

8 Kurtosis=0fora normaldistribution 9 FOV inmm_(horizontal)

[image:48.524.48.482.46.504.2]

10Reference IDNumber _v

Figure 19. Mathcadscreenshot

Theprogram outputs a gloss

image,

a rawdata BRDF (shownonthe

right)

and a

normalizedBRDFcurve(inthe

bottom),

wherethepeakheightisset as 1. Thecoredata

drawn fromtheBRDFanalysisisthefeature_vector,whichcontainsnumericaldescriptors

abouttheBRDFcurve.The featurevectorisshownonthebottomright asthe tableof

referencesampleinto_{consideration,butthereisno visual representation ofthis}calibrated

BRDFinthe

Mathcad

output.Thenumbers,suchaspeak

height,

fromthefeaturevector

cannotbeobservedinthecurve_plots,duetodifferentscales

being

used.Themost

important featurevector_components,relatedtothis_analysis,arelistedin Table 1. In

additionto_these,thefeaturevectorcontainsadditional parametersfor

describing

the

BRDFcurveshape.

Table 1. Feature Vector Components

(Source:

Arney,

2006b)

A AreaunderBRDF

W__f WidthofBRDFathalf-peak

h HeightofBRDF

Aftertheprogram outputstheinitial BRDF_{curve, the}curve shapeis inspectedand

theleftandrightangularboundaries are adjusted.Theprogram calculates theareathatis

betweentheseangularboundaries.BecausetheBRDFcurveisasymptotic,

including

too

muchofthe"tails"inthecalculation would resultin misleadingresults,becauseall of

thesealmost-zero areas would add up.Theangularlimitsare chosen

by

theprogram

operatorsothatsomeofthe tailsare

included,

butnot all.This is becausethecurveis

asymptotic and_summingupallthenear zerodataundertheendsofthetailswoulddistort

thearea measured.Thechoiceforangularlimitaffects_mosdythearea underthecurve and

thewidertheangularrange, thelargerthearea underthecurve willbe. Theangularlimits

tobechosen are evaluatedseparatelyforeachBRDFthatMathcadoutputs. Asan

guL= _{-20 <xR} =

20

-40 -20

0

20

40

fXi,Xb

[image:50.524.132.393.99.345.2]

Chapter 6

Results

DeviceCalibration

Using

aKnown Reference

The blackvinylmaterialwasimaged_usingthemicro-goniophotometer andthe

imageswereinputtoMathcadtoobtainthereference areathatisusedin_analyzingthe

images fromtheother samples.Before Mathcadanalysis, thecapturedimagewas separated

intothe red, green,andbluechannels,resultingin fourpairs ofimagestobeanalyzed.

After

inputting

thedata into

Mathcad,

the

following

resultswere obtained. Figures 21-24

showtheimagesthatMathcad hascalculated

by

_subtractingthebulkreflectedlight image

(polarizers_crossed)fromtheall reflectedtight image (polarizersparallel).

Figure22.Referencesample red channel gloss

As seeninFigures21-24, thereis remainingglossintheleftandrightedges of

the

image,

_signifyingthatthedevicecannot capturetheentire angular range ofthisspecific

reference material. The devicecan_onlycapture a range of+/-₃₀

degrees,

_andthe

reflectionfromthismaterialtakesplace at a wider range.Thismeansthatthearea under

theBRDFcurveisnotcompleteanddoesnot givethereal area.The BRDFcurves,as

output

by

Mathcad,

are shownin Figures 25-28.

0.04

<Xi,Xb

Figure 25. Referencesample compositeimageBRDF

0.04

0.03-t_

=

c

0.02-0.04

[image:54.524.136.378.327.529.2]

fXj,Xb

Figure 27. Referencesample green channelBRDF

0.04

0.03-V 0.02

0.01

-aj.Xb

Figure 28. ReferencesamplebluechannelBRDF

Othermicro-goniophotometer setupsbuilt inthe

laboratory

havedifferentangular

ranges,andthereferencematerialhas beenused with all ofthem. Theangular range

By

measuringthereference materialinadevicethathasa wide enough angularrangeto

capture all ofthereflected

tight,

oneobtains abetterestimateofthearea undertheBRDF

curve.Becausethedeviceusedinthisexperimentcan_onlycapturebetween-30and30

degrees,

thearea underthe"correct"BRDFcurve,betweentheseangles,iscalculatedfrom

measurementsdonewith anotherinstrument. This is done

by

measuringthe sampleonthis

otherinstrumentandthen

defining

theangular ranges as -30and

30,

and_comparingthat

areato theone obtained withtheentire range andfromthiscomparisonacorrectionfactor

is obtained. Figure 29 illustratesthisrelationship.

Figure 29. Area between-30and30

degrees,

comparedto theentire curve area

Inthis case, thecorrectionfactoris

1.9,

meaningthat theentire area underthecurve

is 1.9timesbiggerthan theareajustbetween-30and30degrees.Alltheareas obtained

from_measuringthereference material are multipliedwiththis number,before

being

used

as thereferenceareainother measurements.Thiscompensatesforthelossofdetail dueto

Table2.

Reference

areasmeasured and corrected

Areameasured Areameasured x 1.9

Composite

image 2.39 4.54

Red 2.13 4.04

Green 2.46 4.67

Blue 2.11 4.01

Inall oftheimageanalysisproceduresdone in

Mathcad,

thesecorrected reference

areas are used asthearea whichtocomparethe_{corresponding}sampleimage datato(for

examplethered reference area

being

used withthered channel

images.)

Presenting

theResults

Themeasurement results are presentedfor bothexperiments(page

22)

_usingthe

same structure.Theresultsforthecomposite

image,

red, green,andbluechannel are

grouped_together,sothateach_groupcomparesthetwosamples accordingly.Withineach

group, theresults consistof glossimagesandBRDFcomparisons._{Each group has}two

gloss

images,

oneforeach sample.The BRDFcurves are presentedintwodifferent

figures. The first figureshowstheBRDFcurvesfromcompared samplesinthesame_scale,

sothatthecurve sizes canbecompared visually.Thesame scaleisusedforall ofthese

BRDFcomparison curves(forthedifferentchannels andthecomposite

image)

within each

experiment,sothat thereaderhasa_wayto_visuallycomparetheresultsfromdifferent

channels.ThesecondBRDFcurve showsthetwocurves normalizedtomaximumpeak

Detecting

Difference betweenSampleswithaLarge Visual Difference

Todeterminehowthemicro-goniophotometerbehaveswithmetallics, twovisually

verydifferentsamples were measured.The firstsample was a_{silverycolor,possibly}coated

witha

lacquer,

whiletheotherone was a goldenbronzecolored one with no additional

coating.The silverysamplehad very fine_pigments,whilethebronzeonehad_verycoarse,

largerpigments._{The silvery}sample was_printed,whilethebronzesample was

bronzed,

as

in dustedwithbronze_powder,andisnot_actuallyprinted, thusmakingthe samplesdifferin

color,pigment_size, and coating.Thesamples are referredtoas

"silver"

and

"bronzed"

throughout thedocument. Theanalysis resultedin fourpairs of glossimagestobe

compared,as well asfourpairs offeaturevectors andBRDFcurves.

Composite Image

Thespecular gloss

images,

as output

by

Mathcad,

forthecompositeimageare

Figure 30. Silversample composite glossimage

Figure 31. Bronzedsamplecomposite glossimage

The glossimagesare_{visually very}

different,

whichis tobeexpectedbecauseofthe

differentsurface structure anddifferenceincoating.Thesilverimage hasthegloss

"focused,"

whereasthebronzed image hasthegloss spread out over alargerarea.This

same visualdifferenceis observedinthe red, green,andblue_separations,as well.The

BRDFcurvesare comparedin Figure

32,

andFigure33showsthecurves normalizedto the

same scale.The

key

featurevectorelementsfromthese twoBRDFcurves arepresentedin

Silvervs.

Bronzed,

_{compositeimage} 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05

\

Silver

\

Bronzed 0.00

-30 -20 -10 0 10

Angle,

degrees [image:59.524.134.395.53.319.2]

30

Figure 32. BRDFcomparison ofthesilver andbronzed_sample,

compositeimage

Silvervs.

Bronzed,

compositeimage, normalized

1.0 A 0.9 0.8 * 0.7 cd *-6 __ 0.5 0.4 o z 0.3

\

Silver

\

Bronzed

A

0.2

//

\\

i

0.1

\JJ

Vl

0.0

-30 -20 -10 0 10 20 30

Angle,

degrees

Figure33. Normalized BRDFcurves,stiverandbronzed

[image:59.524.122.394.338.615.2]

Table3.Feature Vector

Comparison,

compositeimage

Silver Sample Bronzed Sample

Area 11.0 4.7

W__f 4.3 9.5

h 0.56 0.14

ThestiverBRDFcurvein Figure 32shows a sharper peakthan thebronzedBRDF

in Figure

30,

_supportingthevisual observation.Thesharper and narrower_{the peak, the}

more

"focused"

theglossis. Thesilver sample alsohas alargerarea undertheBRDF

curve,meaningthatmoretight isreflected andthatit isglossier.

Having

thebronzedcurve

bewideris alsoconsistent,becauseawider curve meansthat theglossismore_evenly

distributedandthat thegloss peakisnot asdistinct. Whilethebronzedsampledoes havea

distinguishablepeak,it isnot as

"sharp"

as withthesilver sample.

Red Channel

Thespecular gloss

images,

as output

by

Mathcad,

forthered channelseparation of

Figure 34. Silversampleredchannelgloss

Figure 35. Bronzedsamplered channel gloss

Thesamedifference insurface structure canbeobservedfromthered channel

images,

as well.Becausethebronzedsampleis very_{reddish, the}red channelisthe

strongest_amongthedifferentseparations.The BRDFcurves are comparedin Figure

36,

andthecurvesnormalizedto thesamescale are presentedinFigure 37. The

key

feature

Silver_red

vs. Bronzed_red

0.60

0.55

0.50

\

Silver

0.45

|>

0.40

\

Bronzed

|

0.35

LI

"S

0.30

jjj

0.25

A

__ 0.20

/

\

0.15

/

\

0.10

/

\

0.05 ._y

v.

-30 -20 -10 0 10 20 30

[image:62.524.121.402.55.312.2]

Angle,

degrees

Figure 36. BRDFcomparison ofthesilver andbronzed

sample red channel

CO T3 T3 -a CD N CO E _. o Z 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

Silver_redvs.

Bronzed_red,

normalized

0.0 -30

\

Silver

\

Bronzed

-20 -10 0

Angle,

degrees [image:62.524.122.404.364.621.2]

30

Figure 37. Nonnalized BRDFcurves,silver andbronzedsample

Table4.Feature Vector

Comparison,

red channel

Silver Sample Bronzed Sample

Area 12.4 10.34

W__f 3.5 8.5

h 0.69 0.33

Thered channelisthedominantone withthebronzedsample, theobtainedvalues

being

significandy higherthanwiththecompositeimage. Whenthecompositeimagehas

thebronzedareabe roughly 50% lessthan thesilverarea,herethedifferenceis

significandy less. Figure 36showsthatthecurve shapes are_nearly

identical,

exceptforthe

sharppeak.Thesmalldifference inshapeis apparentin Figure

37,

which showsthe

bronzedsamplestill

having

thereflectedtightspread more_evenlyacross the range of

angles.

Green Channel

Thespecular gloss

images,

as output

by

Mathcad,

forthegreen channel separation

Figure 38. Silversamplegreen channel gloss

Figure 39. Bronzedsample green channel gloss

The bronzedsample seemstohave less information inthegreenchannelthan the

silverimage

has,

judging

by

evaluatingthepixelbrightnessvalues visually.This is dueto

thecolor ofthesamples. Silveris a neutralcolor,

having

thedifferentchannelsbethesame

intensity,

whilethebronzedsampleisreddishin_color,

having

theredchanneldorninate.

The BRDFcurvesare comparedinFigure 40andthecurves normalizedto thesamepeak

maximumheightarepresentedin Figure 41. The

key

featurevector elementsfromthese

Silver_green vs. Bronzed_green CO a o Si

&

0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

\

Silver

\

Bronzed

-30 -20 20 30

[image:65.524.125.409.48.295.2]

Angle,degrees

Figure 40. BRDFcomparisonofthesilverandbronzedsample

green channel

Silver_greenvs.

Bronzed_green,

normalized

<0 N m E 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

\

Silver

\

Bronzed 0.0

-30 -20 -10 0 10

Angle,degrees

30

Figure 41. Normalized BRDFcurves,silverandbronzedsample

[image:65.524.122.405.357.623.2]

Table 5. Feature Vector

Comparison,

green channel

Silver Sample Bronzed Sample

Area 10.9 3.2

W__f 4.4 10.8

h 0.54 0.09

The bronzedsampledoesnothavea_verydistinctpeak; theglossismore scattered.

Thearea ofthebronzedsampleis_{approximately}one-third ofthesilversample,andthe

peakheight is about one-sixth ofthesilversample peakheight. Theshapesarealso

significandy

different,

thebronzedsample

being

wide andshort, thesilver onenarrowand

tall.Thesilver sample gives a_verysmoothBRDFcurve,butthebronzedsample curvehas

amplifiednoise closerto the tailsofthepeak.Thisnoiseisclippedoutbefore_makingarea

calculations

by

_settingtheangular

limits,

thus_makingthedata in Table5 becomparableto

theotherdata fromthisexperiment.

Blue Channel

Thespeculargloss

images,

as output

by

Mathcad,

forthebluechannel separation of

Figure 42. Silversamplebluechannelgloss

Figure 43. Bronzedsamplebluechannel gloss

The bronzedsamplebluechannelimage is severely_{under-exposed; there}is

hardly

any datawiththeseexposuresettings andthe_resulting analysisdata isincoherentand

mostiy justnoise. It isthusdiscarded from furtheranalysis and comparisons.Whenthis

eff