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 the
fun-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