Image Sticking — Cause, Test and Solutions - Tech Briefs

The cause of LCD image sticking is due to an accumulation of ionic impurities inside the liquid crystal materials. When slight DC voltage occurs ... HomeTest&MeasurementArticle September1,2021 |Photonics/​Optics|Imaging|Test&Measurement ImageSticking—Cause,TestandSolutionsFigure1.ImageStickingLCD(liquidcrystaldisplay)isthemostwidelyuseddisplaytechnology.Theyareusedforautomotive,appliance,telecommunication,homeappliance,industrial,consumerelectronic,militaryetc.ButLCDdisplayshavesomedrawbacks,suchasslowresponse,narrowviewingangle,lowercontrastetc.Oneannoyingphenomenonoftencomplainedaboutbyusersisimagesticking.WhatisImageStickingIfafixedimageremainsonadisplayforalongperiodoftime,thefaintoutlineofthatimagewillpersistonthescreenforsometimebeforeitfinallydisappears.Normally,ithappenstoLCDandplasmascreens,butforthepurposeofourdiscussion,wewillfocusonTFTLCDdisplays.Imagestickingisalsoreferredtoas“imagepersistence”,“imageretention”,“ghosting”or“burn-inimage.”HowAreLCDsConstructed?Figure2.LCDDisplayStructure(ImageSource:ProgrammerSought)Tounderstandbettertheconceptofimagesticking,itisnecessarytounderstandhowLCDsareconstructedandhowtheywork.AnLCDscreenincludesathinlayerofliquidcrystalmaterialsandwichedbetweentwoelectrodesonglasssubstrates,withtwopolarizersoneachside.Apolarizerisanopticalfilterthatletslightwavesofaspecificpolarizationpassthroughwhileblockinglightwavesofotherpolarizations.TheelectrodesneedtobetransparentsothemostpopularmaterialisITO(indiumtinoxide).SinceanLCDcan’temitlightitself,normallyabacklightisplacedbehindanLCDscreeninordertobeseeninadarkenvironment.ThelightsourcesusedforabacklightcanbeLED(lightemittingdiode)orCCFL(coldcathodefluorescentlamps).TheLEDbacklightisthemostpopular.Ofcourse,ifyouwantacolordisplay,alayerofRGBcolorfiltercanbemadeintoanLCDcell.AtouchpanelcanalsobeaddedinfrontofanLCDdisplay.HowLCDsWorkFigure3.HowLCDsWork(ImageSource:JapanDisplayInc.)ThefirstLCDpaneltechnologyinmassproductioniscalledTN(twistednematic).TheprinciplebehindtheLCDsisthatwhenanelectricalfieldisnotappliedtotheliquidcrystalmolecules,themoleculestwist90degreesintheLCDcell.Whenthelighteitherfromambientlightorfromthebacklightpassesthroughthefirstpolarizer,thelightispolarizedandtwistedwiththeliquidcrystalmolecularlayer.Whenitreachesthesecondpolarizer,itgetsblocked.Theviewerseesthedisplayasblack.Figure4.ImageStickingTesting:ChessBoardWhenanelectricfieldisappliedtotheliquidcrystalmolecules,theybecomeuntwisted.Whenthepolarizedlightreachesthelayerofliquidcrystalmolecules,thelightpassesstraightthroughwithoutbeingtwisted.Whenitreachesthesecondpolarizer,itwillalsopassthrough,meaningtheviewerseesthedisplayasbright.BecauseLCDtechnologyuseselectricfieldsinsteadofelectriccurrent(electronpassesthrough),ithaslowpowerconsumption.WhatCausesImageSticking?ThecauseofLCDimagestickingisduetoanaccumulationofionicimpuritiesinsidetheliquidcrystalmaterials.WhenslightDCvoltageoccurs,thechargedimpuritieswillmovetheelectrodesandbuildupareversedvoltagefield.Whenthepowerisremoved,thereversedvoltagewillkickintomaketheLCDmoleculestwisteddifferentfromtheotherpartoftheLCD,whichshowsupastheimagesticking.Thelongerthetime,themoreimpuritieswillmigrate,thelargerthereversedvoltagewillbe,andtheimagingstickingwillappearworse.HowtoTestImageStickingQualityUsingtheblack/whitechessboardimageshownabove:Staticimageitfor2hours,thenchangeto50%grayfor1min.Usean8%neutraldensityfiltertocheckifitisOK.Staticimageitfor8hours,powerofffor30minutes,thenchangeto50%graywithan8%neutraldensityfilter.HowtoAvoidImageStickingTrynottousea“fixed”imageontheTFTLCDdisplayformorethan2hours.UseascreensaverwhentheLCDscreenisnotusedformorethan15minutes,orpowerdownthedisplayifnotusingitformorethananhour.Ifastaticimagemustbedisplayed,trytouseblockpatternsinsteadofdistinctborderlines.Trytousemediumgrayhuesandusecolorsthataresymmetrictothemiddlegreylevelattheboundaryoftwodifferentcolors.Graduallyshifttheborderlinesonceinawhile.ForLCDmanufacturers,trytoprotectliquidcrystalmaterialsexposedtotheairbyusingnitrogengasordriedairtoavoidabsorbingmoisturethatcancreateahugeamountofimpuritiesintheliquidcrystalmaterial,aswaterisanexcellentsolvent.Controllingthehumidityofthefabisalsoveryimportant,asisselectingtherightliquidcrystalmaterialsandtheirmanufacturers.Differentliquidcrystalmaterialshavedifferentmoistureabsorbingabilities.Differentliquidcrystalmaterialfactorieshavedifferentcapabilitiesintermsofcontrollingimpurities.Despitethefactthathighpuritycanmeanhighincost,usinghigherpurityliquidcrystalmaterialsanddesigningthecircuitrytogetridofDCinLCDdisplaydriverscanavoidanimagestickingissue.HowtoFixanImageStickingIssueUnlikethe“burn-in”issuecommonwithCRTs,animagestickingissueisnotpermanent.Itwilleventuallyrecoveraftersometime.Onewaytoexpediteerasingaretainedimageistohaveascreenoninanall-blackpatternfor4-6hours.Ifyouwanttomakeitevenfaster,thedisplaycanbeputintoanenvironmentwithatemperatureofaround35to50°Cfor1-2hours.Asthiselevatedtemperatureiswithintheworkingtemperaturerange,itwillnotdamagetheLCDpanels.ThisarticlewaswrittenbyBillCheung,EngineeringLeadandMarketingManager,OrientDisplay(Bellevue,WA).Formoreinformation,visithere .Topics: DisplaysDisplays/Monitors/HMIsImagingImagingandvisualizationLightemittingdiodes(LEDs)OpticsPhotonicsTelecommunicationsTest&MeasurementMoreFromSAEMediaGroupPhotonics&ImagingTechnologyAPaper-LikeLCD—Thin,Flexible,Tough,andCheapPhotonics&ImagingTechnologySpectroscopyUsinganOpticalMicroscopeTechBriefsUsingSolarPanelTechnologytoCreateUltrahigh-ResolutionOLEDDisplayTechBriefsProductsofTomorrow:May2018MoreTechBriefsMobileRuggedDisplaysforSituationalAwarenessTechBriefsDefectsEnableHigh-PerformanceInfraredPhotodetectorsTechBriefsElectronicPaperDisplaysBrilliantColorsPhotonics&ImagingTechnologyUsingDigitalMicroscopesasInspectionToolsTechBriefsSub-PixelSpatialResolutionInterferometryPhotonics&ImagingTechnologyMicrochipProducesHigh-ContrastOCTImageryTechBriefsDiffractionGratingProvidesWiderAnglesforLightInputPhotonics&ImagingTechnologyNewProducts:September2021Photonics&ImagingTechnologyPhotonics&ImagingTechnologyArtificialIntelligenceMagnifiestheUtilityofElectronMicroscopesTechBriefsAtomicallyThinLight-EmittingDeviceforInvisibleDisplaysPhotonics&ImagingTechnologyMegapixelPhoton-CountingCameraTechnologyPhotonics&ImagingTechnologyResearchersGenerateTerahertzLaserwithLaughingGasTechBriefsManufacturingUltra-ThinCameraLensesMedicalDesignBriefsImprovingMid-InfraredImagingandSensingPhotonics&ImagingTechnologyEnhancingtheCapabilitiesofHigh-SpeedCMOSImageSensorsPhotonics&ImagingTechnology2021OEMPhotonics&ImagingDirectoryPhotonics&ImagingTechnologyMagazineThisarticlefirstappearedintheSeptember,2021issueofPhotonics&ImagingTechnologyMagazine.Readmorearticlesfromthisissue 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